Battery charge distribution based on reactive load

Abstract

There is a battery system configured to manage reactive energy transfer between a plurality of battery cell modules, where a switching circuit is configured to selectively connect any two or more battery cell modules with a reactive circuit, and configure the polarity of each battery cell module connected with the reactive circuit. A controller is configured to control the switching circuit to connect a first group of one or more battery cell modules to drive a reactive current through the reactive circuit. The controller also controls the switching circuit to connect a second group of one or more battery cell modules to the reactive current with a polarity opposite to that of the first cell group. The polarity connection of the second group is based on a determined phase angle of the reactive current.

Description

[0001] Battery charge distribution based on reactive load

[0002] Technical Field

[0003] The invention generally relates to battery storage systems, and in particular to a battery system for managing cell balance using reactive current.

[0004] Background

[0005] Energy storage systems for applications such as full electric vehicles, hybrid electric vehicles, and stationary energy storage in grid connected or off grid applications, frequently include an arrangement of multiple battery cells.

[0006] In multiple battery cell systems, cells connected in series have a problem where cell voltages may become imbalanced, leading to imbalanced cell performance. Battery cell balancing methods are well known, however, there is a problem where known systems require the application of an external power source to the battery cells to charge them to a desired voltage, or require energy from a cell to be dumped into a load if the voltage of the cell is over a desired voltage.

[0007] It is an object of the invention to alleviate or improve upon the aforementioned disadvantages of the prior art, or at least provide the public with a useful choice. Other objects will be apparent to those skilled in the art.

[0008] Summary of the invention

[0009] In an aspect the invention relates to a battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules, a switching circuit, and a reactive load; the switching configured to: selectively configure a circuit operative to deliver current from one or more of the plurality of battery cell modules to a set of output terminals, and selectively configure the relative polarity of each battery cell module configured in the energy path; and a controller configured to: operate the switching circuit to control the connection of a first group of one or more cell modules in series to drive the current through a reactive circuit, and connect a second group of one or more cell modules to the reactive current with an opposing polarity to the first cell group and based on the phase angle of the reactive current.

[0010] In an aspect the invention relates to a battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules; a reactive circuit; a switching circuit configured to: selectively configure any battery cell modules in a series connection with the reactive circuit, and configure the relative polarity of each battery cell module connected in the series connection; and a controller is configured to operate the switching circuit in a first mode and a second mode, wherein: in the first mode: connect one or more battery cell modules to an output for an external load through the reactive circuit, and in the second mode: disconnect the battery cell modules from the output for the external load, select a first group of one or more battery cell modules, select a second group of one or more battery cell modules, alternate, at a frequency, a connection of the first group of battery cell modules with the reactive filter to thereby drive a reactive current in the reactive circuit, determine the phase angle and polarity of the reactive current, and selectively connect the second group of cell modules with the reactive circuit.

[0011] In an aspect the invention relates to a battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules; a reactive circuit; an output circuit; a switching circuit having a set of output terminals and configured to operatively connect one or more battery cell modules with the set of terminals, the operative connections comprising the selective connection of one or more battery cell modules with the set of output terminals, and the selective control of the polarity of each of the one or more battery cell modules relative to the output terminals; wherein the reactive circuit is configured to operatively connect with the set of output terminals.

[0012] In an aspect the invention relates to battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules; a switching circuit configured to: selectively connect any two or more battery cell modules with a reactive circuit, and configure the polarity of each battery cell module connected with the reactive circuit; and a controller configured to: control the switching circuit to connect a first group of one or more battery cell modules to drive a reactive current through the reactive circuit, and control the switching circuit to connect a second group of one or more battery cell modules to the reactive current with an polarity opposite to that of the first cell group, wherein the polarity connection of the second group is based on a determined phase angle of the reactive current.

[0013] In an aspect the invention relates to a battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules; a switching circuit configured to: selectively configure any two or more of the battery cell modules in a series connection, configure the relative polarity of each battery cell module connected in the series connection; and a controller configured to operate the switching circuit to control the connection of a first group of one or more cell modules in series to drive a reactive current through a reactive circuit, and connect a second group of one or more cell modules to the reactive current with an opposing polarity to the first cell group and based on the phase angle of the reactive current.

[0014] In an aspect the invention relates to battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules; a reactive circuit; a switching circuit configured to: selectively configure any battery cell modules in a series connection with the reactive circuit, and configure the relative polarity of each battery cell module connected in the series connection; and a controller is configured to operate the switching circuit in a first mode and a second mode, wherein: in the first mode: connect one or more battery cell modules to an output for an external load through the reactive circuit, and in the second mode: disconnect the battery cell modules from the output for the external load, select a first group of one or more battery cell modules, select a second group of one or more battery cell modules, alternate, at a frequency, a connection of the first group of battery cell modules with the reactive filter to thereby drive a reactive current in the reactive circuit, determine the phase angle and polarity of the reactive current, and selectively connect the second group of cell modules with the reactive circuit.

[0015] In an aspect the invention relates to battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules; a switching circuit configured to: selectively configure any two or more of the battery cell modules in a series connection, configure the relative polarity of each battery cell module connected in the series connection; and a controller configured to operate the switching circuit to control the connection of a first group of one or more cell modules in series to drive a reactive current through a reactive circuit, and connect a second group of one or more cell modules to the reactive current with an opposing polarity to the first cell group and based on the phase angle of the reactive current.

[0016] In an aspect the invention relates to battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules, a switching circuit, and a reactive load; the switching configured to: selectively configure a circuit operative to deliver current from one or more of the plurality of battery cell modules to a set of output terminals, and selectively configure the relative polarity of each battery cell module configured in the energy path; and a controller configured to: operate the switching circuit to control the connection of a first group of one or more cell modules in series to drive the current through a reactive circuit, and connect a second group of one or more cell modules to the reactive current with an opposing polarity to the first cell group and based on the phase angle of the reactive current.

[0017] The below embodiments are intended to be read in combination with any one of the above aspects, alone, or in combination with any other embodiment.

[0018] In some embodiments, the controller is further configured to operate the switching circuit in a first mode and a second mode, wherein: in the first mode the switching circuit is configured to: connect one or more battery cell modules to an output for an external load through the reactive circuit; and in the second mode the switching circuit is configured to: disconnect the battery cell modules from the output for the external load, alternate, at a frequency, the connection of the first group of the one or more battery cell modules with the reactive circuit to drive a reactive current in the reactive circuit, determine the phase angle and polarity of the reactive current, and connect the second group of one or more battery cell modules to the reactive circuit with a polarity opposite to that of the first group, based on the determined phase angle.

[0019] In some embodiments, the reactive circuit comprises at least part of a low pass output filter configured to connect with the output of the one or more switching circuits and filter an output from the plurality of battery cells when the output connection is connected to the load. In some embodiments, the low pass filter comprises an LCL filter.

[0020] In some embodiments, the switching circuit comprises switches configured as a bridge circuit for controlling the polarity of one or more battery cell modules.

[0021] In some embodiments, the switching circuit comprises switches configured to control the interconnection of battery cell modules in a series or parallel circuit arrangement. the controller is further configured to selectively connect and alternate, at an alternation frequency, the polarity of the first and second cell groups and thereby control the transfer of reactive current between cells.

[0022] In some embodiments, the battery system further comprises a resistive component configured for connection in the reactive current path, and the controller is configured to control the resistive current based on connection of the resistive component.

[0023] In some embodiments, the selective connection the second group of cell modules with the reactive circuit is based on the determined phase angle of the reactive current.

[0024] In some embodiments, the switching circuit comprises one or more switching circuits, each configured to:

[0025] • selectively connect two or more battery cells in the series connection,

[0026] • selectively bypass one or more cells from the series connection, and

[0027] • selectively control the forward or reverse polarity of one or more cells in the series connection.

[0028] In some embodiments, the reactive circuit is a low pass filter configured to receive the output of the one or more switching circuits.

[0029] In some embodiments, the controller is further configured to:

[0030] • determine the first cell group based on one or more criteria, comprising one or more battery cells configured with a first polarity,

[0031] • determine, based on the one or more criteria, the second cell group comprising one or more battery cells configured with a second, opposing, polarity,

[0032] • control the one or more switching circuits to selectively connect the first and second cell group in the series connection with the reactive filter, and

[0033] • selectively connect and alternate, at an alternation frequency, the polarity of the first and second cell groups in the series connection to thereby control the transfer of reactive current between cells.

[0034] In some embodiments, the output connection is configured to selectively connect and disconnect the plurality of battery cells to a load.

[0035] In some embodiments, the reactive circuit is configured to filter an output from the plurality of battery cells when the output connection is connected to the load.

[0036] In some embodiments, in the second mode, the reactive circuit comprises a series connected inductive and capacitive component (an LC filter).

[0037] In some embodiments, in the first mode, the reactive circuit comprises an LCL filter.

[0038] In some embodiments, the controller is further configured to modulate the connection of one or more cells in at least one of the first and second cell groups based on one or more modulation parameters.

[0039] In some embodiments, the controller is configured to:

[0040] • determine a measured reactive current, • compare the measured reactive current to a target reactive current, and,

[0041] • based on the comparison, change at least one of: o the alternation frequency, and o the number of series connected cells in at least one of the first and second cell groups.

[0042] In some embodiments, the controller is configured to determine a target reactive current, and control the target reactive current by one or more of:

[0043] • determining a number of cell modules to be included in at least one of the first and second cell groups based on one or more of: o the target reactive current, and o a state of charge criteria of at least some cell modules,

[0044] • determining an alternation frequency based on the target reactive charge current, and

[0045] • controlling the switching circuit to create the series connection of cell modules in the first and second cell group at the determined alternation frequency.

[0046] In some embodiments, the controller is configured to:

[0047] • determine a measured reactive current between the first and second cell groups, and

[0048] • change at least the alternation frequency based on a comparison between the target current and the measured reactive current.

[0049] In some embodiments, a modulation parameter comprises alternating the polarity of the first and second cell groups at a frequency below the resonant frequency of the reactive filter, and increasing the reactive current comprises increasing the alternation frequency.

[0050] In some embodiments, a modulation parameter comprises alternating the polarity of the first and second cell groups at a frequency substantially at the resonant frequency of the reactive filter.

[0051] In some embodiments, a modulation parameter comprises alternating the polarity of the first and second cell groups at a frequency above the resonant frequency of the reactive filter, and increasing the reactive current comprises decreasing the alternation frequency.

[0052] In some embodiments, a modulation parameter comprises a number of series connected cells in one or more of the first and second cell groups, and the controller is configured to control the number of series connected cells in the one or more of the first and second cell groups.

[0053] In some embodiments, the controller is configured to:

[0054] • determine a measured reactive current between the first and second cell groups, and

[0055] • change the number of series connected cell modules in one or more of the first or second cell groups based on the one or more criteria comprising a comparison between a target current and the measured reactive current.

[0056] In some embodiments, the one or more criteria further comprises the state of charge, and the controller is configured to determine the state of charge of cell modules in the battery system, and based on the determined state of charge: change the number of series connected cells, or substitute one or more cell modules with the first or second cell groups with another cell module.

[0057] In some embodiments, a modulation parameter comprises modulation of the connection of one or more cells in one or more of the first and second group of cells, the modulation comprising a connection duty cycle within a connection interval, and the controller is configured to control the connection duty cycle of the one or more cells. In some embodiments, the duty cycle within the connection time comprises one or more of a PWM and PDM based connection modulation during a time interval.

[0058] In some embodiments, the connection interval is based on the reactive current phase angle.

[0059] In some embodiments, the controller is configured to:

[0060] • determine a measured reactive current between the first and second cell groups, and

[0061] • change the duty cycle applied to at least one cell of one or more of the first or second cell groups based on a comparison between a target current and the measured reactive current.

[0062] In some embodiments, a modulation parameter comprises a resistive component configured for connection in the series configuration, and the controller is configured to control the connection of the resistive component.

[0063] In some embodiments, the controller is configured to:

[0064] • determine a measured reactive current between the first and second cell groups, and

[0065] • change the number of cells in one or more of the first or second cell groups based on a comparison between a target current and the measured reactive current.

[0066] In some embodiments, the criteria comprises one or more of voltage, state of charge, temperature, age, measured service life, predicted service life, rate of temperature change, internal resistance, position in a series arrangement, state of health, charge current ability, discharge current ability.

[0067] In some embodiments, the criteria further comprises a weighted combination of two or more criteria.

[0068] In some embodiments, the modulation parameter is based on one or more of:

[0069] • a determined voltage of the one or more cell modules in the first group;

[0070] • a determined voltage of the one or more cell modules in the second group;

[0071] • a target reactive charge current;

[0072] • a determined connection time;

[0073] • the phase angle of the reactive current;

[0074] • the connection phase of the second cell group; and

[0075] • the connection interval of the second cell group.

[0076] In some embodiments, the target reactive charge current is based on one or more of:

[0077] • a threshold reactive charge current;

[0078] • a reactive charge current limit;

[0079] • a target discharge current of the one or more battery cells in the first cell group;

[0080] • a target charge current for the one or more battery cells in the second cell group; and

[0081] • the alternation frequency.

[0082] In some embodiments, the one or more criteria comprises a state of charge for cells in the battery system, and the controller is configured to: determine the first cell group based on identification of one or more cell modules to source energy based on their determined state of charge, determine the second cell group based on identification of one or more cell modules to sink energy based on their determined state of charge, and control the reactive current transfer from the first to the second group based on one or more modulation parameters, to thereby execute a cell balancing operation.

[0083] In some embodiments, the controller is further configured to execute a cell heating function comprising:

[0084] • determining the temperature of one or more battery cells is below a predetermined temperature threshold,

[0085] • alternating the first and second group polarities to transfer energy from the first group to the second group; and • alternating the first and second group polarities to transfer energy from the second group to the first group to thereby execute the cell heating operation wherein the interval of the connection of the second cell group comprises a phase angle that includes a nonreactive current.

[0086] In some embodiments, the controller is further configured to remove one or more cells from the first or second group of cells when a cell temperature reaches one or more of:

[0087] • the predetermined temperature threshold,

[0088] • the predetermined rate of temperature increase, and

[0089] • a voltage threshold.

[0090] In some embodiments, the controller is configured to determine the health status of a combination of one or more switches in the switching circuit and one or more battery cells by the steps of:

[0091] • controlling the switching circuit to thereby create a reactive current between at least two battery cells,

[0092] • determining a reactive current operating in the circuit based on the control of the switching circuit, and

[0093] • determining the health status of at least one of a switch and a battery cell based on the determined reactive current.

[0094] In some embodiments, the controller is configured to determine an operational health status of at least one cell module of the or at least one switch of the switching circuit by executing a method including the steps of:

[0095] • driving the reactive current through a series connection of a first set of components comprising the first and second cell groups, and a plurality of switches of the switching circuit,

[0096] • driving the reactive current through a second set of components comprising the first and second cell groups, and a plurality of switches, wherein the first and second group share a component;

[0097] • comparing the reactive current from each set of components to a predetermined reactive current, and

[0098] • determining the operational health status of the component shared by the first and second group based on the comparison.

[0099] In some embodiments, the controller is configured to execute a cell elimination function comprising:

[0100] • identifying a state of health criteria of at least one cell has failed a health criteria;

[0101] • selecting the identified at least one cell in the first cell group,

[0102] • selecting one or more remaining cells from the battery system in the second cell group, and

[0103] • transferring the energy from the one or more cells in the first cell group to the second cell group.

[0104] In some embodiments, the controller is further configured to disconnect the selected at least one cell from the series connection when a parameter falls below one or more of: o a predetermined temperature threshold, o a predetermined rate of temperature increase, o a voltage threshold.

[0105] In some embodiments, the state of health criteria comprises the temperature of at least one battery cell has at least one of: o exceeded a predetermined temperature threshold, and o exceeded a predetermined rate of temperature increase.

[0106] In some embodiments, the invention relates to any one or more of the above statements in combination with any one or more of any of the other statements. Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings. The entire disclosures of all applications, patents and publications, cited above and below, if any, are hereby incorporated by reference. This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[0107] To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

[0108] The term “and / or” referred to in the specification and claim means “and” or “or”, or both. The term “comprising” as used in this specification and claims means “consisting at least in part of’. When interpreting statements in this specification and claims which include that term, the features, prefaced by that term in each statement all need to be present but other features can also be present. Related terms such as “comprise” and “comprised” are to be interpreted in the same manner.

[0109] As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms “includes,” “comprises,” “including,” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present.

[0110] Brief description of the drawings

[0111] The invention can be better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the invention. Furthermore, like reference numerals designate corresponding parts throughout the several views.

[0112] Figure 1 shows an exemplary battery cell structure with cell modules and a circuit module.

[0113] Figure 2 shows an exemplary diagram of components within an exemplary battery system.

[0114] Figure 3 shows an exemplary output of a reconfigurable battery system.

[0115] Figure 4 shows a filtered output of the reconfigurable battery system of Figure 3.

[0116] Figure 5 shows an example of a reconfigurable battery system including battery cells and a switching circuit.

[0117] Figure 6 shows another example of a reconfigurable battery system including battery cells and a switching circuit.

[0118] Figure 7 shows an exemplary resonant circuit.

[0119] Figure 8 shows another example of a reconfigurable battery system including the resonant circuit.

[0120] Figure 9 shows a graph of voltage and the number of cells connected in series.

[0121] Figure 10 shows a graph of the relationship between reactive current and frequency based on components in a resonant filter circuit.

[0122] Figure 11 shows an exemplary closed loop control strategy for reactive current control.

[0123] Figure 12 shows another exemplary closed loop control strategy for reactive current control.

[0124] Figure 13 shows a simplified system diagram including cells and resonant filter components for the purpose of illustrating current control in the preceding diagrams.

[0125] Figure 14 shows a simplified system diagram including cells and reactive filter components for the purpose of illustrating current control in the proceeding diagrams.

[0126] Figure 15 shows reactive current and the frequency output by the PI controlled with a variable target current.

[0127] Figure 16 shows how the switching frequency control can work together with changing the number of cells connected in series and therefore a stepwise voltage change. Figure 17 shows response graphs of the application of PWM duty cycle control to control effective cell contribution voltages.

[0128] Figure 18 is a waveform indicating that cells see an inductive load as they are switched at a frequency greater than the resonant frequency.

[0129] Figure 19 is the waveform indicating that the battery system sees a capacitive load as it switches at a frequency lower than the resonant frequency.

[0130] Figure 20 shows the state of charge exchange in the series cell configuration. The light green curve is the SOC of the cells to be charged and the yellow curve is the cells to be discharged.

[0131] Figure 21 shows a time domain diagram of when the cells are charged, and voltage and current Vs SOC. Figure 22 is a V-l diagram to show how the current flows in the cell(s) to be charged and in the whole series cell configuration.

[0132] Figure 23 is a V-l diagram to show the entire series cell configuration where the current flows in all four quadrants,

[0133] Figure 24 shows a diagram of passive components of a simplified model of a battery cell where the state of health of a cell correlates to RO.

[0134] Figure 25 is a simulation circuit to demonstrate how RO, RC and C_terminal of Figure 24 can be measured from reactive current flow through the reactive filter.

[0135] Figure 26 shows the cell voltage (terminal + to terminal -) step response when a cell under test is included or excluded from the test circuit, where three different RO cell resistance values are shown.

[0136] Figure 27(A) shows a circuit arrangement whereby a first cell group 271 is alternated to generate alternating voltage 272 and an alternating reactive current.

[0137] Figure 27(B) shows an exemplary circuit configuration whereby a cell under test 274 is connected in series with the circuit.

[0138] Figure 28(A) shows an exemplary voltage (solid line) of the first group of battery cells, and an exemplary current (broken line) showing the phase difference of the reactive current.

[0139] Figure 28(B) shows a time alignment between the test cell connection and the current of Figure 28(A).

[0140] A current sensor 273 may be connected in the circuit for current measurement the controller may use for determination of the current phase angle offset from the voltage.

[0141] Figure 28(C) shows the voltage drop over the instant the target cell is connected to the reactive current flow.

[0142] Figure 28(D) shows the near instant current gain at the time of the voltage drop.

[0143] Detailed Description

[0144] Embodiments of the invention discussed herein relate to reconfigurable battery system technology. In a reconfigurable battery system, there are cells which require balancing, and some embodiments relate to a cell balancing system which minimises or substantially eliminates energy waste, and also makes use of components already present in the system which are primarily implemented for other tasks, or have a secondary function within the system. The cell balancing system utilises a reactive filter circuit primarily used to filter the output of a switched series arrangement of battery cells. In embodiments of the system, control based on reactive filter characteristics manages energy transfer from one or more battery cells with high energy to one or more battery cells with low energy. A controller manages reactive current through the reactive filter to provide energy exchange between battery cells within the battery system, and controls the magnitude of reactive current based on control of switch modulation parameters, and the particular battery cells selected for energy exchange through the flow of the reactive current.

[0145] In this specification, the terms “energy storage module”, "battery cell module", "cell unit" or “cell module” are generally intended to refer to a module which can store an electrical charge and can refer to an individual battery cell or a block of cells connected in series or parallel, or a multitude of individual battery cells, or blocks of parallel cells, or a combination of cells connected in series and / or parallel. Where the terms battery cell, cell or cell unit is used, equally applicable are battery module, module, cell module or module unit, where a module may contain one or more cells in series and / or in parallel.

[0146] In this specification, the term “battery cell module” may also refer to a block of cells connected in parallel and / or series and which further includes circuit components such as fuses, resistors, passively controlled diodes, capacitors or inductors are connected in series and / or parallel with individual cells. Figure 1 shows an exemplary battery cell structure 10 with battery cell modules 30 and a circuit module 20. In the exemplary structure, the battery cell modules contain one or more battery cells and are attached to the circuit modules by cell terminals. The cell module, or combination of modules, may contain any number of cells in series, or parallel, or a combination of series and parallel.

[0147] The circuit module 20 includes a circuit board which supports a circuit path and electronic components which form part of and modify the circuit path. Electronic components include switching devices such as transistors, a controller for operating the transistors, and optional sensing devices. The circuit paths and electronic components together form a switching circuit configured to reconfigure the connections between battery cells in the system.

[0148] In one embodiment, the switching circuit controls the at least the series connection of any one of those cells to the cell terminals, and the polarity of any one or more cells in the series connection. For example, in a reconfigurable battery system it is common for the switching circuit to control the connection of those cells to terminals of the circuit module 20 or for the terminals to be connected such that the cells are bypassed. Polarity control of a cell or group of cells is typically provided by a bridge circuit, and may be provided by, for example, half bridge, full bridge, H-Bridge or 2n+2 inverted H-Bridge circuit topologies. In some embodiments, bridge circuits are configured to share switches between two or more battery cell modules.

[0149] Circuits for supporting cells and the arrangement of switches for controlling connected or bypassed states of cells can have many configurations. In some embodiments, the cells and associated switching circuit have a fixed number of switches, such as two, for each cell. However, in other embodiments, the switching circuit may have combinations of two or more switches which are configured to operate to connect or bypass a cell.

[0150] In preferred embodiments, a controller is provided and configured to control the switching of one or more cell modules in and out of a current path to target a system output voltage and control the polarity of at least some cells in the configuration, and further, to control the connection timing of all cell modules based on the phase of current flowing in the system. The term controller as used in this specification will most commonly be used to describe the functions of one or more processing devices such as microprocessors. A controller may be a single processor, or a combination of multiple processing devices as may be practical. A controller will typically feature output pins which are operably connected to switching devices to control the state of that switching device. For example, the pin of a microprocessor may be connected to operate a switching device. In some embodiments, the controller is configured to control functionality of the system such as the determination of a target voltage, and control output signals required to operate the switching circuit of one or multiple cell modules to generate the target output voltage.

[0151] The term switching devices as used in this specification refers to a component which is capable of making and breaking an electrical connection to allow or prevent current flow, such as a relay or transistor. Various Figures in this specification show a SPST (Single Pole Single Throw) switch as an example of the switches provided. However, the present invention is not limited thereto, and it may be implemented with another switch element, for example, a power switch of MOSFET (Metal Oxide Field Effect Transistor), BJT (Bipolar Junction Transistor), IGBT (Insulated Gate Bipolar Transistor) and the like.

[0152] MOSFET based transistors are particularly advantageous for use in battery storage systems due to their speed of operation, cost and form factor. The presence of body diodes in most MOSFET devices means that two devices configured in anti-series and with common gates are typically used for the switching of bidirectional current as may be found in the system of the embodiments discussed herein.

[0153] Figure 2 shows an exemplary diagram of components within an exemplary battery system 10 including battery cell modules and an associated switching circuit 40, together forming a reconfigurable battery circuit 40. A reactive filter 50 configured to connect with the output of the reconfigurable battery circuit and in turn output a filtered response to an output connection. The output connection 60 is configured to connect the filter 50 with an output 80 which is connected or configured to connect with a load. A controller 70 is connected to at least the reconfigurable battery circuit 40 and the output connection 60. The controller is configured to operate functions of the switching circuit and in particular is configured to control the series connection of one or more battery cell modules with the reactive filter. Further, the controller is configured to control the polarity of at least some cell modules in the reconfigurable battery circuit.

[0154] In some embodiments, the output voltage of the reconfigurable battery system is determined by the controller operating to control specific switches which need to be connected in order to connect a number of cell modules to achieve a target voltage. The target voltage will typically be predetermined on the basis of the output voltage requirement for the system. For example, in some embodiments, the system is configured to generate a mains output voltage which may be a sinusoidal 110 to 240V signal at 50 to 60Hz. Other examples include DC voltage applications such as would be found in an electric vehicle (EV), EV charging device, or other more general DC application. As such, the target output voltage may be time varying or time static. A reconfigurable battery system is most often found with lithium-based cells and as such the voltage of individual cells is typically operated between 2.5 and 4.2V per cell. Hence many cells are required within a system to generate the desired output voltage.

[0155] In some embodiments, there are multiple controllers configured to control groups of switching devices which are arranged to control the series connection, bypass, and polarity of one or more cells. These controllers are referred to as cell controllers. In such embodiments, cell controllers are each connected to one or more switches and figured to control the switches in order to bypass or include any specific one or more cells in order to meet the target output voltage. In some embodiments, each cell controller operates under the control of a master controller which, for example, is configured to send a target output voltage requirement to the cell controllers which in turn interpret that requirement and determine whether to include or bypass cells under its control. The master cell controller is often referred to as a central stack controller. In other embodiments, the cell controllers operate together based on an algorithm which controls or determines the connection of cells. Various other control techniques are possible which operate cell controllers, connect or bypass cells and achieve a target output voltage.

[0156] Figure 3 shows an exemplary output of a reconfigurable battery system including a stepped voltage waveform which represents a battery cell being added or removed from a series connection of cells. Figure 4 shows a filtered output of the reconfigurable battery system of Figure 3 after a filter has smoothed the stepped voltage waveform. A stepped voltage output such as shown by Figure 3 may contain unwanted noise in the form of high frequency harmonics. Further, the shape of the output waveform may be desired to substantially match the smoother nature of a regular mains AC voltage waveform. Hence, filtration of the waveform is desirable in most reconfigurable battery systems.

[0157] In some embodiments, the stepped output voltage comprises a positive and negative going voltage waveform which is created by control of the polarity of one or more cells connected to the system output. For example, voltage point 32 may represent a zero crossing, where upper waveform part 31 represents a positive going voltage, and lower waveform part 33 includes a negative voltage.

[0158] In some embodiments, the battery cell modules are configured to output a positive going voltage only, and a bridge circuit is configured to change the relative polarity of a cell or cells in relation to another cell or cells. In other embodiments, the polarity of each individual cell is controllable. In each configuration, all cells may be utilised for positive and negative going waveforms through polarity control of the cells during phase parts of the output waveform.

[0159] Accordingly, the controller is configured to generate DC current, or AC current based on polarity control of cells in the series configuration. The frequency of the AC current is controlled by the frequency at which the polarity of cells in the series configuration is changed, relative to the output.

[0160] A reconfigurable battery system has a filter configured to receive the switched output voltage from the battery cells and remove high frequency content. A reactive circuit, which may be a reactive filter or part thereof, is a typical example of a filter best suited to this task, having the benefits of adequately performing the intended function and being constructed from reasonably low cost components.

[0161] In some embodiments, the reactive filter is an inductive-capacitive (LC) low-pass filter. In some embodiments, the reactive filter is a LCL filter. In some embodiments, the reactive filter is configured as an LC filter and the output filter is configured as an LCL filter, whereby LC components are shared. The sharing of components and configuration of components in the filter at any time can be provided by switching devices configured to connect L or C components in and out of the current path.

[0162] An LCL filter, also called line filter, is specially designed to suppress the switching frequency components caused by the rectifiers’ semiconductors (IGBTs). It consists of a parallel-series combination of reactors and capacitors adapted to reduce the THD(i) of rectifiers and switching circuits.

[0163] DC current at the output of a battery system may also be achieved via other means such as the use of conversion or transformation based approaches. In these instances, battery cell modules may not be configured in a series connection, but rather parallel, or series parallel as desired, and where the resulting output is controlled in some way by a circuit which generates AC current for the purpose of the conversion or transformation. A reactive circuit may also be contained in the conversion or transformation circuit. Accordingly, in this specification, where reference is made to a series or anti-series arrangement of battery cell modules, equally applicable may be parallel or series-parallel connected cells as would be relevant to the configurable circuit. However, in each circuit example, polarity control of the battery cell modules relative to the current is required.

[0164] Figure 5 shows an exemplary reactive circuit 50 which includes a first inductor 51 and a first capacitor 52. Figure 5 also shows an output connection circuit which includes a second inductor 53 and output switches 61 , 62 configured to disconnect the filter from a load which may be connected to output terminals 80.

[0165] Observable is that the reactive filter is an LC filter which includes an inductor 51 and capacitor 52 when operating a reactive filter for the purposes of transmitting reactive current within the reconfigurable battery system, and further includes inductor 53 during operation to supply current to a load connected with the output terminals 80 to thereby form an LCL filter.

[0166] In some embodiments, the reactive filter 50 further comprises a series resistor. The value of the resistance may be adjusted to change the reactive characteristics of the filter. Inclusion of a resistor may be beneficial in circumstances where alteration of the reactive characteristics is desirable, such as the resonant frequency, and may lack benefit if the value of the resistance is too high such that non-negligible power is dissipated in the resistor, thereby impacting system efficiency.

[0167] While discrete components are shown as the inductor and capacitor in the exemplary Figure, other circuit elements may also provide indictive or capacitive properties useful for forming a reactive circuit. For example, in some embodiments, the indictive component is provided by a printed circuit path, series equivalent indictive aspect of a battery cell, or a transformer device as may be located in a current path where reactive current is generated. Similarly, a capacitive component may be provided by capacitive interactions between circuit features such as tracks of a printed circuit board. While such indictive and capacitive interactions are generally small and may not always be practical, they do exist and therefore may present an opportunity to realise advantages of the invention.

[0168] Figure 6 shows another example of a reconfigurable battery system 40 including the reactive circuit 50. In particular, the battery system 40 depicts series configuration where there is a first one or more battery cells 41 configured with a first polarity, and a second one or more battery cells configured with a second, opposite polarity. An alternating reactive current is generated by rapidly alternating the polarity of each cell in the series configuration with the reactive circuit at a desired alternation frequency.

[0169] Figure 6 shows the battery system 40 connected with the reactive filter 50. When output switches 61 , 62 are closed, the reactive filter includes the second inductor 53 and the battery system supplies current to the load, shown here as the mains grid with nominal series resistance. When the output switches 61 , 62 are open, the reactive filter 50 includes LC components, current is not supplied to the second inductor 53, nor to the output load, and instead reactive charge current flows between battery cells within the battery system. In some embodiments, the system 10 includes a current measurement device 44 and the controller 70 is configured to receive a signal from the device 44 indicative of series current flowing in the circuit. In some embodiments, the controller 70 is configured to execute control of one or more battery system parameters based on the current measurement to change the characteristics of current flow. System parameters include the number of battery cells connected in series, bypassed from the series connection, the polarity of any one or more cells in the series connection, and the frequency of alternating current generated by the cells by control of effective alternating polarity state of cell modules in the series configuration, and the phase timing of the connection and disconnection events of cell modules.

[0170] The battery system of various embodiments includes a circuit module comprising a pair of output terminals configured to provide a target output voltage to a load. The module comprises a string of battery cell modules, each cell module comprises a first terminal and a second terminal, at least one cell, and a switching circuit configured to operate in at least the following states: a state to connect the first terminal to the second terminal thereby bypassing the at least one cell, or a state to connect the at least one cell between the the first terminal and the second terminal. Many other circuit configurations are possible to generate the output voltage. However, it is typical that many battery cell modules are connected in series to produce a target voltage, or that voltage conversion or transformation takes place by way of transformer or boost circuit. In each case, advantages of the invention can be realised by controlling battery cell modules within the energy path of the reactive filter such that reactive current is caused to flow in the battery cell module.

[0171] The controller is configured to control the switching circuit of each cell module to provide the target output voltage. The controller is configured to control of the switching circuit of one or more cell modules to meet the target circuit module output voltage by a combination of: series connection of one or more cell modules with the output terminals, and modulation of the switching state of at least one cell module such that the target voltage is substantially provided at the circuit module output terminals. For example, the controller will determine a set of cell modules for series connection to meet a target output voltage within a predefined time increment, then control the switching circuit of one or more cell modules comprising a combination of series connection of one or more cell modules with the output terminals, and optionally modulation, within the predefined time increment, of the switching state of at least one cell module such that the output voltage is substantially provided at the circuit module output terminals. Such switching events may include the abovementioned voltage conversion or transformation operations.

[0172] Figure 7 shows an example of a reconfigurable battery system 40 including battery cells and a switching circuit which supports the series connection of each cell, the bypassing of each cell from the series connection, and the polarity of each cell in the series connection. In this circuit, there are 2n+2 switches for n battery cells which provides a bridge circuit for each battery cell, where one side of the bridge is shared with a neighbouring battery cell to provide individual cell control with a flexible circuit path.

[0173] A 2n+2 battery circuit has a sequence of cell modules, each with positive and negative terminals. Each terminal is connected to adjacent opposing polarity terminals of another cell module via a switch. Each cell module can therefore be connected to an ‘upstream’ cell terminal by the upstream switches, and connected to a downstream cell terminal by the downstream switches. The end of the sequence has a further two switches for completing the circuit.

[0174] In Figure 7, there are four cells shown, but it will be appreciated that there may be any number of cells configured in the circuit. In one example, each cell may be two or more cells placed in parallel such that they act like a single, larger capacity cell. In another example, each cell may be two or more cells configured in series such that they act like a single cell with a higher voltage. Other combinations of series and / or parallel cells may be used in place of those shown. In this example, the switching circuit is configured such that each cell is connected with four switches. For example, cell 423 is connected to four switches 412, 413, 416 and 417. The four switches are configured such that two switches are directly connected to a first terminal of the cell, and two switches are directly connected to a second terminal of the cell. One of the two switches connected with opposite cell terminals is shared by an upstream adjacent cell. The other of the two switches connected with opposite cell terminals is shared by a downstream adjacent cell. Cells are also arranged such that the cell terminals have opposing polarity compared to adjacent cells.

[0175] The arrangement of the switching circuit is such that any one or more cells may be configured with an opposing polarity to any one or more other cells. This means that any cell may redistribute energy to any other cell. Various cell configurations are possible for connection to the reactive filter 50. Examples are as follows:

[0176] In one exemplary configuration, cells 420, 421 , 422 and 423 can be placed in series with a first polarity by closing switches 410, 415, 412 and 417. A second opposing polarity of these cells can be achieved by closing switches 414, 411 , 416 and 413.

[0177] In another exemplary configuration, cells 420 and 421 can be configured with a first polarity and in series with cell 423 which are in a second, opposing polarity. In this example, switches 414, 411 , 412 and 417 are closed. Or alternatively, switches 410, 415, 416 and 413 are closed to achieve the opposite polarity. In practice, to achieve alternating current, these two sets of closed switches are alternated between at a frequency operated by the controller to achieve an alternating current which flows through the reactive filter 50.

[0178] Alternating current is achieved by rapid switching between the two cell polarity arrangements in the series configuration.

[0179] Figure 8 shows another example of a reconfigurable battery system 40 including battery cells and a switching circuit. In this example, there are two sets 71 , 73 of battery cells configured where any cell may be connected in series with any other cell, or bypasses any cell from the series configuration of the set. Each set of battery cells is connected with a full bridge circuit 72, 74 such that the resulting polarity output from each string of cells is able to be alternated. In this way, the first string of cells 71 is able to be connected in series with the second string of cells 73. The polarity of each set of cells in the series connection may also be set by the configuration of each bridge such that the sets may be connected with like polarity, or opposing polarity. In such embodiments, the controller 70 is connected to the full bridge circuits and configured to control the output polarity from each bridge.

[0180] Other similar circuit configurations are possible while achieving the requirement of cells being controlled to have the same or opposing polarity with other cells. For example, the circuit of Figure 8 may be altered such that only one bridge circuit is present on the output of one cell string.

[0181] Other circuit configurations are possible where there are at least two cell modules (or groups of cell modules) are connected by way of a bridge circuit such that reactive current can be distributed between the groups by configuration of the opposing polarity with respect to the phase of the reactive current.

[0182] In addition to control of the switching circuit, the controller 70 is configured to execute a number of control strategies in order to effect the distribution of energy within the battery cells in the system via the control of reactive current. In one embodiment, the controller is configured to control the magnitude of current flow from one or more cells configured with a first polarity to one or more cells configured with a second polarity.

[0183] Accordingly, in some embodiments the controller is configured to operate in at least two modes. In the first mode, the output switches 61 , 62 are closed such that the battery cells are connected to the output through the reactive filter, and the controller is configured to operate the switching circuit of the reconfigurable battery cells in order to produce a target output voltage and connect the cells to the output. The target output voltage may be DC or AC, which is a time varying DC target such as the time varying output such as that observable in Figure 3.

[0184] In the second mode, the output switches 61 , 62 are open such that cells are disconnected from the output 80, and the controller is configured to target a reactive current and is configured to do so by one or more control strategies. The reactive current is generated by switching the first group of cells in and out of series with the reactive filter to create a variable voltage over time. The controller is said to be driving a reactive current by the control of the number of cells in the first group, polarity of the cells in the first cell group, and the timing of the connection of cells in the first cell group, to the effect of generating an AC reactive current through the reactive filter. In some embodiments, the second cell group is connected based on the phase angle of that reactive current.

[0185] In the second mode, the controller is configured to connect two groups of cells, a first group of one or more cells which are intended to supply energy, and a second group of one or more cells which are intended to receive energy. In some embodiments, the timing of the connection of cells from each cell group will depend on the frequency of operation relative to the resonant frequency of the filter, and whether the cells supplying energy see a primarily capacitive or inductive load, and therefore whether reactive current will lead or lag the voltage. Accordingly, the controller is configured to connect cells in the second cell group based on a phase angle determination and the frequency of operation. The phase angle of connection of the second group of cells in the series configuration, relative to the first group, depends on the cyclic and alternating connection of the cells, and therefore the frequency of alternating reactive current generated in the system. Figures 18 and 19, discussed in further detail below, illustrate examples of the relative connection phase angles of first and second cell groups. In other embodiments, the cells of the first and second group are connected at the same time.

[0186] The controller is therefore able to make use of the reactive filter as an impedance to control the magnitude of current flowing in the circuit which allows current to flow in the battery system from the first cell group into the second cell group. Although some power is stored in the LC circuit during resonance, it is reactive power only and not relevant. The reactive filter used as an impedance control device produces a very high current capability and ability to operate with arbitrarily high current (limited only by the Q of the resonant filter) and therefore facilitates rapid charge redistribution between cells.

[0187] In some embodiments, the controller is configured to determine cell data from each cell the cell data indicating criteria which may be used to select cells for a cell group, comprising one or more of: voltage, state of charge, temperature, age, measured service life, predicted service life, rate of temperature change, internal resistance, position in a series arrangement, state of health, charge current ability, discharge current ability. The controller may be configured to determine a rank for each cell module based on the determined cell data, the rank also being a selection criteria. In some embodiments, the controller is configured to determine a rank for one or more cells based on a weighted combination of two or more determined cell data parameters.

[0188] The controller is configured to control one or more modulation parameters within the system. The parameters are those which have an effect on the magnitude of the current which flows from cell modules in the first group (cells to be discharged, or source cells) and cell modules in the second group ( cells be charged, or sink cells). Parameters include the switching frequency which builds the AC reactive current, the number of cell modules in the first group and therefore the voltage and potential current, the connection interval of the first and / or second cell module groups, and the connection timing relative to the current phase of the second cell group and therefore the power factor and current transfer efficiency. Any of the above parameters may be combined as desired to control current transfer between the first and second cell module groups. The following exemplary strategies provide an overview of how each parameter may be controlled.

[0189] In a first exemplary reactive current control strategy, the controller is configured to control the number of cells connected in series with a first polarity and a number of cells connected in series with a second opposing polarity. The voltage differential of the two cell groups is a contributing factor in the reactive current which is generated and therefore the balancing charge magnitude which determines the rate of energy transfer from cells to be discharged to cells to be charged. Figure 9 shows a graph of reactive current for a number of cells connected in series at an arbitrary switching polarity frequency of 3 kHz. It is observable that the reactive current will ‘step’ between levels based on the number of cells connected in series. The magnitude of any given step will be determined by the voltage of the cell being connected in the series configuration. Accordingly, in some embodiments, the controller is configured to determine a target reactive current, and configure a series connection of cells which closely matches that target. In some embodiments, the controller is configured to measure the voltage of cells available for discharge and for connection in the series configuration, and connect those cells which best matches the reactive current target. For example, cells to be discharged are selected based on a combined voltage output.

[0190] In a second exemplary reactive current control strategy, the controller is configured to control the polarity switching frequency of cells connected in series with a first polarity (first group of cells to be discharged), and the number of cells connected in series with a second opposing polarity (second group of cells be charged). The reactive current magnitude is dependent on the switching frequency (AC current frequency) and the resonant frequency of the reactive filter. Figure 10 shows a graph of the relationship between reactive current and frequency based on components in a reactive filter circuit. It is observable that the resonant frequency of the reactive filter provides the largest possible reactive current magnitude.

[0191] In some embodiments, a range of frequencies (f min to f max) would be such that they provide an equivalent change in the fine control of current (when applied to the LC circuit) compared to adding or subtracting one cell from the coarse control of the first exemplary reactive current control strategy. In some embodiments, the f min and f max would be extended to include a range of current adjustment which exceeds that of the first exemplary reactive current control strategy amount to ensure that component variations in the LC circuit are tolerated. In some embodiments, the controller is configured to commence frequency based control from f min. In some embodiments, the controller is configured to commence frequency based control from a frequency approximately midway between f min and f max.

[0192] In a third exemplary reactive current control strategy, the controller is configured to control a modulated connection of one or more cells in the series connection. For example, the modulated connection is PWM based modulation, where the duty cycle of the connection provides for a connection time variable from 0- 100% of any cell relative to the connection time of another cell. Accordingly, PWM connection control is operable to provide a variable contribution to RMS current based on the PWM connection duty cycle.

[0193] In some embodiments, the controller is configured to commence PWM control with duty cycle = 0 which indicates the effective rms stack voltage is 0V. The rms voltage of the combined series cells is then increased so that a target current can be achieved. In some embodiments, the controller is configured to determine the difference between a target voltage and a measured voltage, then based on that difference, change the duty cycle to modulate the output voltage to get a higher voltage from the combined series cell combination.

[0194] The controller is configured to control the AC current frequency either below, at, or above the resonant frequency. Below the resonant frequency, the switching frequency is slower which may be beneficial based on the available processing speed of the controller. However, switching devices such as MOSFETS are required to be ‘hard switched’ which may lead to switching induced energy inefficiencies. Above the resonant frequency, the cell switching devices in the switching circuit see an inductive load. The switching devices can be at a zero crossing of the AC output voltage, or current, to reduce switching related energy inefficiency. However, transient voltages may be generated meaning voltage clamping is needed to protect the switching devices. For optimised energy efficiency, operating above the resonant frequency is desirable. Operating at the resonant frequency offers the largest amount of reactive current transfer. This may be desirable where cell heating, or where time critical tasks are desired.

[0195] Figure 17 shows response graphs of the application of PWM duty cycle control to control effective cell contribution voltages. Observable is the controller configured to vary the duty cycle of cells and therefore the effective voltage to control the total current. In particular, the top graph shows a varying PWM duty cycle and the bottom graph shows the corresponding change in reactive current. In a fourth exemplary reactive current control strategy, the controller is configured to control the phase timing and interval of connection of one or more cells in the series connection with the reactive filter based on a power factor determination. While this control strategy is similar to that of the PWM based example above, here, the timing of connection is based on connecting one or more cells relative to the phase timing of the current and voltage, and therefore the power factor. The power factor is defined as the cosine of the phase angle between voltage and current and represents the instantaneous power and its direction. A lagging power factor is one in which the current is lagging behind the voltage and is characteristic of an inductive load. A leading power factor is one in which the current is leading the voltage and is characteristic of a capacitive load. Therefore the controller is configured to determine a connection time of the one or more cells relative to the phase angles of the voltage and current, which are determined by the frequency of the AC current relative to the resonant frequency of the filter. Figure 18 shows the current leading the voltage by 90 degrees, and the connection interval of the cells centred around the current peak where the power factor is highest. Other connection intervals and timing of connections are possible based on a desired power transfer. For example, the connection timing and interval is aligned according to a desired power factor which is a method of controlling the rate of charge transfer.

[0196] In a fifth exemplary reactive current control strategy, the controller is configured to control the connection of a resistive device in series with the LC components of the reactive filter. As mentioned above, the resonant characteristics may be alternated by the addition of series resistance in the reactive filter, and in particular to reduce the sensitivity of the resonant peak.

[0197] In some embodiments, the controller is configured to change the number of series connected cells based on a reactive current measurement, such as from the current measurement device 44. In some embodiments, the controller is configured to change the resonant frequency based on a reactive current measurement, such as from the current measurement device 44. In some embodiments, the controller is configured to connect a resistive device in series with the LC components of the filter based on a reactive current measurement, such as from the current measurement device 44.

[0198] In some embodiments, the controller is configured to combine any two or more of the above reactive control strategies. For example, the controller is configured to begin a charge redistribution process by the connection of a number of cells in series, then based on a reactive current measurement, change one or more of the resonant frequency and / or number of cells in series. In this specification, the ability of the controller to change a configuration or operational characteristic is known as a modulation parameter. Therefore, in some embodiments, the controller is configured to operate in a closed loop based on a reactive current feedback signal and adjusts on or more modulation parameters based on that signal. In one exemplary embodiment, the controller is configured to configure a predetermined number of cells in series in accordance with the first exemplary reactive current control strategy. The number of cells is determined by a table which correlates between a current target and number of cells required to substantially effect that current target. The number of cells can be changed or the implementation of any one or more other exemplary reactive current control strategies based on a feedback signal indicative of the current in the circuit.

[0199] In another exemplary embodiment, the controller is configured to configure a first cell to be discharged in series, then incrementally add one or more cells to the series configuration in order to incrementally increase the current. The number of cells can be changed or the implementation of any one or more other exemplary reactive current control strategies based on a feedback signal indicative of the current in the circuit.

[0200] In another exemplary embodiment, the controller is configured to configure a first cell to be discharged in series, then, based on the second exemplary reactive current control strategy, tune the switching frequency to tune the magnitude of current in the circuit. If the target current is unable to be reached by frequency adjustment, the controller is configured to incrementally add one or more cells in the series configuration. Again, the controller is configured to tune the switching frequency to tune the magnitude of current in the circuit. This process is repeated until a target current is met. In some embodiments, tuning the frequency includes starting at a first switching frequency and adjusting the frequency toward the resonant frequency. This may mean adjusting the frequency up or down, depending on the first / starting frequency being above or below the resonant frequency.

[0201] In some cases, the resonant frequency is predetermined. In some cases, the frequency is changed to be higher, and if current trends away from the target current, the frequency is changed to be lower. In some cases, the frequency is changed to be lower, and if current trends away from the target current, the frequency is changed to be higher. Predetermination of the resonant frequency would not be required in this instance.

[0202] In some instances, the switching frequency will have a limited range so that the controller is able to determine whether the complex impedance is predominantly inductive or capacitive; and the working frequency may stay away from the resonant point.

[0203] In some embodiments, the state of charge (SOC) of one or more cells to be charged acts to interrupt a balancing charge control routine. For example, if a cell that has lower SOC (charging) reaches the target SOC, the controller will stop the battery system from generating current. In another example, if one or more cells that have a higher SOC (discharging) reaches a SOC limit, the controller is configured to select a new cell to be discharged to provide energy to the cell that needs to be charged. Accordingly, in some embodiments, the controller is configured to operate the switching circuit to configure cells into and out of the series configuration based on the SOC of any cell.

[0204] Various embodiments discussed in this specification may use a cell ranking process as the determination for which cells should be prioritised to be charged, discharged, or both. The aforementioned voltage target is defined by time varying amplitude and phase, and the controller is configured to control, at predetermined time intervals, the series connection of one or more cell modules and the modulation of the switching state over time based on the time varying amplitude and phase.

[0205] The controller is further configured to determine the target circuit module output voltage; and control the modulation duty cycle of the switching state of at least one cell, or cell module to meet the target circuit module output voltage. To do so, the controller is configured to determine the target circuit module output voltage and control the modulation duty cycle of the switching state of one or more cell modules based on the determined rank to meet the target circuit module output voltage. The determined rank of each cell module can be used to prioritise a higher modulation duty cycle of the switching state to those cell modules with a higher rank and / or a lower modulation duty cycle of the switching state to those cell modules with a lower rank. Modulation of the stitching state includes pulse width modulation (PWM), pulse density modulation (PDM), and any other form of modulation where connection time can be varied across a known interval.

[0206] In some embodiments, the controller is configured to determine a weighting factor attributing a modulation duty cycle priority to each cell module based on the determined rank, and control the modulation duty cycle of the switching state of each ranked cell module based on the weighting factor. The weighting factor may define an even distribution across the ranked cell modules. However, the weighting factor may also be used to define a nonlinear distribution across the ranked cell modules. Therefore, the controller is is configured to control the energy distribution across multiple cells by executing a process with the following steps:

[0207] • determine a weighting factor assigning a cell contribution to one or more cells;

[0208] • determine a number of cells for connection to meet the target output voltage;

[0209] • control the switching circuit each cell module comprising a combination of: o series connection of one or more cell modules with the output terminals, and o modulation of the switching state of at least one cell module based on the weighting factor such that the output voltage is substantially provided at the circuit module output terminals;

[0210] • determine a new weighting factor based on cell data and / or cell rank; and

[0211] • determine a new number of cells for connection to meet the target output voltage;

[0212] • control the switching circuit of each cell module such that the output voltage is substantially provided at the circuit module output terminals based on the new number of cells. In some embodiments, the controller is configured to control modulation applied to any one or more cells based on the weighting factor. For example, the controller can determine the target circuit module output voltage, determine the output voltage, determine the difference between the output voltage and the target voltage, then, based on the difference: adjust the weighting factor attributing the modulation duty cycle priority to each cell module based on the determined rank, and control the modulation duty cycle of the switching state of each ranked cell module based on the weighting factor. The weighting factor can be directly attributed to the modulation duty cycle applied to the connection of any cell in the series configuration. Accordingly, the controller is configured to map the weighting factor to the modulation duty cycle priority to each cell module based on the determined rank, and control the modulation duty cycle of the switching state of each ranked cell module based on the weighting factor. A feedback signal, such as one or more of reactive current flow in the series configuration and the voltage of any one or more cells can be used to adjust the modulation duty application to any one or more cells. Therefore, in some embodiments, the controller is further configured to measure a feedback signal from the system, and adjust the modulation duty cycle applied to any one or more cell modules based on the feedback signal.

[0213] Other cell ranking based functions are possible for implementation, such as those described in Australian patent application number 2023902371 , filed 25 July 2023, the entirety of which is incorporated into this specification by reference.

[0214] In some embodiments, the controller is configured to adjust one or more modulation parameters based on a feedback signal. The following diagrams outline possible closed loop control strategies operable by the controller to target a desired AC current flow.

[0215] Figure 11 shows an example diagram of a closed loop controller process where a proportional-integral (PI) control loop is implemented as part of the feedback control to prevent or minimise fluctuations and target a setpoint. In particular, the controller is configured to receive or determine a target requirement for reactive current distribution between two or more cells in the system (lstack_req). The controller is configured to implement one or more reactive current control strategies as discussed above. The reactive current measurement is determined from a current sensor signal and combined with the target requirement. The PI control loop is configured to filter the reactive current control strategy based on its tuning parameters.

[0216] Figure 12 shows another exemplary closed loop control strategy diagram for reactive current control. A PI control loop is implemented as part of the feedback control to prevent or minimise fluctuations and target a setpoint. The Ki / s term is the integral over time of an error term with a constant gain. This helps the PI controller to converge the error over time to zero, that is when the measured current is equal to the desired target current. The gain is selected to bring convergence in sufficient time without unstable or oscillatory side effects.

[0217] Figure 13 shows another exemplary closed loop control strategy for reactive current control. A PIR control loop is implemented as part of the feedback control to prevent or minimise fluctuations and target a setpoint. The PIR control loop includes a feed forward term, where a Kf term provides an estimate of the compensation required, leaving only a small error for the Kp and Ki / s functions to address. This control loop is optimised to compensate for a high Q of an LC filter and may be advantageous when operating a wide range of the frequency that is close to the resonant frequency. The speed of current change of frequency becomes large around the resonant peak of the reactive filter, and the gain of the controller may not be not enough to follow the shape of the current. Therefore, a high gain around the resonant frequency of the reactive filter is advantageous to compensate for a lack of response speed.

[0218] Figure 14 shows a simplified system diagram including cells and reactive filter components for the purpose of illustrating current control in the preceding diagrams. In the exemplary system there are three cells B2, B8 and B9 connected in series, and in series with a reactive filter L1 and C2. R1 represents a simulated internal resistance of the non-ideal components. In this example, B8 is the cell (or cells) determined to require energy input / require charging, and B2 and B9 have been determined to supply energy / require discharging. The SOC of each cell is a simple example of a cell parameter determinative of the cell charge or discharge status. B8 is shown configured in the opposite polarity to that of B2 and B9. In this example, a PI filter is implemented by the controller to adjust the polarity switching frequency of cells B2, B8 and B9.

[0219] Figure 15 shows reactive current and the frequency output by the PI controlled with a variable target current. The close-loop control of the reactive current is by frequency and / or voltage control. In this simulated example, the upper graph shows rms reactive current (Blue) that follows a request-target current (Red) by changing the switching frequency (Brown) as shown in the lower graph in accordance with the second reactive current control strategy as discussed above.

[0220] Figure 16 shows how the switching frequency control can work together with changing the number of cells connected in series and therefore a stepwise voltage change. In particular, the top graph of Figure 16 shows how the frequency response of the PI control responds with the different number of cells (8 cells- lower line, 10 cells and 20 cells-top line) in the series configuration to get a 25A reactive current in the series configuration. The bottom graph of Figure 16 shows the response time of the control loop to track to a target reactive current.

[0221] Figure 18 and Figure 19 demonstrate the phase difference between the current and voltage in the circuit, depending on the AC current frequency relative to the resonant frequency of the filter. Figure 18 is a waveform indicating that cells see an inductive load as they are switched at a frequency greater than the resonant frequency. The combined cell voltage (Brown) and combined cell current (Blue) of the cells which supply current have a 90-degree phase shift in which the voltage leads the current. However, the voltage of the cells to receive (Red) have a 0-degree phase shift to the current. It should be noted that regardless of whether the cells supplying current see an inductive or capacitive load, the cells that receive current always see a resistive load (0 degrees phase shift). Figure 19 is the waveform indicating that the battery system sees a capacitive load as it switches at a frequency lower than the resonant frequency. The Stack voltage (Vstack - Brown) and stack current (I (B8) - Blue) have a 90-degree phase shift, in which the current leads the voltage. However, the voltage of the cells to be charged (Red) still has a 0-degree phase shift to the current.

[0222] The phase angle of the current, relative to the voltage, is shown in Figures 18 and 19 as being about 90 degrees apart. However, as the AC current frequency tends toward the resonant frequency of the filter, the phase difference will tend toward zero. At the resonant frequency, the phase angle will be zero such that the reactive current is in phase with the voltage. Embodiments of the invention include connection of cell modules based on the phase angle of the reactive current relative, which is in turn based on the AC frequency and resonant frequency of the filter.

[0223] It should also be noted that the voltage of the cells supplying energy (first group of cells), and those receiving energy (second group of cells) are each shown as positive voltages for the purpose of illustrating the voltage of the combined cells. However, the polarity of the cell receiving energy is connected opposite to that of the current and hence the relative voltages of cells connected in the circuit are the opposite at any one time. The connection timing between the first and second cell groups may overlap such that each cell is connected in the series configuration at the same time, or they may not overlap such that the first group of cells is disconnected from the series configuration at the time when the second group is connected. The connection timing depends on the phase of the current, with the second group of cells connected based on the phase of the current, the connection interval, and the desired function to be achieved by the energy transfer.

[0224] Figure 20 shows the state of charge exchange in the series cell configuration. The SOC of the cells being charged is shown to be increasing and the SOC of the cells being discharged is shown to be decreasing. The battery cell module configuration uses the reactive filter as the load to alter the charge in the series cell combination.

[0225] Figure 21 shows a time domain diagram of when the cells are charged, and voltage and current Vs SOC. Regardless of which direction of the current flows in the circuit, as shown in Figure 19, the cells to be charged always see a resistive load and can be charged at any time when included in the series cell configuration.

[0226] Figure 22 is a V-l diagram to show how the current flows in the cell(s) to be charged and in the whole series cell configuration. The diagram has four quadrants which indicate the voltage and current direction. If the current is in Q1 or Q3, the cells are charged and if the current is in Q2 and Q4, the cell is discharged. So, the current of the cell(s) to be charged is only in Q1 and Q3, which indicates in a steady state, the cell(s) is constantly charged.

[0227] Figure 22 is a V-l diagram to show the entire series cell configuration where the current flows in all four quadrants, which indicates the series cell configuration is charged and discharged with the LC load. In particular, this shows that the series cell configuration is operating in all four quadrants, and therefore has no net change of energy (unlike Figure 22 which shows operation in Q1 and Q3 and a net accumulation of charge).

[0228] The controller is configured to operate the switching circuit to control the connection of cells in phase and out of phase with the current over the phase of an AC cycle, such as by connection of series or anti-series cells. As discussed elsewhere in this specification, the phase angle of the AC reactive current will lead or lag the voltage depending on the switching frequency relative to the resonant frequency of the reactive circuit. In most circumstances, operating the reactive circuit predominantly in the inductive regime is mostly more advantageous, meaning the polarity switching frequency and resulting AC reactive current frequency is above the resonant frequency of the reactive circuit. The phase angle alignment and connection interval of each of at least the second group of anti-series configured cells is controllable to achieve many advantageous functions. The controller can measure the reactive current and thereby determine the phase angle of the current by use of a current sensor configured in series with the cell modules. In some embodiments, the controller is configured to calculate a predicted phase angle of the current based on known or predetermined, or determined component values in the circuit.

[0229] In some embodiments, one function of the controller is to optimise the power transfer between series cells and anti-series cells by, for example, connection of anti-series cells around the phase angle aligned with peak current. Anti-series cells are those configured with a polarity opposite to those of the series cells, and are orientated negatively for a positively going current. This means that a first group of one or more cells are configured with a first polarity, and a second group of one or more cells are configured with a second, opposing polarity. However, the configuration and timing of connection in the series configuration off cell groups depends on the phase angle of the current. Figure 18 shows one such example where the connection phase angle of anti-series cells is connected for an interval substantially centred about the maxima of the AC current as that is where the power factor is the highest at that reactive current frequency. The second group of anti-series cell connection interval also provides another controllable parameter to aid in the rate of charge transfer between cells. For example, rate of current transfer between cells can be controlled by the connection phase and interval across the AC current waveform as detailed above as one of several modulation techniques. In the Figure 18 example, the anti-series cells are shown to be connected for an interval where the series cells are not simultaneously connected. However, the connection interval of the anti-series cells, and series cells, and operation frequency control whether there overlap in the connection timing occurs. At or near the resonant frequency of the reactive filter, where current transfer is highest, will require the series cells and anti-series cells to be connected at the same time.

[0230] In some embodiments, other functions of the controller are to avoid or promote cell heating by connecting cells based on a high or low power factor. Non-ideal components in the system will have some finite resistance which results in a slight shift in the optimum phase angle, such as from 90 degrees (ideal) to 89 degrees (non-ideal). The non-ideal nature of real components can be effectively leveraged where cell heating is desired. For example, operating the connection of anti-series cells around 1 degree offset from the zero crossing which will promote cell heating behaviour. In practice, this occurs by switching around the zero crossing of the voltage of the series cells. This function is described further in relation to a cell heating function operated by the controller, below. In some embodiments, the controller is configured to blend a cell heating function with a charge balancing function. This can be achieved by connection including a switch timing event which includes the zero crossing, and also extends through the phase angle which includes maximum current for effective power transfer.

[0231] Accordingly, there are two major functions for the battery cell configurations in the system to elicit some useful tasks. The first group of cells are connected and alternated in polarity to form a reactive current with reactive circuit having magnitude and frequency. The second group of cells are to perform a task with the current generated by the first group. The tasks include self testing, cell balancing, self heating, impedance measurement and others. When the second group of cells are connected in the series configuration, there will be a disturbance of the current, for example, a 4V cell will cause a 4V bias in the AC waveform. The controller is configured to compensate for the impact of the bias to keep the reactive current as desired. Compensation may be by means of adding cells in the opposite polarity of the second group to the first group, or changing the strategy of the group 1 switching to cancel or mitigate the impact.

[0232] The connection timing of the second group of cells is relative to the phase angle of the current which in reality will include some small amount of non-reactive current due to the non-ideal nature of components having a small resistance. The small resistance causes the phase shift between voltage and current to reach a maximum of about 89 degrees, rather than the full 90 degrees possible with ideal components. The non-reactive phase interval of the current phase angle is to be avoided where resistive losses are not desired. However, in some cases, the resistive losses can be used advantageously such as for cell heating - discussed further below.

[0233] According to one embodiment which performs a cell balance charging operation, energy is transferred from one or more cells to one or more other cells in a reconfigurable battery system such that the distribution of energy over cells in the battery system becomes more balanced. The controller is configured to execute a series of steps, outlined as follows.

[0234] 1 . At a first step, the controller is configured to identify one or more cells that need charging.

[0235] Identification is based on a measurement of the state of charge of a cell. In some embodiments, the controller is configured to rank cells according to state of charge, where cells lowest in the rank are prioritised to be charged.

[0236] 2. At a second step, the controller is configured to Identify one or more cells that are adequate for supplying charge.

[0237] Identification is based on a measurement of the state of charge of a cell. In some embodiments, the controller is configured to rank cells according to state of charge, where cells highest in the rank are prioritised to be discharged into those cells with the lowest rank.

[0238] In some varied embodiments, a rank determination is also used to determine a cell contribution amount over a time interval such as the switching period, such as PWM or PDM duty cycle based cell contribution amount. For example, to assign a higher duty cycle to those cells ranked higher.

[0239] The cells supplying charge are considered to be the first group of one or more cells and the cells receiving charge are considered to be the second group of one or more cells.

[0240] 3. At a third step, the controller is configured to connect an arrangement of above cells, where the cells to be charged are configured as a first group with a first polarity, and the cells to be discharged are configured as a second group with a second polarity.

[0241] The connection configuration of cells is a dynamic configuration and subject to change at any time based on current flow and cell related factors such as the SOC. In some embodiments, the second group of cells is connected around the phase of the reactive current corresponding with the peak current to effect optimum charge transfer. The phase of the reactive current, relative to the voltage, depends on the operation frequency. A rate of current transfer can be achieved by adding more cells to the first cell group, and by operating the switching frequency at or near the resonant frequency.

[0242] The number of cells to be charged and the number to be discharged are selected based on numerous criteria, including the number of cells determined to be imbalanced within the system. A measure of the standard deviation or similar representation may indicate the system imbalance, where comparison of the representation to a threshold is instructive in initiating this balancing action.

[0243] 4. At a fourth step, the controller is configured to determine and implement modulation parameters which control AC current flow between the cells in the series configuration. The modulation parameters are as described in detail above and include the AC current frequency, the resonant frequency of the filter, the connection interval of first and second group cells and other parameters.

[0244] Cells from step 1 can be considered as a first group and cells from step 2 can be considered as a second group. As discussed above, the polarity of cells in each cell group in the connected configuration is rapidly alternated by operation of the switching circuit to generate the AC current. The reactive circuit provides that the charge cell(s) are at a different phase angle to the discharge cell(s). The degree of simultaneity may vary by some finite phase angle, such as 180 degrees which may represent a so-called anti-series configuration for the discharge cell(s) versus the charge cell(s). By nature of the AC current, the switching of cells is not a static configuration - rather a substantially repetitive cycle of positive and negative excitation of both charge cell(s) and discharge cell(s) at a chosen frequency.

[0245] 5. At an optional fifth step, the controller is configured to adjust one or more modulation parameters based on a sensed feedback signal and thereby target a current.

[0246] Closed loop control of the modulation parameters to tune the current toward a target takes as described above in relation to the outlined closed loop control strategies. In some embodiments, the controller is configured to adjust one or more modulation parameters when the measured AC current flow exceeds a threshold value from a desired target current.

[0247] In some embodiments, the controller is configured to adjust one or more modulation parameters based on other sensed inputs including temperature. For example, a temperature sensor is configured to indicate the temperature of one or more cells, and the controller is configured to add or remove cells from a group based on cell temperature. Other inputs are also possible, including other cell SOC and state of health indicators.

[0248] In some embodiments, the anti-series connection of cell module group 2 is made at a phase angle and for an interval which aligns with the peak region of the reactive current, noting that the power factor will fall away as the phase angle differs further from peak. Hence, connection of second group of cells may only be useful for a fraction of the each polarity phase interval of the AC waveform, such as shown by at least Figures 18 and 19. Further, due to the nonideal nature of components, it may be preferable to connect anti series cells with a phase timing that avoids phase angles proximate the zero-crossing of the AC waveform since that is the region where resistive losses may occur. For example, if a zero crossing occurs at -90 and +90 degrees, an ideal connection phase angle would be 1 or more degrees after or before, such as more than -89 and less than +89 degrees. In practice, the phase angle at which switching is best timed will depend on the resonant frequency of the filter, the frequency of the AC current, and the lead or lag phase difference between voltage and current.

[0249] In the above described method, the output connection to a load is disconnected such that the reactive current flows only in the battery system and reactive filter.

[0250] According to another embodiment, the controller performs a cell safety function when a faulty cell is identified. Energy is transferred from one or more faulty cells to one or more other cells in a reconfigurable battery system in order to avoid a cell fault producing a system failure. The controller is configured to execute a series of steps, outlined as follows.

[0251] 1 . At the first step, the controller is configured to identify one or more cells that require a change to its SOC (source cells).

[0252] One indicative parameter is the temperature of a cell exceeding a threshold associated with safe operating temperature threshold. In some embodiments, the temperature is based on the load applied to a cell such that a higher temperature threshold is operable for cells under load compared with those not under load. In some embodiments, the temperature threshold is based on time spent under load to account for recent high load use, and subsequent cooling, over time. In some embodiments, the controller is configured to determine the temperature of one or more cells based on receiving a signal from a temperature sensing device which is indicative of cell temperature. Cell faults can be determined based on measures other than temperature, generally referred to in the art as state of health determinations. For example, the internal resistance profile of a cell is another state of health indicator deterministic of a fault in the cell.

[0253] In one example, while the battery pack is at rest with no load, a cell may be detected to have a significantly higher temperature than other cells, indicating a possible internal fault in the cell which could pose a risk of thermal runaway. In this exemplary situation, it is desirable to extract charge from the identified high temperature cell and deposit it into other cells in order to reduce the risk of thermal runaway or further failure modes.

[0254] 2. At the second step, the controller is configured to identify one or more cells in the system which are able to accept charge (sink cells).

[0255] In some embodiments, cells meeting a requirement to accept charge are determined based on those which are below a temperature threshold. In some embodiments, cells meeting a requirement to accept charge are determined based on having less than 100% SOC. In some embodiments, cells meeting a requirement to accept charge are determined based on those determined to tolerate a brief excursion beyond 100% SOC - while this is ordinarily to be avoided, it may be preferable to avert a catastrophic situation in the battery system. In some embodiments, the controller is configured to combine any two or more of the above determinations in order to determine the particular cell or cells meeting the requirement to accept charge.

[0256] 3. At the third step, the controller is configured to configure the connection of the sink cell group and source cell group with an opposing polarity relative to the phase of the reactive current, and excite an AC current in the battery pack.

[0257] As discussed above, the polarity of cells in the connection configuration is rapidly alternated to generate the AC current. The reactive filter provides that the charge cell(s) are at a different phase angle to the discharge cell(s). The degree of simultaneity may vary by some finite phase angle, such as 180 degrees which may represent a so-called anti series configuration for the discharge cells versus the charge cell(s). By nature of the AC current, the switching of cells is not a static configuration - rather a substantially repetitive cycle of positive and negative excitation of both charge cell(s) and discharge cell(s) at a chosen frequency.

[0258] 4. At the fourth step, the controller is configured to regulate the AC current through the reactive circuit by one or more of: a. the total number of cells in the series configuration; b. the frequency which excites current in the reactive filter; and c. any of the exemplary methods described above.

[0259] In some embodiments, the controller is configured to control one or more modulation parameters to control the discharge rate of the selected at least one cell.

[0260] 5. At the fifth step, the controller is configured to control the energy distribution from the discharge cell(s) over the range of charge cell(s) by one or more of: a. monitoring the SOC of cells to be discharged, in use, and until a threshold minimum voltage is reached: b. monitoring the SOC of cell(s) to be charged, and when a maximum SOC threshold is reached, substituting other cell(s) to be charged into the AC excitation circuit.

[0261] In some embodiments, the fifth step further includes periodically and continuously replacing one or more of the cells in the group of cell(s) to be charged in the series configuration. In this way, charge energy from the discharging cell is distributed until the cell(s) to be charged reach their maximum SOC and must be excluded from the series configuration.

[0262] The above described process therefore takes stored energy from a cell determined to be faulty, and distributes that charge to other cells in the battery system. Where cells receiving energy reach a maximum SOC, they are substituted for other cells. To evenly distribute charge across many cells, it is desirable to switch multiple different cells into the connection configuration such that many cells receive a small amount of charge, rather than a small group of cells receiving all of the charge.

[0263] The above control method has particular advantages including that current can still be regulated to flow in the circuit regardless of a low terminal voltage of a cell being discharged. The other cells switched in configured in anti-series make the current flow and the cell with the low voltage is forced into the circuit by the switches. This means that the transfer of energy from the cell that has a low SOC can occur to a cell that has higher SOC. Resistive based cell balancing technology cannot achieve this advantage.

[0264] Accordingly, in a varied embodiment the controller is configured to rectify a fault with at least one battery cell by executing a method including the steps of:

[0265] • identifying at least one battery cell has one or more of: o exceeded a predetermined temperature threshold based on comparing a temperature measurement to a threshold, o exceeded a predetermined rate of temperature increase based on temperature measurements over time based on comparing a temperature measurement to a threshold rate of change, and o exceeded an internal resistance limit based on comparing a the resistance measurement to a threshold;

[0266] • selecting the identified at least one cell as the determined one or more battery cells to be discharged,

[0267] • connecting the determined at least one cell in a series configuration with one or more battery cells to be charged and the reactive filter to complete a circuit and thereby control the transfer of reactive charge current between cells.

[0268] In some embodiments, the controller is further configured to execute a method including disconnecting the selected at least one cell from the series connection when a parameter falls below one or more of: o a predetermined temperature threshold, o a predetermined rate of temperature change, o a voltage threshold.

[0269] In the above described method, the output connection to a load is disconnected such that the reactive current flows only in the battery system and reactive filter.

[0270] A known limitation of many cell technologies is that performance degrades at low cell temperatures. For example, most lithium based cells can store energy and operate well in the standard temperature range of 20-60 °C, but performance significantly degrades when the temperature drops below zero. Performance includes current delivery ability, and can also include capacity. In some instances, it is advantageous to heat cells to ensure adequate performance should their resting temperature be low enough to affect performance for an intended application. In some embodiments, the controller is configured to execute a cell heating process. In this process, select cells undergo a net discharge as stored energy is converted to heat. Although it is possible for some cells to be charged while others are discharging (redistribution of charge), it is not necessary for merely heating up cells. Accordingly, there are at least two methods of cell heating, a first method where cells of a first group transfer energy to cells of a second group, and the second method where the cells of the first group and the second group exchange energy back and forth. In each method, the magnitude of reactive current is controlled by the controller. Further, the controller is configured to connect the second group of cells to the current in phase with a non-reactive region of the phase cycle, typically occurring at around 89 degrees. In some embodiments, the connection of the second group to the non-reactive region of the phase cycle occurs at a short interval centred about that region, and in other embodiments, the interval spans the entire phase cycle including the non-reactive region.

[0271] The controller is configured to execute a series of steps, outlined as follows.

[0272] 1 . At the first step, the controller is configured to identify the temperature of one or more battery cells is below a predetermined temperature threshold.

[0273] The temperature measurement may be made by a temperature sensor placed proximate to one or more cells. In an electric vehicle having a cooling system connected to a bank of cells, the temperature measurement may be of a cooling system heat transfer medium. For example, the temperature of fluid coolant or metallic busbars may be indicative of cell temperature. In some embodiments, the controller is configured to determine cell temperature based on an ambient temperature measurement, and at a time the cells are dormant, or have not been subject to any substantial charge or discharge energy. The time may be indicative of the cells substantially matching the ambient temperature.

[0274] 2. At the second step, the controller is configured to select the identified one or more battery cells as the determined one or more battery cells to undergo ohmic heating.

[0275] In some embodiments, a first group of cells has excess charge, and that excess charge is used to generate heat in a second group of cells, heating and balancing can occur by this process. In some embodiments, two or cells which are below the temperature threshold are configured to transfer energy back and forth. Here, the controller is configured to split the low temperature cells into a first and second group which exchange energy.

[0276] 3. At the third step, the controller is configured to connect the first group of cells in anti series with one the second group of cells and the resonance circuit and rapidly alternate the polarity of the cells and the connection timing of the cells, to thereby control the transfer of reactive current in the battery pack.

[0277] The controller is configured to connect the second group of cells which include a phase angle which incorporates the phase shift caused by non-ideal components having a resistive response component. Operating the connection of the second group of anti-series cell modules around 1 degree offset from the zero crossing which will promote cell heating behaviour. In practice, this occurs by switching in the second group of cells with a phase timing that includes the zero crossing of the voltage of the connected cell groups.

[0278] 4. At an optional fourth step, the controller is configured to disconnect the selected at least one cell from the connection when a cell temperature reaches one or more of:

[0279] • the predetermined temperature threshold,

[0280] • the predetermined rate of temperature increase,

[0281] • a voltage.

[0282] The above steps transfer energy from some cells to others. In one example, half of a battery pack is configured as a first cell group, and the other half of the battery pack is configured as the second cell group. Charge may be continuously transferred back and forth between the first and second group of cells such that the balancing of cells is unaffected, all cells undertaking energy transfer undergo net discharging. In some embodiments, the controller is configured to enact a cell balancing operation in conjunction with an ohmic heating operation. Here, the first group of cells comprises those determined to have the greatest SOC such that charge is transferred from higher SOC cells to lower SOC cells.

[0283] Accordingly, in a varied embodiment the controller is configured to heat cells in a reconfigurable battery system by executing a method including the steps of:

[0284] • Identifying the temperature of one or more battery cells is below a predetermined temperature threshold.

[0285] • Selecting the identified one or more battery cells as the determined one or more battery cells to undergo ohmic heating. Then:

[0286] • Connecting the determined at least one cell in anti-series, with one or more other battery cells and the resonance circuit and rapidly alternating the polarity of the cells, to thereby control the transfer of reactive current in the battery pack.

[0287] In some embodiments, the controller is further configured to execute the further step of disconnecting the selected at least one cell from the series connection when a cell temperature reaches one or more of:

[0288] • the predetermined temperature threshold,

[0289] • the predetermined rate of temperature increase,

[0290] • a voltage.

[0291] In the above described method, the output connection to a load is disconnected such that the reactive current flows only in the battery system and reactive filter.

[0292] The above-described system potentially includes many battery cells and switches, any of which represents a possible failure point. It would therefore be advantageous to have a process operable to determine the operational status of any cell in the battery system, and any switch in the switching circuit. Accordingly, in some embodiments, the controller is configured to arrange the series configuration of cells and switches, and by changing the combination of cells and switches is able to determine that any particular cell or switch is operating as expected, or whether there may be a fault. For example, any one cell may be configured to discharge into one or more other cells. By measuring the current drawn from the discharging cell, the controller can determine whether the current is as expected, such as comparing the current to predetermined current profiles, which may be based on the SOC of the cell. Further, the switches of the switching circuit may simultaneously be determined to be operating as expected if current flows as expected. However, if a switch fault occurs, there may be an open circuit or short circuit caused by that faulty switch. Accordingly, the controller may arrange a series configuration of cells which includes and excludes the (suspected) faulty switch. The controller may be configured to do so if current is above or below what might be expected from any particular cell combination.

[0293] Referring to Figure 7, if, for example, switch 411 was to have a shorted fault condition, then cells 421 and 422 would always be connected, and closing switch 415 would cause a shorting of the cells 421 and 422. The controller may be configured to determine an output voltage from the closing of switch 415, and if the voltage was lower than expected, as caused by the shorted cells, then a fault would be noted.

[0294] In the above examples, one or more cells would always be configured in anti-series with a cell under test and the reactive current generated. In this way, short circuits may be tested and the current controlled by the AC current control strategies described above.

[0295] Accordingly, in some other embodiments, the controller is configured to determine the health status of a combination of one or more switches in the switching circuit and one or more battery cells by a process which includes controlling the switching circuit to create a reactive current between a first series configuration of at least two battery cells and one or more switches of the switching circuit, determining a reactive current operating in the circuit based on the control of the switching circuit, and determine the health status of at least one of a switch and a battery cell based on the determined reactive current. In some embodiments, the controller is configured to control the switching circuit to configure a second series configuration of at least two battery cells and one or more switches of the switching circuit, where at least one switch or battery cell is shared with the first series configuration. For each of the first and second series configurations, the controller is configured to compare generated reactive currents with predetermined expected reactive currents. Where the generated current substantially matches the expected current, the controller is configured to determine that at least the component shared between the first and second series configurations is operational. However, in instances where there is an unexpected reactive current measurement, indicated by the current not substantially matching a predetermined expected current, the controller is configured to arrange circuit paths including different combinations of cells and switches, with the combinations sharing different components, to thereby isolate the component which is at fault.

[0296] Therefore, in one embodiment the controller is configured to determine the health status of a combination of one or more switches in the switching circuit and one or more battery cells by the steps of:

[0297] • controlling the switching circuit to create a reactive current between at least two battery cells,

[0298] • determining a reactive current operating in the circuit based on the control of the switching circuit, and

[0299] • determining the health status of at least one of a switch and a battery cell based on the determined reactive current.

[0300] In one embodiment, the controller is further configured to determine an operational health status of at least one cell or at least one switch in a reconfigurable battery system by executing a method including the steps of:

[0301] • Configuring a series connection of a first group of components, the first group comprising a one or more cells and a plurality of switches.

[0302] • Configuring a series connection of a second group of components, the first group comprising one or more cells and a plurality of switches, where the first and second group share a component;

[0303] • Connecting the determined at least one cell in anti-series, with one or more other battery cells and the resonance circuit and rapidly alternating the polarity of the cells, to thereby control the transfer of reactive current in the battery pack.

[0304] • Comparing the reactive current to a predetermined reactive current, and

[0305] • Determining a health status of the component shared by the first and second group based on the comparison.

[0306] In the above described method, the output connection to a load is disconnected such that the reactive current flows only in the battery system and reactive filter.

[0307] Cell impedance measurement can be an important factor in determining the state of health of a cell since it is understood by those skilled in the art that cell impedance, and change of impedance over time, is related to cell health. In some embodiments, the controller is configured to determine the impedance of a cell module based on control of reactive current in the circuit. Figure 24 shows a diagram of passive components of a simplified model of a battery cell where the state of health of a cell correlates to RO. Figure 25 is a simulation circuit to demonstrate how RO, RC and C_terminal of Figure 24 can be measured from reactive current flow through the reactive filter. Load 11 is representative of what the battery cell will see when it is included in the test circuit. In practice, a reactive current is built by a first cell group, and that current is then directed to a cell under test when it is switched into the series configuration of the circuit. A measurable voltage response is used to derive cell impedance.

[0308] Figure 26 shows the cell voltage (terminal + to terminal -) step response when a cell under test is included or excluded from the test circuit, where three different RO cell resistance values are shown.

[0309] The following table represents a relationship between an observable voltage dip of the terminal voltage and different cell resistances RO. The current and voltage data may also be used to deduce the equivalent circuit including the lumped passive components R, L and C in Figure 24 by curve fitting, if desired.

[0310] The controller is configured to determine the cell impedance measurement by implementing the steps based on the following.

[0311] At a first control step, the controller is configured to build a reactive current in the battery system at a desired frequency. Higher current produces more voltage dip than lower currents for any given resistance, therefore the controller is configured to build sufficient current by including more cells in the connection configuration, and / or by alternating the polarity of cells at or near the resonant frequency of the filter, as may be required. Accordingly, the controller configures a first group of battery cell modules 271 connected in series, and alternates the polarity of the first group at a desired frequency to thereby generate an alternating voltage 272 in the circuit. It should be noted that the circuit output switches 61 , 62 are open such that the external load, such as the grid, is disconnected.

[0312] Figure 27(A) shows a circuit arrangement whereby a first cell group 271 is alternated to generate alternating voltage 272 and an alternating reactive current. A current sensor 273 may be connected in the circuit for current measurement the controller may use for determination of the current phase angle offset from the voltage. Figure 28(A) shows an exemplary voltage (solid line) of the first group of battery cells, and an exemplary current (broken line) showing the phase difference of the reactive current.

[0313] At a second control step, the controller is configured to connect a target cell for testing. The controller is configured to connect the target cell in series with and in the reverse polarity of the cells of the first cell group. The timing of the connection is based on the phase angle of the current. The most desirable connection time is shortly before the peak current occurs in the current waveform as this will ensure a sufficient current is applied to the cell. However, the particular phase timing will depend on desired current to be connected with the cell under test, and other circuit factors such as how much current is available for testing. Figure 27(B) shows an exemplary circuit configuration whereby a cell under test 274 is connected in series with the circuit.

[0314] Figure 28(B) shows a time alignment between the test cell connection and the current of Figure 28(A). Figure 28(B) also shows the resulting voltage across the test cell 274 (solid line) and the current delivered to the cell (broken line) during the connection interval.

[0315] Figure 28(C) shows the voltage drop over the instant the target cell is connected to the reactive current flow. Figure 28(D) shows the near instant current gain at the time of the voltage drop. The controller is configured to measure the voltage drop and current over this time period to thereby determine a correlation with cell impedance. In some embodiments, the controller is configured to deduce the equivalent circuit consisting of lumped passive components R L and C by curve fitting. The majority parameter that impacts the SOH is RO, where RO is defined as follows RO = Vceii (step) I lceii

[0316] The controller is configured to determine the RO data to thereby determine a measure of the SOH of the cell. The current data seems to indicate it is a linear relationship where the relationship of SOH vs RO required minimal data to be sufficiently mapped and therefore allow the predetermination of a range of values based on measurement of a select few values. The predetermined range may be stored by the controller as a look-up table directly translating voltage drop to impedance or another SOH metric.

[0317] Data recorded during testing has indicated there is a linear relationship between SOH and RO such that RO can directly correlate to SOH. Accordingly, the controller can determine a SOH indication directly from RO determinations. Within a battery system having many cells, each cell may be ranked based on RO measurements. The measured voltage and current of the target cell can be used to determine real values of RO based on one or more further processes, discussed as follows.

[0318] The cell impedance can be determined from a variety of controller implemented methods, outlined as follows. In the first method, the controller is configured to operate the battery system to conduct the following steps:

[0319] • Building the reactive current by alternating the first group of cell modules at a desired frequency, such as less than 100Hz.

[0320] • Selecting the cell for the impedance measurement and configuring that cell as the cell in the second cell group.

[0321] • Operating the switching circuit to configure the second cell group in phase with the peak of the reactive current. In some embodiments, the first group of cells connected in the circuit is configured to include cells with a combined voltage which cancels the DC voltage of the second group. High current provides for the best signal to noise ratio.

[0322] • Measuring the target cell voltage and reactive current with a sensing circuit operably connected with the controller.

[0323] • Performing phasor analysis to determine the real and imaginary part of the impedance.

[0324] • Determining the RO based on the complex impedance, where RO is the impedance when the imaginary part is zero.

[0325] The controller is then configured to determine the SOH of the cell based on the RO determination.

[0326] The target voltage cell measurement need only take place for as long as required to measure the voltage drop after the cell is connected. In practice, this period is about 10 microseconds since the current cannot rapidly change in a short period of time as it is limited by the inductance of the filter (di / dt). The voltage drop data then determines the SOH of the cell.

[0327] In second method, the controller is configured to operate the battery system to conduct the following steps:

[0328] • Building the reactive current by switching the stack in the desired frequency to get a high current

[0329] • Selecting the cell in which impedance needs to be measured.

[0330] • Including the target cell with a lower frequency (this frequency is the fO of the frequency signal).

[0331] • Measuring the cell voltage and reactive current with a sensing circuit operably connected with the controller.

[0332] • Performing FFT analysis to determine the magnitude and phase of the impedance in frequency domain.

[0333] • The RO of the impedance is at the resonant frequency where the phase = 0.

[0334] In the above described methods, the output connection to a load is disconnected such that the reactive current flows only in the battery system and reactive filter.

[0335] While the invention has been described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims

Claims1 . A battery system configured to manage reactive energy transfer between battery cells, the system comprising: a plurality of battery cell modules; a switching circuit configured to: selectively connect any two or more battery cell modules with a reactive circuit, and configure the polarity of each battery cell module connected with the reactive circuit; and a controller configured to: control the switching circuit to connect a first group of one or more battery cell modules to drive a reactive current through the reactive circuit, and control the switching circuit to connect a second group of one or more battery cell modules to the reactive current with a polarity opposite to that of the first cell group, wherein the polarity connection of the second group is based on a determined phase angle of the reactive current.

2. The battery system as claimed in claim 1 , wherein the controller is further configured to operate the switching circuit in a first mode and a second mode, wherein: in the first mode the switching circuit is configured to: connect one or more battery cell modules to an output for an external load through the reactive circuit; and in the second mode the switching circuit is configured to: disconnect the battery cell modules from the output for the external load, alternate, at a frequency, the connection of the first group of the one or more battery cell modules with the reactive circuit to drive a reactive current in the reactive circuit, determine the phase angle and polarity of the reactive current, and connect the second group of one or more battery cell modules to the reactive circuit with a polarity opposite to that of the first group, based on the determined phase angle.

3. The battery system as claimed in claim 1 or claim 2, wherein the reactive circuit comprises at least part of a low pass output filter configured to connect with the output of the one or more switching circuits and filter an output from the plurality of battery cells when the output connection is connected to the load.

4. The battery system as claimed in claim 3, wherein the low pass filter comprises a an LCL filter.

5. The battery system as claimed in any one of claims 1 to 4, wherein the switching circuit comprises switches configured as a bridge circuit for controlling the polarity of one or more battery cell modules.

6. The battery system as claimed in any one of claims 1 to 5, wherein the switching circuit comprises switches configured to control the interconnection of battery cell modules in a series or parallel circuit arrangement.

7. The battery system as claimed in any one of claims 1 to 6, wherein the controller is further configured to selectively connect and alternate, at an alternation frequency, the polarity of the first and second cell groups and thereby control the transfer of reactive current between cells.

8. The battery system as claimed in any one of claims 1 to 7, wherein the controller is further configured to modulate the connection of one or more battery cell modules in at least one of the first and second groups based on one or more modulation parameters.

9. The battery system as claimed in any one of claims 1 to 8, wherein the controller is configured to determine a target reactive current, and control the target reactive current by one or more of: determining a number of cell modules to be included in at least one of the first and second cell groups based on one or more of the target reactive current, and a state of charge criteria of at least some cell modules; determining an alternation frequency based on the target reactive charge current, and controlling the switching circuit to create a connection of cell modules in the first and second cell group at the determined alternation frequency.

10. The battery system as claimed in claim 9, wherein the controller is configured to determine a measured reactive current between the first and second cell groups, and change at least the alternation frequency based on a comparison between the target current and the measured reactive current.11 . The battery system as claimed in claim 9, wherein the controller is configured to determine a measured reactive current between the first and second cell groups, and change the number of series connected cell modules in one or more of the first or second cell groups based on the one or more criteria comprising a comparison between a target current and the measured reactive current.

12. The battery system as claimed in claim 10, wherein the one or more criteria further comprises the state of charge, and the controller is configured to determine the state of charge of one or more battery cell modules, and based on the determined state of charge, change the number of connected cells, or substitute one or more battery cell modules within the first or second cell groups with another battery cell module.

13. The battery system as claimed in claim 8, wherein the battery system further comprises a resistive component configured for connection in the reactive current path, and the controller is configured to control the resistive current based on connection of the resistive component.

14. The battery system as claimed in claim 8, wherein the modulation parameter is based on one or more of:• a determined voltage of the one or more battery cell modules in the first group;• a determined voltage of the one or more battery cell modules in the second group;• a target reactive charge current;• a determined connection time;• the phase angle of the reactive current;• the connection phase of the second cell group; and• the connection interval of the second cell group.

15. The battery system as claimed in claim 9, wherein the target reactive charge current is based on one or more of:• a threshold reactive charge current;• a reactive charge current limit;• a target discharge current of the one or more battery cell modules in the first group;• a target charge current for the one or more battery cell modules in the second group; and• the alternation frequency.

16. The battery system as claimed in any one of claims 1 to 15, wherein the controller is further configured to: determine battery cell modules for the first group based on identification of one or more battery cell modules to source energy based on their determined state of charge,determine battery cell modules for the second group based on identification of one or more battery cell modules to sink energy based on their determined state of charge, and control the reactive current transfer from the first group to the second group based on one or more modulation parameters, to thereby execute a cell balancing operation.

17. The battery system as claimed in any one of claims 1 to 16, wherein the controller is further configured to execute a cell heating function comprising:• determining the temperature of one or more battery cell modules is below a predetermined temperature threshold,• alternating the first and second group polarities to drive reactive current from the first group to the second group, and• controlling the timing of the connection of the second group to the reactive circuit such that a portion of the energy transfer occurs during a phase angle that results in non-reactive current flow, thereby executing a cell heating operation through controlled bidirectional energy transfer..

18. The battery system as claimed in any one of claims 1 to 17, wherein the controller is configured to determine the health status of a combination of one or more switches in the switching circuit and one or more battery cell modules by the steps of:• controlling the switching circuit to thereby create a reactive current between at least two battery cell modules,• determining a reactive current operating in the circuit based on the control of the switching circuit, and• determining the health status of at least one of a switch and a battery cell module based on the determined reactive current.

19. The battery system as claimed in any one of claims 1 to 18, wherein the controller is configured to determine an operational health status of at least one battery cell module or at least one switch of the switching circuit by executing a method including the steps of:• driving the reactive current through a series connection of a first set of components comprising the first and second cell groups, and a plurality of switches of the switching circuit,• driving the reactive current through a second set of components comprising the first and second cell groups, and a plurality of switches, wherein the first group and second group share a component;• comparing the reactive current from each set of components to a predetermined reactive current, and• determining the operational health status of the component shared by the first and second group based on the comparison.

20. The battery system as claimed in any one of claims 1 to 19, wherein the controller is configured to execute a cell elimination function comprising:• identifying at least one battery cell module has failed a state of health criteria,• connecting the identified at least one battery cell module in the first group,• connecting one or more remaining battery cell modules in the second group, and• driving reactive current from the first group to the second group.

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