Accumulator with a plurality of battery cells and method for operating such a

Abstract

Accumulator, in particular a lithium-ion accumulator, with a plurality of battery cells (20, 22, 24) which are connected in parallel and / or in series, a battery management system (26, 28, 30) which detects a value representing the charge state of the battery cell (20, 22, 24) from a plurality of battery cells (20, 22, 24) and which connects the battery cells (20, 22, 24) to the remaining battery cells in a timed manner according to their detected value, characterized in that a. the battery management system (26, 28, 30) detects the voltage value of each battery cell (20, 22, 24) as a measure of its discharge state and connects each battery cell (20, 22, 24) to the other battery cells (20, 22, 24) in a timed manner according to its detected voltage value, b. the battery cell (20, 22, 24) or the battery cells (20, 22, 24) with the highest voltage value of all battery cells (20, 22, 24) are connected without interruption to the other battery cells (20, 22, 24) and c. a battery cell (20, 22, 24) whose detected voltage value deviates from the highest voltage value by more than a predetermined difference value is connected to the other battery cells (20, 22, 24) in a clocked manner such that the duration of the connection to the other battery cells, based on a total clock duration, corresponds to the voltage value of the battery cell (20, 22, 24) based on the highest voltage value of all battery cells.

Description

[0001] The present invention relates to a rechargeable battery, in particular a lithium-ion rechargeable battery, comprising a plurality of battery cells connected in parallel and / or in series. The invention also relates to a method for operating a rechargeable battery, in particular a lithium-ion rechargeable battery, comprising a plurality of battery cells connected in parallel and / or in series.

[0002] When using rechargeable batteries, especially lithium-ion batteries, it is common practice to symmetrize the voltage of the individual battery cells. This involves extracting charge from a battery cell with a higher voltage, temporarily storing it in a coil or capacitor, and then supplying it to cells with a lower voltage. This process is also known as inductive or capacitive cell balancing. Cell balancing is used when the cell voltages across the battery cells differ too significantly from each other.

[0003] DE 10 2008 021 090 A1 discloses a circuit arrangement and a method for exchanging electrical charge between accumulators of a battery arrangement. In the known circuit arrangement, electrical charge is exchanged between series-connected accumulators, each of which has a relay-connected switching element in a load path and an inductive storage element. The individual accumulators are inductively coupled to one another in order to exchange charges with one another by controlling the switching element.

[0004] DE 10 2012 223 482 A1 discloses that the voltage of a battery string is not recorded, but rather the charge states of individual battery cells. A battery string consists of several battery cells, but the charge states of the individual battery cells are not recorded.

[0005] From DE 10 2010 041 036 A1 a control of individual cells according to their charge states is known.

[0006] DE 10 2010 041 059 A1 discloses a method for setting a target output voltage of a controllable energy storage device. A coupling unit is provided for this purpose, which is switched in a clocked manner.

[0007] The invention is based on the object of providing an alternative method for cell balancing in an accumulator with a plurality of battery cells, which ensures a uniform and gentle load on the battery cells using simple means.

[0008] According to the invention, the object is achieved by an accumulator having the features of claim 1 and a method for operating an accumulator having the features of claim 8. Advantageous embodiments are the subject of the subclaims.

[0009] The accumulator according to the invention has a plurality of battery cells that are connected in parallel and / or in series to achieve the desired output voltage. Lithium-ion accumulators, in particular, are constructed with a plurality of battery cells that are themselves designed as lithium-ion battery cells. Furthermore, the accumulator according to the invention has a battery management system that detects a value representing the state of charge of the battery cells from several battery cells. The battery management system connects the battery cells to the other battery cells in a timed manner according to their detected value. A timed connection means that, based on a repeating time interval (total cycle length), the battery cells are connected to the other battery cells for a specific period of time (cycle length) and are disconnected from them for the remaining time of the time interval.In the accumulator according to the invention, battery cells are connected to one another according to their charge level. The accumulator according to the invention is based on the idea of ​​not inductively or capacitively balancing a charge imbalance that arises between the cells, as is the case with known inductive or capacitive cell balancing, but rather of preventing such an imbalance from occurring. This is achieved by interconnecting the individual battery cells with the other battery cells according to their charge level.

[0010] In the invention, the battery management system records a voltage value from each battery cell, preferably the battery cell's output voltage. Each battery cell is then connected to the other battery cells, preferably in a timed manner according to its recorded voltage value. The current voltage value of the battery cell is a measure of the remaining capacity of the battery cell, thus preventing overloading of the battery cells during the charging or discharging process.

[0011] In a preferred embodiment of the accumulator according to the invention, the voltage values ​​are measured at regular intervals and / or when no power is being drawn. The clock cycle for controlling the battery cells is re-determined based on the recorded values. Since it can be assumed that the electrical behavior of a battery cell is generally stable over a longer period of time, it is sufficient to repeat the voltage value measurement at regular intervals. It is also sufficient to measure the voltage values ​​when no power is being drawn. The clock cycle for controlling the battery cells can be re-determined completely or in the form of a correction to the existing clock cycle. The clock cycle indicates the ratio of the time period t within a clock cycle T in which the battery cell is connected to the time period of the clock cycle T.

[0012] In the invention, the battery cell or cells with the highest voltage value of all battery cells are connected seamlessly to the remaining battery cells. This battery cell or cells have the best state of charge and the highest remaining capacity, so they can be connected seamlessly to the remaining battery cells. For assignment to the highest voltage value, an interval can also be provided, in which an interval of voltage values ​​is considered the highest voltage value.

[0013] In the invention, a battery cell whose detected voltage value deviates from the highest voltage value by more than a predetermined difference value is clocked in such a way that the duration t of the connection with the other battery cells, based on a total duration T, corresponds to the voltage value u of the battery cell, based on the highest voltage value of all battery cells (t / T = u / u maxThis means that the battery cell or cells with the highest voltage value are connected to the other battery cells at 100%, i.e. permanently and without interruption. The duty cycle is interpolated so that each battery cell can be assigned a duty cycle for the clocked connection of the battery cell to the other battery cells. Battery cells that have no voltage are naturally defective and must be switched off. The same applies to battery cells that fall below a minimum voltage. The value of a duty cycle of zero is assumed solely for the purposes of interpolation.

[0014] In a preferred embodiment, the battery management system has a memory in which the value for the maximum charge state (C max) is stored. The current state of charge C(u) dependent recorded voltage value is used as a measure of the state of discharge and is compared to the maximum state of charge in order to determine the duty cycle when controlling the battery cell. Similar to the recorded voltage values, the duty cycle for the battery cells is preferably interpolated linearly. The maximum state of charge for each battery cell is stored in a memory. The recorded voltage value is converted into its current state of charge so that the ratio of the current to the maximum state of charge specifies the duty cycle with which the battery cells are connected or disconnected.

[0015] In a preferred embodiment, the value for the maximum state of charge of the battery cell is adjusted based on the measured voltage values ​​and stored in the battery management system. Such continuous adjustment of the maximum state of charge also allows aging processes and other influences on the battery cells to be taken into account. In a practical embodiment, the value for the maximum state of charge is adjusted by the battery management system at the start of charging, when no power is being drawn, and / or at specific time intervals.

[0016] The timing in question is expediently a pulse-width modulation, in which the battery cell is connected to the other battery cells for a period of time t, based on a total clock duration T. As is usual with pulse-width modulation, a uniform total clock duration T is specified for all battery cells. Depending on the specific design of the pulse-width modulation, the clock ratio specifies the duration of t, for example, at the beginning of the total clock duration T, at which the battery cells are connected or disconnected. Preferably, the clock is the same and synchronous for all battery cells.

[0017] In a preferred embodiment, the battery management system can, in response to a predetermined load signal, connect each battery cell to the remaining battery cells without interruption. In this embodiment, in response to the predetermined load signal, all battery cells are permanently connected to one another, so that the accumulator delivers its maximum possible power without taking the charge level of the individual battery cells into account.

[0018] The object of the invention is also achieved by a method having the features of claim 8. Advantageous embodiments form the subject matter of the dependent claims referring back thereto.

[0019] The method according to the invention is provided and intended for operating a rechargeable battery, in particular a lithium-ion rechargeable battery, which has a plurality of battery cells connected in parallel and / or in series. The method provides the method step of measuring a value representing the state of charge of a battery cell. The measurement is carried out for several battery cells, with the value being measured for each of these several battery cells. In a further method step, the duty cycle of each of the several battery cells is determined depending on the measured value. Furthermore, the method according to the invention provides for connecting each of the several battery cells to the other battery cells according to their determined duty cycle.The method according to the invention treats the battery cells of the accumulator individually and, via the clock ratio, ensures that each of the measured battery cells is connected to the other battery cells of the accumulator according to its state of charge and thus no large charge differences can arise between the battery cells.

[0020] In a preferred further development, the method is carried out for all battery cells of the accumulator.

[0021] In a preferred embodiment, the voltage values ​​are measured at regular intervals and / or when no power is being drawn. The timing for controlling the battery cells is redetermined accordingly, so that the method according to the invention adapts to aging phenomena of the battery cells in the accumulator.

[0022] In the invention, the duty cycle for each of the battery cells is determined such that the duration of the connection of the battery cell to the other battery cells, relative to a total duty cycle, corresponds to the voltage value of the battery cell relative to the highest voltage value of all battery cells. In the advantageous method step, the duty cycle is determined relative to the highest voltage value of all battery cells. This approach assumes that those battery cells with the highest voltage value can deliver the maximum power and can therefore be connected without interruption, while the other battery cells deliver a lower voltage value and are therefore only connected to the other battery cells with a certain duty cycle relative to the total duty cycle. An interval of voltage values ​​can also be assigned to the highest voltage value.

[0023] In an alternative embodiment, the clock ratio can be set in relation to a maximum state of charge.

[0024] In a preferred embodiment, the value for the maximum state of charge of the battery cells is adjusted accordingly at the start of charging, when no power is drawn and / or at predetermined time intervals.

[0025] The invention is explained in more detail below using an exemplary embodiment. It shows: Fig. Figure 1 shows a schematic view of several battery cells connected in series, containing a parallel path with diodes.

[0026] Fig. 1 shows a motor 10 connected to an inverter 12. The inverter 12 is powered by a lithium-ion battery 18 via its terminals 14 and 16. The lithium-ion battery 18 has, by way of example to better explain the invention, three individual battery cells 20, 22 and 24. In the illustrated embodiment, the battery cells 20, 22, 24 are made of Fig. 1 connected in series. Each of the battery cells has its own battery management system 26, 28, 30, with the individual battery management systems being connected to a higher-level battery management system 32.

[0027] The battery management system 26 measures the voltage of the corresponding battery element at points A and B. The measured voltage value is forwarded to the higher-level battery management system, where it is compared to the voltage values ​​measured by the battery management systems 28 and 30. The resulting ratio is returned to the battery management system 26, from where it is used to open and close the switch 34 in a timed manner according to the ratio. The switching elements 36 and 38 of the battery cells 22 and 24, respectively, are also controlled accordingly based on the measured voltage of the respective battery element.

[0028] When the switching elements 34, 36, and 38 are closed, the battery cells 20, 22, and 24, connected in series, contribute to the input voltage 14, 16 of the converter 12. If one of the battery cells is weaker, i.e., has a lower voltage than the others, it is temporarily disconnected by opening switch 34, 36, or 38 and does not contribute to the voltage applied to the converter 12. In this case, the current flowing to the converter 12 takes its path via one of the diodes 46, 48, and 50 arranged in the parallel paths 40, 42, and 44. The parallel paths and the corresponding diodes ensure that opening a switch does not interrupt the entire voltage supply to the converter 12, but rather that each battery cell can be disconnected individually.

[0029] The invention is based on the realization that battery cells can have different chemical, physical, and thus electrical properties, such as capacity and internal resistance. In order to provide the necessary voltage and capacity at the terminals of the lithium-ion battery, several battery cells are connected in parallel and in series. The cell voltage of a battery cell is a measure of the remaining capacity of the battery cell. The battery can be operated until the weakest cell reaches a lower voltage limit. To do this, a battery management system checks the cell voltage and switches off the individual battery cells to prevent damage. Other battery cells may still have a significantly better state of charge at this switch-off point.The invention prevents a charge imbalance from occurring in the battery cells. Instead, only enough charge is drawn from the individual battery cells so that, ideally, all cells reach their lowest possible state of charge (SOC) simultaneously. The same applies to the charging process, where, ideally, all battery cells reach their maximum state of charge simultaneously.

[0030] To ensure that all battery cells are discharged according to their capacity during operation, each cell can be assigned a switching element 34, 36, 38 in the form of a transistor, and this can be subjected to pulse width modulation. This means that a cell that has, for example, 90% of the capacity of the most powerful cell only delivers 90% of its power and, through appropriate timing of its transistor, is only 90% connected to the other battery cells. The cells with the highest voltages are operated at 100%. To determine the pulse width modulation for the battery cells, it is possible to continuously monitor the voltage in the battery cells and estimate the battery cell's state of charge based on the measured voltage.As soon as the deviation deviates by more than one differential value from the highest cell voltage of all battery cells, the pulse width modulation can be reduced via the battery cell's associated transistor. This means that less charge is drawn from the cell than before, allowing the cell voltage to equalize again. In an alternative approach, the battery management system can record the discharge of a battery cell over a longer period of time. The stored measured values ​​can then be used to determine the remaining capacity of the battery cell and thus the pulse width modulation for the next discharge.

Claims

[1] Accumulator, in particular lithium-ion accumulator, with a plurality of battery cells (20, 22, 24) connected in parallel and / or in series, a battery management system (26, 28, 30) which detects a value representing the state of charge of several battery cells (20, 22, 24) and which connects the battery cells (20, 22, 24) to the other battery cells in a timed manner according to their detected value, characterized by , that a. the battery management system (26, 28, 30) detects the voltage value of each battery cell (20, 22, 24) as a measure of its state of discharge and connects each battery cell (20, 22, 24) to the other battery cells (20, 22, 24) in a timed manner according to its detected voltage value, b. the battery cell (20, 22, 24) or the battery cells (20, 22, 24) with the highest voltage value of all battery cells (20, 22, 24) are continuously connected to the other battery cells (20, 22, 24) and c. a battery cell (20, 22, 24), whose detected voltage value deviates from the highest voltage value by more than a predetermined difference value, is connected to the other battery cells (20, 22, 24) in such a clocked manner that the duration of the connection to the other battery cells, based on a total clock duration, corresponds to the voltage value of the battery cell (20, 22, 24) based on the highest voltage value of all battery cells. [2] Accumulator according to claim 1, characterized by , that the voltage values ​​are measured at regular intervals and / or when there is no power draw and the clocking for the control of the battery cells (20, 22, 24) is redefined. [3] Accumulator according to one of claims 1 or 2, characterized by , that the battery management system (26, 28, 30) has a memory in which the maximum state of charge of each of the clocked controlled battery cells (20, 22, 24) is stored and the detected voltage value is used as a measure of their state of discharge to the maximum state of charge in order to determine the clock ratio when controlling the battery cell (20, 22, 24). [4] Accumulator according to claim 3, characterized by , that the value for the maximum charge level of the battery cell (20, 22, 24) is adjusted based on the measured voltage values ​​and stored in the battery management system (26, 28, 30). [5] Accumulator according to claim 4, characterized by , that the value for the maximum state of charge is adjusted by the battery management system (26, 28, 30) at the start of charging, when there is no power draw and / or at predetermined time intervals. [6] Accumulator according to any one of claims 1 to 5, characterized by , that the clocking is carried out via pulse width modulation, in which, for a time period t, based on a total clock period T, the battery cell (20, 22, 24) is connected to the other battery cells (20, 22, 24). [7] Accumulator according to any one of claims 1 to 6 characterized by , that the battery management system (26, 28, 30), responding to a load signal, can connect all battery cells (20, 22, 24) to the other battery cells without interruption. [8] Method for operating an accumulator, in particular a lithium-ion accumulator, comprising a plurality of battery cells (20, 22, 24) connected in parallel and / or in series, comprising the following method steps: - Measuring a value representing the state of charge of a battery cell (20, 22, 24), whereby the measurement is carried out for several battery cells (20, 22, 24), - Determining a duty cycle ratio for each of the several measured battery cells (20, 22, 24), depending on their measured value and - Connect each of the multiple battery cells (20, 22, 24) to the remaining battery cells (20, 22, 24) according to their switching ratio, - characterized by , that the switching ratio for each battery cell (20, 22, 24) is determined such that the duration of the connection of the battery cell (20, 22, 24) with the other battery cells, based on a total switching duration, corresponds to the voltage value of the battery cell (20, 22, 24) as a measure of its state of discharge, based on its maximum state of charge, where the maximum state of charge is a quantity stored for each battery cell (20, 22, 24). [9] Method according to claim 8, characterized by , that the process steps are carried out for each battery cell (20, 22, 24) of the accumulator. [10] Method according to claim 8 or 9, characterized by, that the measurement of the voltage values ​​permanently representing the state of charge is carried out at regular intervals and / or when there is no power draw and the clocking for the control of the battery cells (20, 22, 24) is redefined. [11] Method according to claim 10, characterized by , that the value for the maximum charge level of the battery cell (20, 22, 24) is adjusted at the start of charging, when there is no power draw and / or at predetermined time intervals.

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