Method and system for balancing electrical energy storage cells

Abstract

The invention concerns a system for balancing charge over rechargeable energy storage devices coupled in series, comprising : balancing units assigned to the rechargeable energy storage devices; an AC signal generator providing an AC signal to the balancing units for balancing the charge on their assigned rechargeable energy storage devices; and a capacitive coupling between the AC signal generator and the balancing units for common mode blocking; wherein the balancing units comprise switching circuits for transferring charge from the AC signal generator to said energy storage devices, and wherein each switching circuit comprises : a first transistor, a first a diode, and a second diode, arranged to transfer, when said first transistor is in a conducting state, charge from the AC signal generator to a respective energy storage device; and a second transistor and a first resistor arranged to discharge, when said second transistor is in a conducting state, charge from said respective energy storage device over said first resistor. A corresponding method is also disclosed.

Description

[0001] Method and system for balancing electrical energy storage cells

[0002] Field of the Invention

[0003] The present invention is related to energy storage in series- coupled rechargeable electrical energy storage devices , also called cells , and in particular to balancing systems for increasing or decreasing the voltage of a limited number of such electrical cells . The balancing systems provide means for balancing charge (voltage ) differences between cells in a rechargeable energy storage system . The invention is particularly related to a balancing system and to a method of balancing for a series connection of electrochemical cells , such as , but not limited to , ultracapacitors or lithium cells .

[0004] Background

[0005] European patent application publication no . EP 2 302 757 Al , in the name of the present applicant , discloses a system for charge balancing over a plurality of rechargeable energy storage devices coupled in series , said system comprising a plurality of balancing units each assigned to one of the rechargeable energy storage devices , an AC signal generator for providing an AC signal to the plurality of balancing units , and capacitive coupling between the AC signal generator and each of the plurality of balancing units for common mode blocking .

[0006] It is a disadvantage of the known system that it requires a specifically adapted microcontroller to perform the desired balancing scheme . The desired balancing schemes cannot be implemented in the known system with a standard battery management system ( BMS ) chip without the use of additional components such as drivers for operating the switches present in the circuit .

[0007] Hence , there is a need for an improved system for charge balancing over a plurality of rechargeable energy storage devices coupled in series , which can operate on the basis of a standard battery management system ( BMS ) chip .

[0008] International patent application publication no .

[0009] WO 2017 / 109006 Al , in the name of the present applicant , discloses a connectivity check for a rechargeable energy storage system comprising a string of rechargeable energy storage cell modules in series . Each cell module comprises an individual rechargeable energy storage cell or plurality of rechargeable energy storage cells , a switch for use in a connection check, a monitoring and control circuit adapted for measuring a cell module voltage , a single common connection between a terminal of a rechargeable energy storage cell and the monitoring and control circuit within each module . To perform the connectivity check the switch is opened in one module , a cell voltage is measured in the module to give a first measured value , the switch is closed . The cell voltage is measured again to provide a second measured value , and a determination is made if there is a difference between the first and second measured values which would indicate a bad connection .

[0010] European patent application publication no . EP 3 061 197 Bl , in the name of the present applicant , discloses a method and system for providing pulsed power and data from a main control unit to slave units via a first bus . The main control unit comprises an AC signal generator for providing a plurality of first pulses on the bus for providing the power to the slave units . Each slave unit is AC-coupled to the bus via a first series capacitor arranged for converting the first pulses into second pulses . Data communication from the main control unit to the slave units is established by modulating the first pulses , and by demodulating the second pulses . The modulation may be based on Pulse Position Modulation, Pulse Width Modulation, Pulse Count Modulation, Pulse Amplitude modulation . Zero , one or more bits may be communicated per first pulse . Optionally the slave units may communicate to the main control unit via a second bus . United States patent application publication no . US 2018 / 152028 Al discloses a battery active-balancing system comprising an external balancing power; a buck converter having a first side-winding and second side-winding induced thereby, the first and second sidewindings each having positive and negative terminals ; a battery comprising series-connected cell-units each having positive and negative terminals ; a cell voltage-sensing unit coupled to the battery to sense voltage of the cell-units ; a main switch component coupled to first side-winding, battery, and external balancing power, to first side-winding and external balancing power in ON state , and to first side-winding and battery in OFF state ; a cell-switch unit comprising first and second cell-switch components , with first cell-switch components coupled to positive terminals of the cell-units and second side-winding, respectively, and second cell-switch components coupled to negative terminals of the cell-units and second side-winding , respectively; and a microcontroller for controlling main switch component and cellswitch unit , and determining voltage levels fed back from cell voltage-sensing unit .

[0011] United States patent application publication no . US 2012 / 313441 Al discloses a battery cell balancing system including N switching circuits connected to first terminals and second terminals of N battery cells , respectively, where N is an integer greater than one . A first transformer includes a first core and N sets of windings wound around the first core . The N sets of windings are connected to the N switching circuits , respectively . A first control module controls switching of the N switching circuits to reverse first connections between the first terminals and the second terminals of the N battery cells and the N sets of windings , respectively, at a first frequency to balance charge levels of the N battery cells . Summary of the Invention

[0012] According to an aspect of the present invention, there is provided a system for balancing charge over a plurality of rechargeable energy storage devices coupled in series , the system comprising : a plurality of balancing units each assigned to one of the rechargeable energy storage devices ; an AC signal generator for providing an AC signal to the plurality of balancing units for balancing the charge on their assigned rechargeable energy storage devices ; and a capacitive coupling between the AC signal generator and each of the plurality of balancing units for common mode blocking , wherein the balancing units comprise a plurality of switching circuits for transferring charge from the AC signal generator to respective ones of said energy storage devices ; wherein each of said plurality of switching circuits comprises : a first transistor , a first a diode , and a second diode , arranged to transfer , when said first transistor is in a conducting state , charge from the AC signal generator to a respective one of said energy storage devices over the first diode DI and over the second diode D2 ; and a second transistor and a first resistor arranged to discharge , when said second transistor is in a conducting state , charge from said respective one of said energy storage devices over said first resistor .

[0013] In accordance with the present invention, said first transistor of the system is a MOSFET ; said first diode is connected in series between said capacitive coupling and a positive terminal of said respective one of said energy storage devices ; said first transistor and said second diode are connected between a negative terminal of said respective one of said energy storage devices and said capacitive coupling ; a second resistor is connected between said positive terminal of said respective one of said energy storage devices and a gate driver of said first transistor ; and said switching circuit further comprises a switch which is connected to the negative terminal , and operatively connected to a gate of said first transistor via the gate driver configured to bring said first transistor (Ml ) into a conducting or nonconducting state dependent on the status of the switch .

[0014] The term "transistor" , as used herein, is not limited to any particular technology . Hence , unless otherwise indicated, the term covers inter alia bipolar j unction transistors ( BJT ) , insulated gate bipolar transistors ( IGBT ) , and field effect transistors ( FET ) such as MOSFETs .

[0015] It is an advantage of the present invention that the circuit for each energy storage device can easily be switched between a charging and a discharging state by controlling the state of a limited number of switches / transistors ) , in a way that can be achieved by very simple off-the-shelf logic, such as a standard battery management system ( BMS ) chip ( e . g . the LTC®6811 multicell battery monitor, available from Analog Devices of Wilmington, Massachusetts ) .

[0016] It is an advantage of the present invention that the circuit for each energy storage device can easily be switched to a charging state by controlling the state of a single switch . Preferably, said switch is comprised in an integrated battery management system ( BMS ) component . The BMS typically comprises plural switches for plural switching circuits allowing plural rechargeable energy storage devices to be charged and discharged controlled by the BMS .

[0017] According to an aspect of the present invention, there is provided a method of balancing the charge of a rechargeable storage device having two terminals and a balancing unit comprising a switching circuit for transferring charge from an AC signal generator to said energy storage devices ; wherein said switching circuit comprises : a first transistor, a first a diode , and a second diode , arranged to transfer, when said first transistor is in a conducting state , charge from the AC signal generator to a respective one of said energy storage devices over the first diode ( DI ) and over the second diode ( D2 ) ; and a second transistor and a first resistor arranged to discharge , when said second transistor is in a conducting state , charge from said respective one of said energy storage devices over said first resistor; the method comprising : measuring an instantaneous electrical parameter across the terminals of the energy storage device ; comparing the measured electrical parameter to a pre-determined parameter value ; and depending on the comparison result , transferring charge to or away from the energy storage device by actuating said first transistor and / or said second transistor; wherein transferring charge comprises providing an AC signal to a balancing unit coupled to the rechargeable energy storage device , the balancing unit applying the AC signal to the rechargeable energy storage device or not depending on the signal of the AC signal and the comparison result , transferring charge including common mode blocking by capacitive coupling .

[0018] According to the method of the present invention, said first transistor of the system is a MOSFET ; said first diode is connected in series between said capacitive coupling and a positive terminal of said respective one of said energy storage devices ; said first transistor and said second diode are connected between a negative terminal of said respective one of said energy storage devices and said capacitive coupling ; a second resistor is connected between said positive terminal of said respective one of said energy storage devices and a gate driver of said first transistor ; and said switching circuit further comprises a switch which is connected to the negative terminal , and operatively connected to a gate of said first transistor via the gate driver configured to bring said first transistor (Ml ) into a conducting or non-conducting state dependent on the status of the switch .

[0019] In a specific embodiment , said switch is comprised in an integrated battery management system ( BMS ) component .

[0020] The technical features and advantages of embodiments of the method according to the present invention correspond, mutatis mutandi s, to those of the corresponding embodiments of the system according to the present invention .

[0021] Brief Description of the Figures

[0022] These and other technical features and advantages of embodiments of the present invention will now be described with reference to the accompanying drawings , in which :

[0023] Figure 1 schematically illustrates a rechargeable energy storage system provided with a balancing system according to an embodiment of the present invention;

[0024] Figure 2 schematically illustrates the balancing system of Figure 1 in more detail ;

[0025] Figure 3 presents a schematic of an example of a switching circuit ; and

[0026] Figure 4 presents a schematic of an embodiment of a switching circuit of the system according to the invention as claimed; Figures 5 and 6 illustrate the switching circuit presented in fig . 4 with different implementations of a gate driver .

[0027] Detailed Description of Embodiments

[0028] Figure 1 illustrates a rechargeable energy storage system 10 including a system 13 for balancing charge over the plurality of rechargeable energy storage devices 12 , according to an embodiment of the present invention . The illustrated system 10 is similar to the one presented in Figure 1 of European patent application no . EP 2 302 757 Al in the name of the present applicant , but differs in the configuration and the operation of the balancing units 15, as will be made clear hereinbelow .

[0029] The illustrated system 13 comprises a series string 11 of a plurality of energy storage devices 12 , also called cells . The energy storage devices 12 are rechargeable . Each one energy storage device 12 can comprise a single energy storage cell or a parallel connection of multiple energy storage cells ( not illustrated) . The series-connected rechargeable energy storage devices 12 together form a battery, a pack or a stack . The rechargeable energy storage devices 12 may be any type of rechargeable energy storage device . The energy storage devices 12 may in particular be ultracapacitors , also known as supercapacitors or electric or electrochemical double-layer capacitors ( EDLCs ) . These are electrochemical capacitors that have an unusually high energy density when compared to common capacitors ; typically on the order of thousands of times greater than a high-capacity electrolytic capacitor . Typical capacitance values may range from a few farads to a few thousands of farads . Alternatively, the rechargeable energy storage devices 12 may be batteries , for example high energy density batteries , such as for example Lithium or Lithium Ion ( Li-ion) batteries . Other kinds of rechargeable energy storage devices can equally be used . Preferred applications are in the fields of energy smoothing and momentary load devices , which require very high instant powers compared to the average power ratings .

[0030] Each rechargeable energy storage device 12 has a limited operating voltage , e . g . between 2 and 4 V . In order to obtain a useful operating voltage of , for example , a few tens to a few hundreds of Volts or even more , useful e . g . as an energy source in vehicle applications , a plurality, and sometimes a large number of rechargeable energy storage devices 12 need to be coupled in series . Slight differences between the energy storage devices 12 in a string or series-coupling can produce imbalances in the cell voltages , which may greatly reduce the charge capacity .

[0031] In order to alleviate this , a rechargeable energy storage system 10 also comprises a system 13 for balancing charge over the plurality of rechargeable energy storage devices 12 .

[0032] The system 13 for balancing charge comprises an AC signal generator 14 for charging one or more of the plurality of rechargeable energy storage devices 12 . The system 13 furthermore comprises a plurality of balancing units 15. Each balancing unit 15 is assigned to one of the rechargeable energy storage devices

[0033] 12 . Fo r optimal performance , each rechargeable energy storage device 12 is assigned a balancing unit 15.

[0034] Each energy storage device 12 in the string 11 is provided with a first terminal 16, which is its positive terminal , and a second terminal 17 which is its negative terminal . The energy storage devices 12 are coupled in the string 11 such that a positive terminal 16 of a first energy storage device 12 is coupled to a negative terminal 17 of a second energy storage device , and so on . The negative terminal 17 of the very first energy storage device 12 in the string, and the positive terminal 16 of the very last energy storage device 12 in the string are accessible from outside .

[0035] As shown in Figure 2 , a balancing unit 15 has a first input port S+ and a second input port S- . The first and second terminals 16 , 17 of an energy storage device 12 are connected to the first and second input ports S+, S- of the balancing unit 15 assigned to that energy storage device 12 , so that the positive terminal 16 of the energy storage device 12 is connected to the first input port S+ and the negative terminal 17 of the energy storage device 12 is connected to the second input port S- . The balancing unit 15 is adapted for measuring an instantaneous electrical parameter of the associated energy storage device 12 , e . g . the voltage across the two input ports S+ and S- . In order to obtain this , the balancing unit 15 is provided with suitable measurement circuitry . Preferably, this measurement circuitry is an analog input of a battery management system ( BMS ) component 18 present in the balancing unit 15 .

[0036] The balancing unit 15 according to embodiments of the present invention furthermore comprises an input terminal P for receiving an input signal . This input terminal is electrically connected, over a switching circuit , to the first or second input ports S+ and S- , respectively . The input signal is received from the AC signal generator 14 . The balancing unit 15 is adapted for controlling the switching circuit in function of the voltage level at input terminal P . This control of the switching circuit is controlled by the battery management system ( BMS ) component 18 . The battery management system ( BMS ) component 18 may draw power from the input terminal P by rectifying the AC signal , e . g . a block wave .

[0037] According to the invention, the switching circuit comprises a first transistor , a first diode , and a second diode , arranged to transfer , when said first transistor is in a conducting state , charge from the AC signal generator 14 to a the energy storage device 12 . The first transistor is preferably a MOSFET connected in said circuit with its source and its drain . The switching circuit further comprises a second transistor and a first resistor R1 arranged to discharge , when said second transistor is in a conducting state , charge from said energy storage device 12 over said first resistor .

[0038] Examples of the switching circuit will now be described with reference to Figures 3 and 4 , where the switching circuit corresponds to the portion of the schematic enclosed in a rectangle drawn in dashed lines . For the sake of internal consistency of Figures 3 and 4 (which have the traditional form of electronic schematics ) and without loss of generality, the AC signal generator 14 has been represented as voltage source VI , the energy storage device 12 of interest has been represented as voltage source V2 , the collective effect of the remaining energy storage devices 12 has been represented as voltage source V3 , and the capacitive coupling 19 has been represented as capacitor Cl .

[0039] A first example of the switching circuit will now be described with reference to Figure 3 .

[0040] As stated above , the first transistor Ml , the first a diode DI , and the second diode D2 are arranged to transfer , when said first transistor Ml is brought into a conducting state between its source and its drain by applying an appropriate gate-source voltage , charge from the AC signal generator 14 to the energy storage device 12 . To that end, the first transistor Ml is a MOSFET connected in said circuit with its source connected to the first diode DI and its drain connected to the capacitive coupling 19 of the AC signal generator 14 . The first transistor Ml is illustrated as an n-channel MOSFET ; this choice ensures minimal power loss in the circuit branch switched by the first transistor Ml . It is , however , also possible to use a p-channel MOSFET as the first transistor Ml , in which case it may easily be switched on and off by an output of the battery management system ( BMS ) component 18 suitably connected to its gate by means of basic components to convert the BMS signal to the gate-source voltage required to control the MOSFET ( not illustrated) .

[0041] Furthermore , the second transistor M2 and the first resistor R1 are arranged to discharge , when said second transistor M2 is in a conducting state between its source and its drain by applying an appropriate gate-source voltage , charge from the energy storage devices 12 over said first resistor R1 . In the illustrated case , the second transistor M2 is an n-channel MOSFET .

[0042] In this first example , the first transistor Ml and the first diode DI are connected in series between the capacitive coupling 19 and the positive terminal 16 of the energy storage device 12 . The second diode D2 is connected between the negative terminal 17 of the energy storage device 12 and the capacitive coupling 19 . The second transistor M2 and the first resistor R1 are connected in series between the positive terminal 16 and the negative terminal 17 of the energy storage device 12 .

[0043] A second example , being an embodiment of the switching circuit in accordance with the invention as claimed, will now be described with reference to Figures 4 to 6 .

[0044] As stated above , the first transistor Ml , the first diode DI , and the second diode D2 are arranged to transfer, when said first transistor Ml is brought into a conducting state between its source and its drain by applying an appropriate gate-source voltage , charge from the AC signal generator 14 to the energy storage device 12 .

[0045] In this embodiment , the first diode DI is connected in series between said capacitive coupling 19 and the positive terminal 16 of the energy storage device 12 . The first transistor Ml and the second diode D2 are connected between the negative terminal 17 of the energy storage device 12 and the capacitive coupling 19. The second transistor M2 and the first resistor R1 are connected in series between the positive terminal 16 and the negative terminal 17 of the energy storage device 12 .

[0046] A second resistor R2 is connected between the positive terminal 16 of the energy storage device 12 and a gate driver of the first transistor Ml .

[0047] A switch SI is connected to a drain of the first transistor Ml , and operatively connected to a gate of the first transistor Ml via the gate driver ( as shown in fig . 4 ) .

[0048] When switch SI is open, the gate driver drives the gate-source tension existing over the first transistor Ml causing the first transistor Ml to be in a conducting state , such that charge can be transferred from the AC signal generator 14 to the energy storage device 12 . There will be conduction over the first diode DI when the instantaneous voltage of the AC signal presented via the capacitive coupling Cl exceeds the voltage of the energy storage device 12 . Conversely, there will be conduction over the second diode D2 when the instantaneous voltage of the AC signal presented via the capacitive coupling Cl is less than the voltage of the energy storage device 12 .

[0049] When switch SI is closed, the gate driver drives the gate-source tension of the first transistor Ml to a low voltage level such that the first transistor Ml returns to a non-conductive state , whereby no charging of the energy storage device 12 will take place .

[0050] The switch SI is preferably comprised in an integrated battery management system ( BMS ) component .

[0051] The gate driver of the switching circuit as shown in fig . 4 can be implemented in various ways such as for example illustrated in figures 5 and 6 .

[0052] In the example as illustrated in fig . 5 , the gate driver comprises a third transistor QI . The third transistor QI is a bipolar NPN- transistor connected as follows : with its collector to the gate of the first transistor Ml , with its emitter to the source of the first transistor Ml , and with its base , over a third resistor R3 , to the switch SI . In the illustrated example , the second transistor M2 is an n-channel

[0053] MOSFET .

[0054] In the example as illustrated in fig . 6 , the gate driver comprises a MOSFET (M3 ) . The MOSFET (M3 ) is connected as follows : with its source to the gate of the first transistor Ml , with its drain to a source of the first transistor Ml , and with its gate , over a third resistor R3 , to the switch SI .

[0055] As before , the second transistor M2 and the first resistor R1 are arranged to discharge , when said second transistor M2 is in a conducting state between its source and its drain by applying an appropriate gate-source voltage , charge from the energy storage devices 12 over said first resistor R1 . In the illustrated case , the second transistor M2 is an n-channel MOSFET .

[0056] A balancing unit 15 according to embodiments of the present invention performs at least the following functions : measuring an instantaneous electrical parameter, e . g . the voltage across the input ports S+, S-of the associated energy storage device 12 , and actuating the switching circuit in function of the measured parameters .

[0057] According to embodiments of the present invention, all balancing units 15 of the system 13 for balancing are coupled to the AC signal generator 14 . This AC signal generator 14 can be a block wave generator , a sinusoidal signal source , a saw tooth generator, or any other type of AC signal generator . It is advantageous if the AC signal generator 14 is a high frequency signal generator, as this reduces component size . A sinusoidal signal generator is more efficient , has low peak currents hence low heat generation, and does not substantially present any switching losses . However, a block wave generator is easier to implement than a sinusoidal signal generator . The AC signal generator 14 can be powered from an external source ( not illustrated) , e . g . a charger, or from the series string 11 of energy storage devices 12 . The AC signal generator 14 can be controlled by the main controller 20 . It is advantageous to have a single AC signal generator 14 for the balancing system 13.

[0058] According to embodiments of the present invention, a plurality of AC signal generators are provided ( not illustrated ) , and the plurality of balancing units are grouped into groups , the balancing units of one group being coupled to one of the AC signal generators . Also in this case it can be said that the system for balancing comprises a single AC signal generator , the signal generator and the group of balancing units coupled thereto forming the system for balancing . No signal generator is required in the balancing units .

[0059] According to embodiments of the present invention, the balancing units 15 are coupled to the AC signal generator by means of a capacitor 19. The capacitive coupling provided by the capacitors 19 is used to block common mode voltages . This is required because the energy storage devices 12 are at different and varying potential levels .

[0060] The balancing units 15 , or thus the system 13 for balancing, functions as follows . Each balancing circuit 15 in first instance measures a local instantaneous electrical parameter of the associated energy storage device 12 , e . g . the local voltage across its first and second terminals 16, 17 , i . e . the voltage across the associated energy storage device 12 . This measuring can be performed via a method and device as disclosed in WO 2006 / 058394 . In accordance with embodiments of the present invention, the measurement can be performed by means of an AD converter , for example an AD converter 21 which forms part of battery management system ( BMS ) component 18 , local to the balancing unit 15.

[0061] In accordance with embodiments of the present invention, a measuring circuit may digitize the measured values , e . g . voltages , and transmit them to the battery management system ( BMS ) component 18 . The measured parameter value , e . g . voltage , determines whether the cell 12 is operating in a region where charging or discharging is required .

[0062] When the comparison identifies that a cell 12 needs to be charged, the battery management system ( BMS component ) 18 actuates the switching circuit of that cell 12 as follows : if the switching circuit is configured according to the first example : o the first transistor Ml is brought into a conducting state between its source and its drain by applying an appropriate gate-source voltage ; o the second transistor M2 is kept in a non-conducting state between its source and its drain; if the switching circuit is configured according to the second example being an embodiment according to the invention as claimed : o the first transistor Ml is brought into a conducting state between its source and its drain by applying an appropriate gate-source voltage by opening the switch SI ; o the second transistor M2 is kept in a non-conducting state between its drain and its source .

[0063] When the comparison identifies that a cell 12 needs to be discharged, the battery management system ( BMS component ) 18 actuates the switching circuit of that cell 12 as follows : if the switching circuit is configured according to the first example : o the first transistor Ml is kept in a non-conducting state between its source and its drain; o the second transistor M2 is brought into a conducting state between its source and its drain by applying an appropriate gate-source voltage ; if the switching circuit is configured according to the second example being an embodiment according to the invention as claimed : o the first transistor Ml is kept in a non-conducting state between its source and its drain by closing the switch SI ; o the second transistor M2 is brought into a conducting state between its source and its drain by applying an appropriate gate-source voltage .

[0064] When a cell 12 does not need any charge nor discharge , the battery management system ( BMS component ) 18 actuates the switching circuit of that cell 12 as follows : if the switching circuit is configured according to the first example : o the first transistor Ml is kept in a non-conducting state between its source and its drain; o the second transistor M2 is kept in a non-conducting state between its source and its drain; if the switching circuit is configured according to the second example being an embodiment according to the invention as claimed : o the first transistor Ml is kept in a non-conducting state between its source and its drain by closing the witch SI ; o the second transistor M2 is kept in a non-conducting state between its source and its drain .

[0065] Controlling the switching circuit in this way in one balancing unit 15 allows to transfer charge from the AC signal generator to the associated energy storage device 12 , or to drain charge from an energy storage device 12 over its associated resistor R1 . Hence the voltage of individual cells with a low voltage can be increased, and the voltage of individual cells with a high voltage can be decreased . This provides a gain of balancing speed and efficiency relative to traditional systems that only rely on discharging of the storage devices .

[0066] Due to the capacitive coupling 19 of the individual balancing units 15, charge transfer can take place between any energy storage device 12 and the AC signal generator 14 , regardless of individual energy storage devices ' 15 potential with respect to each other .

[0067] According to embodiments of the present invention, decisions on charging or discharging of energy storage devices or on energy storage devices remaining unaffected, may be taken locally in the balancing units 15 , for example by the local battery management system ( BMS ) components 18 .

[0068] Strategies for balancing a system of energy storage devices 12 are disclosed in the aforementioned European patent application publication no . EP 2 302 757 Al , in the name of the present applicant . Such strategies can be implemented in the system disclosed herein .

[0069] The balancing method and device according to embodiments of the present invention allows to obtain overvoltage and undervoltage protection . It is advantageous that a balancing system 15 in accordance with embodiments of the present invention provides such overvoltage and undervoltage protection, without supplementary hardware features being required . A concrete example of a scheme to prevent overvoltage and undervoltage is presented in the aforementioned European patent application publication no .

[0070] EP 2 302 757 Al , in the name of the present applicant , and applies in the same way to the present invention .

[0071] Although the invention has been described hereinabove with reference to specific embodiments , this was done to clarify and not to limit the invention, the scope of which is determined by the accompanying claims .

Claims

Claims1. A system (13) for balancing charge over a plurality of rechargeable energy storage devices (12) coupled in series, the system (13) comprising: a plurality of balancing units (15) each assigned to one of the rechargeable energy storage devices (12) ; an AC signal generator (14) for providing an AC signal to the plurality of balancing units (15) for balancing the charge on their assigned rechargeable energy storage devices (12) ; and a capacitive coupling (19) between the AC signal generator (14) and each of the plurality of balancing units (15) for common mode blocking; wherein the balancing units (15) comprise a plurality of switching circuits for transferring charge from the AC signal generator (14) to respective ones of said energy storage devices (12) ; characterized in that each of said plurality of switching circuits comprises : a first transistor (Ml) , a first a diode (DI) , and a second diode (D2) , arranged to transfer, when said first transistor (Ml) is in a conducting state, charge from the AC signal generator (14) to a respective one of said energy storage devices (12) over the first diode (DI) and over the second diode (D2) ; and a second transistor (M2) and a first resistor (Rl) arranged to discharge, when said second transistor (M2) is in a conducting state, charge from said respective one of said energy storage devices (12) over said first resistor (Rl) ; wherein said first transistor (Ml) is a MOSFET; wherein said first diode (DI) is connected in series between said capacitive coupling (19) and a positive terminal (16) of said respective one of said energy storage devices (12) ; wherein said first transistor (Ml) and said second diode (D2) are connected between a negative terminal (17) of said respective one of said energy storage devices (12) and said capacitive couplingwherein a second resistor (R2) is connected between said positive terminal (16) of said respective one of said energy storage devices (12) and a gate driver of said first transistor (Ml) ; and wherein said switching circuit further comprises a switch (SI) which is connected to the negative terminal (17) , and operatively connected to a gate of said first transistor (Ml) via the gate driver configured to bring said first transistor (Ml) into a conducting or non-conducting state dependent on the status of the switch (SI) .

2. The system (13) according to claim 1, wherein said switch (SI) is comprised in an integrated battery management system (BMS) component .

3. The system (13) according to any preceding claim, wherein said first transistor (Ml) is an n-channel MOSFET; wherein said gate driver comprises a third transistor (QI) , said third transistor being a bipolar NPN-transistor ; and wherein said third transistor (QI) is connected: with its collector to said gate of said first transistor (Ml) , with its emitter to a source of said first transistor (Ml ) , and with its base, over a third resistor (R3) , to the switch ( SI ) .

4. The system (13) according to claim 1 or claim 2, wherein said gate driver comprises a MOSFET (M3) ; and wherein said MOSFET (M3) is connected: with its source to said gate of said first transistor (Ml) , with its drain to a source of said first transistor(Ml ) , and with its gate, over a third resistor (R3) , to the switch ( SI ) .

5. A method of balancing the charge of a rechargeable storage device (12) having two terminals (16, 17) and a balancing unit (15) comprising a switching circuit for transferring charge from an AC signal generator (14) to said energy storage devices (12) ; wherein said switching circuit comprises : a first transistor (Ml) , a first a diode (DI) , and a second diode (D2) , arranged to transfer, when said first transistor (Ml) is in a conducting state, charge from the AC signal generator (14) to a respective one of said energy storage devices (12) over the first diode (DI) and over the second diode (D2) ; and a second transistor (M2) and a first resistor (Rl) arranged to discharge, when said second transistor (M2) is in a conducting state, charge from said respective one of said energy storage devices (12) over said first resistor (Rl) ; the method comprising: measuring an instantaneous electrical parameter across the terminals (16, 17) of the energy storage device (12) ; comparing the measured electrical parameter to a predetermined parameter value; and depending on the comparison result, transferring charge to or away from the energy storage device (12) by actuating said first transistor (Ml) and / or said second transistor (M2) ; wherein transferring charge comprises providing an AC signal to a balancing unit coupled to the rechargeable energy storage device, the balancing unit applying the AC signal to the rechargeable energy storage device or not depending on the signal of the AC signal and the comparison result, transferring charge including common mode blocking by capacitive coupling; wherein said first transistor (Ml) is a MOSFET; wherein said first diode (DI) is connected in series between said capacitive coupling (19) and a positive terminal (16) of said respective one of said energy storage devices (12) ; wherein said first transistor (Ml) and said second diode (D2) are connected between a negative terminal (17) of said respective one of said energy storage devices (12) and said capacitive coupling (19) ;wherein a second resistor (R2) is connected between said positive terminal (16) of said respective one of said energy storage devices (12) and a gate driver of said first transistor (Ml) ; and wherein said switching circuit further comprises a switch (SI) which is connected to the negative terminal (17) , and operatively connected to a gate of said first transistor (Ml) via the gate driver configured to bring said first transistor (Ml) into a conducting or non-conducting state dependent on the status of the switch (SI) .

6. The method according to claim 5, wherein said switch (SI) is comprised in an integrated battery management system (BMS) component .

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