Battery cell charge balancing device, motor vehicle and associated methods

Abstract

The device for balancing the charge of cells connected in series in a battery (2) comprises: - charging means (3) configured to charge the battery cells at a constant voltage, and to stop charging the cells when the current flowing through the terminals of the battery (2) is equal to or less than a threshold (Sc), - allocation means (12) configured to assign to each cell (6, 7, 8, 9) of the battery (2) a state of charge derived from the voltage measured across the open-circuit terminals of said cell and from a curve (CSOC) relating voltage values ​​across said cell to states of charge, and - balancing means (13) configured to balance the states of charge of the battery cells according to the state of charge assigned to each cell. Figure for the abstract: Fig 1

Description

Title of the invention: Device for balancing the charge of battery cells, associated motor vehicle and method

[0001] The present invention relates to balancing the charge of cells of a battery.

[0002] The present invention relates more particularly to a method for balancing the charge of cells connected in series of a battery, a device for balancing the charge of cells connected in series of a battery, and a motor vehicle comprising such a device.

[0003] Generally a lithium-ion battery is formed by a plurality of electrochemical cells connected together, for example in series.

[0004] The charge states of the cells are kept as close as possible to each other so that the battery operates optimally and sustainably.

[0005] On the one hand, due to the manufacturing processes of the cells, they do not have exactly the same storage capacity. On the other hand, due to the aging of the cells, the states of charge of the battery cells tend to diverge from each other during the lifetime of the battery, despite substantially identical stress. They do not have the same self-discharge over time, generating differences in state of charge.

[0006] It is therefore necessary to periodically balance the charge states of the cells to keep them as close to each other as possible.

[0007] Conventionally, the states of charge of lithium-ion battery cells are determined from the open circuit voltage OCV.

[0008] Indeed, the OCV voltage at the terminals of each cell can be easily associated with a state of charge of said cell.

[0009] A commonly implemented method of balancing charge states generally involves discharging the cells with the highest voltages, such that the gap between the cells with the highest voltages and the cells with the lowest voltages becomes less than or equal to a predetermined threshold.

[0010] The balancing method works for electrode materials whose association in an electrochemical cell gives an open circuit voltage profile OCV having a sufficient slope so that a voltage measurement accurate to a few tens of millivolts makes it possible to deduce the state of charge of the cell precisely, for example to within a few percent, with an error of the order of plus or minus 5% over the entire operating range of the cell, i.e. between 0 and 100% state of charge, 0% corresponding to a fully discharged cell and 100% corresponding to a fully charged cell.

[0011] However, when the combination of electrode materials does not make it possible to obtain an open circuit voltage profile having a sufficient slope over the operating range, as is particularly the case for Lithium Iron Phosphate (LFP) batteries, the method of balancing the charge states does not operate with sufficient precision, the error generally being greater than 5%.

[0012] Document CN108780927 proposes implementing a method for balancing the states of charge for states of charge of lithium iron phosphate batteries (LiFePO4 batteries) in the range 10% and 40%. In this range, the open circuit voltage profile OCV has a sufficient slope to obtain a voltage measurement accurate to within a few tens of millivolts in order to determine the state of charge of each cell accurately.

[0013] However, it is not possible to determine a precise state of charge of the cells outside this range, in particular for batteries having a functional need to remain charged at a charge level greater than 40%, and consequently to carry out a balancing of the states of charge of the cells of the battery.

[0014] The aim of the invention is to overcome all or part of these drawbacks.

[0015] The subject of the invention is a method for balancing the load of cells of a battery, the cells being connected in series between two terminals of the battery.

[0016] The method comprises:

[0017] - (a) constant voltage charging of the battery cells,

[0018] - (b) stopping the charge when the intensity of the current flowing through the terminals of the battery is equal to or less than a threshold,

[0019] - (c) for each cell of the battery, an allocation of a state of charge to said cell deduced from the voltage measured at the terminals of said cell and from a curve associating values ​​of the voltage measured at the open circuit terminals of said cell with states of charge, and

[0020] - (d) balancing the charge states of the battery cells according to the state of charge assigned to each cell.

[0021] Advantageously, the curve associates each value included in a first voltage domain with a variable state of charge included between a first predetermined state of charge and a second predetermined state of charge, said first domain extending between a minimum voltage value and a first reference voltage value, the curve associates the first reference value with the second predetermined state of charge, and the curve associates each value included in a second voltage domain with a variable state of charge included between a third predetermined state of charge and a fourth predetermined state of charge, said second domain extending between the first reference value and a second maximum voltage value, the first reference value being greater than the minimum value and less than the maximum value, the second predetermined state of charge being greater than the first predetermined state of charge and less than the third predetermined state of charge, and the fourth predetermined state of charge being greater than the third predetermined state of charge.

[0022] Preferably, the balancing of the charge states of the cells comprises: - when the assigned state of charge of at least one cell is lower than the second predetermined state of charge, discharging each cell having an assigned state of charge higher than the second predetermined state of charge (SOC2) until the state of charge of said cell is substantially equal to the assigned state of charge of the cell lower than the second predetermined state of charge,

[0023] - when the state of charge assigned to at least one cell of the battery is equal to the second predetermined state of charge, discharging each cell having an assigned state of charge greater than the third predetermined state of charge until the state of charge of said cell is substantially equal to the third predetermined state of charge and repeating steps (a) to (d), and

[0024] - when the state of charge assigned to each of the battery cells is greater or equal to the third predetermined state of charge, determining the lowest assigned state of charge, and discharging each cell having an assigned state of charge greater than the lowest state of charge until the state of charge of said cell is substantially equal to the lowest state of charge.

[0025] The state of charge of a cell is substantially equal to another state of charge when the difference in absolute value of the state of charge of said cell and the other state of charge is for example less than 1%.

[0026] Advantageously, the discharge of each cell comprises the supply of a resistive circuit by said cell for a predetermined discharge duration.

[0027] Preferably, the supply of a resistive circuit by said cell for the predetermined discharge duration is repeated until the assigned state of charge of said cell is substantially equal to the third predetermined state of charge or until the state of charge of said cell is substantially equal to the lowest state of charge.

[0028] Advantageously, the battery comprises lithium iron phosphate / graphite cells.

[0029] A device is also provided for balancing the charge of cells of a battery, the cells being connected in series between two terminals of the battery.

[0030] The device comprises:

[0031] - charging means configured to charge the cells of the battery, and to stop charging the cells when the current flowing through the battery terminals is equal to or less than a threshold,

[0032] - allocation means configured to allocate to each cell of the battery a state of charge deduced from the voltage measured at the terminals of said cell and from a curve associating values ​​of the voltage measured at the open circuit terminals of said cell with states of charge, and

[0033] - balancing means configured to balance the charge states of the cells of the battery according to the state of charge assigned to each cell.

[0034] Advantageously, the device further comprises storage means configured to store the curve, the curve associating each value included in a first voltage domain with a variable state of charge between a first predetermined state of charge and a second predetermined state of charge, said first domain extending between a minimum voltage value and a first reference voltage value, the curve associating the first reference value with the second predetermined state of charge, and the curve associating each value included in a second voltage domain with a variable state of charge between a third predetermined state of charge and a fourth predetermined state of charge, said second domain extending between the first reference value and a second maximum voltage value, the first reference value being greater than the minimum value and less than the maximum value,the second predetermined state of charge being greater than the first predetermined state of charge and less than the third predetermined state of charge, and the fourth predetermined state of charge being greater than the third predetermined state of charge.

[0035] Preferably, the balancing means are configured to: - when the assigned state of charge of at least one cell is lower than the second predetermined state of charge, discharging each cell of the battery having an assigned state of charge higher than the second predetermined state of charge until the state of charge of said cell is substantially equal to the assigned state of charge of the cell lower than the second predetermined state of charge,

[0036] - discharging each cell of the battery having an assigned state of charge greater than third predetermined state of charge until the state of charge of said cell is substantially equal to the third predetermined state of charge when the state of charge assigned to at least one cell of the battery is equal to the second predetermined state of charge and controlling the charging means and the allocation means when the state of charge of said cell is substantially equal to the third predetermined state of charge, and

[0037] - determining the lowest assigned state of charge among the states of charge and discharging each cell of the battery having an assigned state of charge greater than the lowest state of charge until the state of charge of said cell is substantially equal to the lowest state of charge when the state of charge assigned to each of the cells of the battery is greater than or equal to the third predetermined state of charge.

[0038] Advantageously, the device further comprises a resistive circuit, the balancing means being configured to supply the circuit from each cell for a predetermined discharge duration.

[0039] There is also provided a motor vehicle comprising a battery comprising cells connected in series, and a device as defined previously connected to the cells.

[0040] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

[0041] [Fig.l]

[0042] schematically illustrates an example of a motor vehicle according to the invention;

[0043] [Fig.2]

[0044] schematically illustrates an example of a curve linking cell charge states to voltages at the terminals of the cells according to the invention; and

[0045] [Fig.3]

[0046] schematically illustrates a method of balancing the load of cells according to the invention.

[0047] Reference is made to [Fig.l] which schematically illustrates an example of a motor vehicle 1.

[0048] The vehicle 1 comprises a battery 2, charging means 3 comprising for example an alternator of the vehicle 1 or a charger, a control system 4 of the battery 2, and a resistive circuit 5.

[0049] The battery 2 comprises for example a lithium-ion battery formed by a plurality of electrochemical cells 6, 7, 8, 9, for example of the Lithium Iron Phosphate LFP type.

[0050] Of course, the battery 2 can be a battery of another type, for example sodium-ion.

[0051] The charging means 3 are capable of charging the cells 6, 7, 8, 9 of the battery 2 at constant voltage.

[0052] When the charging means 3 charge the cells 6, 7, 8, 9 of the battery 2, the voltage at the terminals of the battery 2 increases until it reaches a predetermined charging voltage, for example equal to 13.6 V.

[0053] The charging means 3 are capable of stopping the charging of the cells 6, 7, 8, 9 of the battery 2 when the current flowing through the terminals of battery 2 is equal to or less than a threshold Sc.

[0054] Since the charge states of cells 6, 7, 8, 9 are different, the cell having the highest charge state will reach the maximum charge state (100%) before the other cells so that if the charging of said cell continues to charge the other cells, said cell is overloaded and deteriorates.

[0055] The control system 4 is for example a battery control system known by the English acronym BMS “Battery Management System”.

[0056] The control system 4 comprises first means 10 for measuring the voltage at the terminals of each cell 6, 7, 8, 9 and at the terminals of the battery 2, second means 11 for measuring the current flowing through the terminals of the battery 2.

[0057] The control system 4 further comprises allocation means 12, balancing means 13, storage means 14, and control means 15 capable of controlling the charging means 3.

[0058] The resistive circuit 5 is made from resistors.

[0059] The storage means 14 are for example formed by a non-volatile memory and can be arranged outside the control system 4 in the vehicle 1.

[0060] The storage means 14 store a curve CSoc associating values ​​of the open circuit voltage Vco measured at the terminals of each cell 6, 7, 8, 9 with charge states SOC of said cell 6, 7, 8, 9.

[0061] The storage means 14 can further store the threshold Sc.

[0062] [Fig.2] illustrates an example of the CSoc- curve

[0063] The curve CSoc associates each value included in a first voltage domain DI with a variable state of charge between a first predetermined state of charge SOC1 and a second predetermined state of charge SOC2.

[0064] The first domain DI extends between a minimum voltage value Vmin and a first reference voltage value VL

[0065] The curve CSoc associates the first reference value VI with the second predetermined state of charge SOC2.

[0066] The curve CSoc associates each value included in a second voltage domain D2 with a variable state of charge between a third predetermined state of charge SOC3 and a fourth predetermined state of charge SOC4.

[0067] The second domain D2 extends between the first reference value VI and a second maximum voltage value Vmax.

[0068] The first reference value V1 is greater than the minimum value Vmin and less than the maximum value Vmax.

[0069] The second predetermined state of charge SOC2 is greater than the first predetermined state of charge SOC1 and less than the third predetermined state of charge SOC3.

[0070] The fourth predetermined state of charge SOC4 is greater than the third predetermined state of charge SOC3.

[0071] The minimum value Vmin is for example equal to 0 V, the first reference value V1 is for example equal to 3.35 V and the maximum value is for example equal to 3.6 V.

[0072] The first state of charge SOC1 is for example equal to 65%, the second state of charge SOC2 is for example equal to 75%, the third state of charge SOC3 is for example equal to 95% and the fourth state of charge SOC4 is for example equal to 100%.

[0073] The variable state of charge SOC of the first domain DI is determined from a function Fl linking the variable state of charge SOC to a voltage Vc of the first voltage domain DI so that:

[0074] y(7C = Fl(Vc)(l)

[0075] The function Fl is chosen such that:

[0076] SOC 1 = FKVmüî) (2)

[0077] SOC2 = F1(V1)(3)

[0078] The variable state of charge SOC of the second domain D2 is determined from a function F2 linking the variable state of charge SOC to a voltage Vc of the first second voltage D2 so that:

[0079] SOC = F2(Vc) (4)

[0080] The function F2 is chosen such that:

[0081] SOC3 = F1(V1)(5)

[0082] SOCA = Fl(Vmax) (6)

[0083] The functions Fl and F2 are for example linear functions.

[0084] The charging means 3, the allocation means 12, and the balancing means 13 form a device for balancing the charge of the cells 6, 7, 8, 9 of the battery 2.

[0085] In order to prevent the cells from being overcharged, the maximum value of the predetermined voltage Umax applied to the terminals of the battery 2 is chosen so that

[0086] Umax = (Ne -1) • V1 + Vmax 0)

[0087] Ne being the number of cells 6, 7, 8, 9 of battery 2.

[0088] [Fig.3] schematically illustrates a method of balancing the load of cells 6, 7, 8, 9 implementing the device.

[0089] It is assumed that the state of charge of cells 6, 7, 8, 9 is lower than the maximum charge level.

[0090] During a step 20, the charging means 3 controlled by the control means 15 charge the cells 6, 7, 8, 9 at constant voltage.

[0091] As long as the intensity of the current flowing through the terminals of battery 2 measured by the second measuring means 11 is greater than the threshold Sc (step 21), the charging of the cells by the charging means 3 continues.

[0092] When the intensity of the current flowing through the terminals of the battery 2 measured by the second measuring means 11 is equal to or less than the threshold Sc (step 21), during a step 22, the control means 15 control the charging means 3 so that the charging of the cells 6, 7, 8, 9 stops.

[0093] During a step 23, the allocation means 12 allocate to each cell 6, 7, 8, 9 of the battery 2 a state of charge deduced from the open circuit voltage Vco measured at the terminals of said cell 6, 7, 8, 9 and from the curve CSoc stored in the storage means 14.

[0094] The open circuit voltage Vco measured at the terminals of each cell 6, 7, 8, 9 is measured by the first measuring means 10 when the charging of the cells 6, 7, 8, 9 is stopped.

[0095] For each cell 6, 7, 8, 9, depending on the value of the voltage Vco measured at the terminals of the cell 6, 7, 8, 9 in open circuit, the allocation means 12 allocate the state of charge to the cell 6, 7, 8, 9 associated by the curve CSoc with said voltage Vco in open circuit.

[0096] If the open-circuit voltage Vco across the terminals of the cell 6, 7, 8, 9 is in the first voltage range D1, the allocation means 12 determine a state of charge of the cell from equation (1) and allocate said state of charge to the cell.

[0097] If the voltage across the terminals of the cell 6, 7, 8, 9 is equal to the first reference voltage value VI, the allocation means 12 allocate to the cell a state of charge equal to the second state of charge SOC2.

[0098] If the voltage across the terminals of the cell 6, 7, 8, 9 is in the second domain D2, the allocation means 12 determine a state of charge of the cell from equation (4) and allocate said state of charge to the cell.

[0099] The state of charge of battery 2 is equal to the minimum state of charge of cells 6, 7, 8, 9.

[0100] When the allocation means 12 have allocated a state of charge to each cell 6, 7, 8, 9 of the battery 2, the balancing means 13 balance the cells 6, 7, 8, 9 of the battery 2.

[0101] The balancing means 13 identify three cases (step 24).

[0102] When the state of charge assigned to at least one cell 6, 7, 8, 9 of the battery 2 is equal to the second predetermined state of charge SOC2 (step 25), the balancing means 13 discharge each cell 6, 7, 8, 9 of the battery 2 having an assigned state of charge greater than the third predetermined state of charge SOC3 until the state of charge of said cell is substantially equal to the third state of charge predetermined S0C3 and the balancing means 13 control the charging means 3 and the allocation means 12 so as to repeat steps 20 to 24.

[0103] The method continues at step 20.

[0104] When the state of charge assigned to each of the cells 6, 7, 8, 9 of the battery 2 is greater than or equal to the third predetermined state of charge SOC3 (step 26), the balancing means 13 determine the lowest assigned state of charge among the assigned states of charge of the cells 6, 7, 8, 9, and discharge each cell having an assigned state of charge greater than the lowest state of charge until the state of charge of said cell is substantially equal to the lowest state of charge.

[0105] When the assigned state of charge of at least one cell 6, 7, 8, 9 is lower than the second predetermined state of charge SOC2 (step 27), the balancing means 13 discharge each cell 6, 7, 8, 9 of the battery 2 having an assigned state of charge higher than the second predetermined state of charge SOC2 until the state of charge of said cell is substantially equal to the assigned state of charge of the cell lower than the second predetermined state of charge SOC2.

[0106] The balancing means 13 discharge the cells 6, 7, 8, 9 requiring discharge by successively connecting the terminals of said cells to the resistive circuit 5 for a predetermined discharge duration.

[0107] The discharge duration is chosen so that when the terminals of the cell 6, 7, 8, 9 are disconnected from the resistive circuit 5 at the end of the discharge duration, the state of charge of said cell is reduced by a predetermined discharge value, for example 4%.

[0108] If, at the end of the discharge of the cell 6, 7, 8, 9 during the discharge duration, the state of charge of the discharged cell 6, 7, 8, 9 is greater than the third predetermined state of charge SOC3 (step 25) or the state of charge of said cell is greater than the lowest state of charge (step 26), the balancing means reconnect the cell to the resistive circuit 5 during the discharge duration.

[0109] If necessary, the balancing means 13 repeat the connection of the cell to the resistive circuit 5 until the state of charge of said discharged cell is substantially equal to the third predetermined state of charge SOC3 (step 25) or is substantially equal to the lowest state of charge (step 26).

[0110] The device for balancing the charge of the cells 6, 7, 8, 9 of the battery 2 uses equipment available on board the motor vehicle 2 so that the balancing method can be implemented on any motor vehicle having a battery 2 without adding additional equipment.

[0111] Since the balancing method does not use the cell voltage profile for determining the state of charge, the method is suitable for balancing battery cells of any type connected in series.

[0112] The balancing method uses the section of the cell voltage profile close to 100% of the SOC for the determination of the state of charge of said cells.

[0113] And the section close to 100% of the SOC for a battery of any type has a sufficient slope which allows the determination of the state of charge of the battery cells.

[0114] As the open circuit voltage profile of the cells varies during battery aging, it is necessary to take into account the variations in the voltage profile. The section of the cell voltage profile close to 100% of the SOC does not vary during battery cell aging so that the method operates identically throughout the battery life simplifying the implementation of cell charge balancing.

[0115] As soon as the constant voltage charge of the cells 6, 7, 8, 9 of the battery 2 and a state of charge is assigned to each cell 6, 7, 8, 9, and as the reading of the voltage at the terminals of the cells 6, 7, 8, 9 is no longer necessary, the balancing method can be implemented independently of the use of the battery 2, for example when the motor vehicle 2 is in the driving phase.

Claims

Claims

1. Method for balancing the charge of cells (6, 7, 8, 9) of a battery (2), the cells being connected in series between two terminals of the battery, characterized in that the method comprises: - (a) constant voltage charging of the battery cells (2), - (b) a stoppage of the charge when the intensity of the current flowing through the terminals of the battery (2) is equal to or less than a threshold (Sc), - (c) for each cell (6, 7, 8, 9) of the battery (2), an at assignment of a state of charge (SOC1, SOC2) to said cell deduced from the voltage measured at the terminals of said cell and from a curve (CSoc) associating values ​​of the voltage measured at the open circuit terminals of said cell with states of charge, and - (d) balancing of the charge states of the battery cells according to the state of charge assigned to each cell.

2. Method according to claim 1, the curve (CSoc) associating each value included in a first voltage domain (Dl) with a variable state of charge included between a first predetermined state of charge (SOC1) and a second predetermined state of charge (SOC2), said first domain (Dl) extending between a minimum voltage value (Vmin) and a first reference voltage value (VI), the curve (Csoc) associating the first reference value (VI) with the second predetermined state of charge (SOC2), and the curve associating each value included in a second voltage domain (D2) with a variable state of charge included between a third predetermined state of charge (SOC3) and a fourth predetermined state of charge (SOC4), said second domain (D2) extending between the first reference value (VI) and a second maximum voltage value (Vmax),the first reference value (VI) being greater than the minimum value (Vmin) and less than the maximum value (Vmax), the second predetermined state of charge (SOC2) being greater than the first predetermined state of charge (SOC1) and less than the third predetermined state of charge, (SOC3), and the fourth predetermined state of charge (SOC4) being greater than the third predetermined state of charge (SOC3).

3. The method of claim 2, wherein balancing the states of charge of the cells (6, 7, 8, 9) comprises: - when the assigned state of charge of at least one cell (6, 7, 8, 9) is lower than the second predetermined state of charge (SOC2), discharging each cell having an assigned state of charge higher than the second predetermined state of charge (SOC2) until the state of charge of said cell is substantially equal to the assigned state of charge of the cell lower than the second predetermined state of charge, - when the state of charge assigned to at least one cell (6, 7, 8, 9) of the battery (2) is equal to the second predetermined state of charge (SOC2), discharging each cell having an assigned state of charge higher than the third predetermined state of charge (SOC3) until the state of charge of said cell is substantially equal to the third predetermined state of charge (SOC3) and repeating steps (a) to (d),and - when the state of charge assigned to each of the cells (6, 7, 8, 9) of the battery (2) is greater than or equal to the third predetermined state of charge (SOC3), determining the lowest assigned state of charge among the states of charge, and discharging each cell having an assigned state of charge greater than the lowest state of charge until the state of charge of said cell is substantially equal to the lowest state of charge.,

4. A method according to claim 3, wherein the discharging of each cell (6, 7, 8, 9) comprises supplying a resistive circuit (5) with said cell for a predetermined discharge duration.

5. A method according to claim 4, wherein the supply of a resistive circuit (5) by said cell (6, 7, 8, 9) for the predetermined discharge time is repeated until the assigned state of charge of said cell is substantially equal to the third predetermined state of charge (SOC3) or until the state of charge of said cell is substantially equal to the lowest state of charge.

6. A method according to any one of claims 1 to 5, wherein the

7.

8. battery (2) includes lithium iron phosphate / graphite cells. Device for balancing the charge of cells (6, 7, 8, 9) of a battery (2), the cells being connected in series between two terminals of the battery, characterized in that the device comprises: charging means (3) configured to charge the cells of the battery (2) at constant voltage, and to stop charging the cells when the intensity of the current flowing through the terminals of the battery (2) is equal to or less than a threshold (Sc), allocation means (12) configured to allocate to each cell (6, 7, 8, 9) of the battery (2) a state of charge (SOC1, SOC2) deduced from the voltage measured at the terminals of said cell and from a curve (CSoc) associating values ​​of the voltage measured at the open-circuit terminals of said cell with states of charge, and balancing means (13) configured to balance the charge states of the battery cells according to the charge state assigned to each cell. Device according to claim 7, further comprising means of storage (14) configured to store the curve (Csoc), the curve associating each value included in a first voltage domain (Dl) with a variable state of charge between a first predetermined state of charge (SOC1) and a second predetermined state of charge (SOC2), said first domain (Dl) extending between a minimum voltage value (Vmin) and a first reference voltage value (VI), the curve (CSoc) associating the first reference value (VI) with the second predetermined state of charge (SOC2), and the curve associating each value included in a second voltage domain (D2) with a variable state of charge between a third predetermined state of charge (SOC3) and a fourth predetermined state of charge (SOC4), said second domain (D2) extending between the first reference value (VI) and a second maximum voltage value (Vmax),the first reference value (VI) being greater than the minimum value (Vmin) and less than the maximum value (Vmax), the second predetermined state of charge (SOC2) being greater than the first predetermined state of charge (SOC1) and less than the third, predetermined state of charge (SOC3), and the fourth predetermined state of charge (SOC4) being greater than the third predetermined state of charge (SOC3).

9. Device according to claim 8, wherein the balancing means (13) are configured to: - when the assigned state of charge of at least one cell (6, 7, 8, 9) is lower than the second predetermined state of charge (SOC2), discharge each cell (6, 7, 8, 9) of the battery (2) having an assigned state of charge higher than the second predetermined state of charge (SOC2) until the state of charge of said cell is substantially equal to the assigned state of charge of the cell lower than the second predetermined state of charge (SOC2), - discharge each cell (6, 7, 8,9) of the battery (2) having an assigned state of charge higher than the third predetermined state of charge (SOC3) until the state of charge of said cell is substantially equal to the third predetermined state of charge (SOC3) when the state of charge assigned to at least one cell of the battery is equal to the second predetermined state of charge (SOC2) and controlling the charging means (3) and the allocation means (12) when the state of charge of said cell is substantially equal to the third predetermined state of charge (SOC3), and - determining the lowest assigned state of charge among the states of charge and discharging each cell (6, 7, 8,9) of the battery (2) having an assigned state of charge greater than the lowest state of charge until the state of charge of said cell is substantially equal to the lowest state of charge when the state of charge assigned to each of the cells of the battery is greater than or equal to the third predetermined state of charge (SOC3).,

10. Device according to claim 9, further comprising a resistive circuit (5), the balancing means (13) being configured to supply the circuit from each cell (6, 7, 8, 9) for a predetermined discharge duration.

11. Motor vehicle (1) comprising a battery (2) comprising cells (6, 7, 8, 9) connected in series, and a device according to any one of claims 7 to 10 connected to the cells.

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