FR3148176A1 - METHOD FOR BALANCING AN ENERGY STORAGE SYSTEM - Google Patents

Abstract

The present invention relates to a method for balancing a set of electrochemical cells of an energy storage system, comprising the following steps of determining (E1) a first cell having the minimum useful capacity among the set of cells, determining in a first sequence (SQ1) a first balancing requirement for said first cell and in a second sequence (SQ2) second balancing requirements for second cells of the set, the first requirement and each second balancing requirement being calculated from remaining discharge and charge capacities of each of said second cells relative to a remaining discharge and charge capacity of the first cell. The invention finds an application for electrified vehicles, in particular in the automotive field. Figure 2.

Description

METHOD FOR BALANCING AN ENERGY STORAGE SYSTEM

The field of the invention relates to a method for balancing an energy storage system comprising electrochemical cells.

When cells in a battery have non-uniform states of charge, the least charged or most charged cells can limit the discharge or charge capacity of all the cells because they reach their maximum or minimum state of charge limit before the others. In order to optimize the available energy of a battery system, there are passive balancing solutions where the system dissipates energy in order to obtain a state of charge balance across the entire cell pack making up the battery and active solutions where the system can transfer energy from one cell or group of cells to others.

Current strategies for balancing cells in a battery seek to maintain maximum state-of-charge dispersion around the least charged cell. This means that the strategy seeks to bring the state-of-charge of all cells to within +1% of the state-of-charge of the least charged cell.

Further known from the prior art is EP-A1-3093947, which describes a battery balancing system that uses a method for determining the minimum voltage threshold of each cell of the battery. This method prevents the degradation of the useful capacity of the battery by ensuring that the balancing function does not discharge each cell below a minimum voltage threshold. Furthermore, even when there is a battery cell in a battery unit with an abnormality, the storage battery management apparatus can suppress the propagation of the influence of the abnormality and can improve availability.

We also know the patent document US-A1-202132844 which teaches a technique for balancing the cells of a battery which consists in detecting a trigger voltage of a balancing procedure. The balancing procedure also consists in determining a quantity of energy to be transferred to a cell under charge or discharge so as to balance the voltage of the cells.

These strategies have the disadvantage that they do not optimize the useful capacity of the battery pack. In addition, the strategies described above may lead to discharging the cell with the lowest capacity when it does not have the lowest state of charge, thus resulting in a loss of useful capacity. This situation may result in a decrease in the overall performance of the battery.

There is therefore a need to overcome the above problems and to develop new balancing strategies for optimizing the useful capacity of the battery.

More specifically, the invention relates to a method for balancing a set of electrochemical cells of an energy storage system. According to the invention, the method comprises the following steps:

- determining a first cell having the minimum useful capacity among the set of cells,

- determining in a first sequence a first balancing requirement for said first cell and in a second sequence second balancing requirements for second cells of the set, the first requirement and each second balancing requirement being calculated from remaining discharge and charge capacities of each of said second cells relative to a remaining discharge and charge capacity of the first cell.

The method according to the invention may include the following additional characteristics, alone or in combination:

- The first sequence comprises determining remaining discharge capacities up to a minimum functional limit for the first cell and for the second cells of said set of cells, and assessing the first balancing need of said first cell as a function of the remaining discharge capacity of said first cell and the remaining discharge capacity of each second cell, the first balancing need being detected when at least one of said second cells has a remaining discharge capacity lower than the first cell, the second sequence comprises determining remaining charge capacities up to a maximum functional limit for the first cell and for the second cells of said set of cells, and assessing the second balancing needs for each of the second cells as a function of the difference between the remaining charge capacity of each second cell and the remaining charge capacity of the first cell, a second balancing need being detected when at least one of said second cells has a remaining charge capacity lower than the first cell.

- The first sequence further comprises the calculation of the first balancing requirement corresponding to a first quantity of energy to be discharged from the first cell, the first quantity of energy being equal to the difference between the remaining discharge capacity of said first cell and the remaining discharge capacity of said cell among said second cells whose remaining discharge capacity is the lowest.

- The second sequence further comprises the calculation of the second balancing requirement corresponding to a second quantity of energy to be discharged from a second cell, the second quantity of energy being equal to the difference between the remaining charge capacity of the first cell and the remaining charge capacity of the second cell whose remaining charge capacity is lower than that of the first cell.

- Each balancing requirement of a cell determines a discharge duration to be controlled in a discharge circuit of said cell, said duration being calculated from a predetermined discharge current value dependent on the discharge circuit and the quantity of energy to be discharged for said cell.

- A step of controlling the discharge of the cells of said set for which a balancing need is detected, and in which a discharge command comprises the actuation of a discharge circuit during the calculated discharge duration.

The invention further provides an energy storage system comprising a set of electrochemical cells and a control unit, in which the control unit comprises means for implementing the balancing method according to any one of the preceding embodiments.

The invention further provides an electrified vehicle comprising such an energy storage system.

The invention further provides a computer program product comprising instructions which, when the program is executed by a control unit of an energy storage system comprising a set of electrochemical cells, cause the latter to implement the balancing method according to any one of the preceding embodiments.

The method according to the invention makes it possible to optimize the useful storage capacity of a battery system by ensuring that the cell with the minimum useful capacity is a cell that limits the discharge or charge of the storage system. This therefore makes it possible to increase the autonomy of an electrified vehicle.

Other features and advantages of the present invention will appear more clearly on reading the detailed description which follows, comprising embodiments of the invention given as non-limiting examples and illustrated by the appended drawings, in which:

schematically represents an energy storage system comprising a set of electrochemical cells intended for the implementation of the invention.

represents a functional diagram of an embodiment of the balancing method according to the invention.

represents a simplified example of implementation of the balancing method according to the invention in the form of graphs for the step of evaluating the balancing need of the cell having the minimum useful capacity.

represents a simplified example of implementation of the balancing method according to the invention in the form of graphs for the step of evaluating the balancing need of the other cells.

The invention applies to electrical energy storage systems comprising electrochemical cells, preferably for power battery systems having a storage capacity of several kilowatt-hours. The invention finds an application in particular for electrified vehicles, i.e. comprising an electric motor machine and power electronics, with a completely electric or hybrid motor, preferably motor vehicles, but not only such as an airplane, tractor, electric bicycle. The invention proposes a cell balancing technique aimed at optimizing the useful capacity of a battery.

In , an energy storage system 1 intended to implement the invention is shown. The storage system 1 is a battery comprising a set of several electrochemical cells, for example n cells referenced from C1 to Cn electrically connected in series to deliver the voltage adapted to the electrical systems that it supplies. An electrochemical cell Ci is an electrical energy storage element, for example of the Lithium-ion, Ni-mh, Lithium-Iron-Phosphate type. A cell Ci comprises at least one cell or a cluster of cells connected in parallel, for example two, three, four, five or more cells connected in parallel. In a non-limiting example, the battery system 1 delivers a voltage of between 350 volts and 450 volts. Alternatively, the voltage may be higher, of the 800 volt type (between 700 volts and 900 volts), or even more.

The battery system 1 comprises for each cell Ci a discharge circuit CDi comprising a controllable switch and a discharge resistor Ri. The discharge circuit CDi is adapted to discharge electrical energy into the resistor Ri when a balancing need is detected for the associated cell. Each cell Ci or each cluster of cells is provided with a discharge circuit. The invention provides a balancing method for determining balancing needs for the cells of the battery system. The balancing means may be a passive or active balancing circuit.

The battery system 10 further comprises a control unit 2 provided for managing the battery system 1 (commonly designated by the acronym BMS for “Battery Management System” or TBCU for “Traction Battery Control Unit” in the case of an application for an electrified vehicle) and adapted to supervise the parameters specific to the battery in coordination with the operation of the electrical systems.

The control unit 2 comprises sensors and means for measuring, estimating or determining the electrical parameters of each cell and of the storage system as a whole, such as for example the state of charge SOC (“State of Charge”) of a cell expressed by a ratio between the quantity of energy stored at a given time and the maximum quantity of energy storable at a given time, the open circuit voltage OCV (“Open Circuit Voltage”) expressed in volts, the charging current expressed in amperes, the state of health SOH (“State of Health”), which designates the aging level parameter of a cell or battery expressing a ratio between the maximum quantity of energy storable at a given time and the maximum quantity of energy storable in the new state of the battery, the useful capacity of the battery expressed in ampere-hours and designating the quantity of energy storable by a cell, the minimum functional discharge limit for a cell established by the manufacturer, the maximum functional charge limit established by the manufacturer.

The useful capacity of each cell can be determined at any time by the control unit 2 from maps, matrices or vectors provided for this purpose and updated periodically during use of the battery to monitor this parameter. The useful capacity corresponds to the total capacity in new condition of a cell minus the capacity lost due to wear and aging. The useful capacity of each cell can be known from algorithms for estimating the aging state known to those skilled in the art. One technique can consist of fully charging the battery and then determining, when the battery wakes up, after a sufficient relaxation period, an initial state of charge level by measuring the no-load voltage, then again determining a final state of charge level for a given period. By coulometric measurement and by calculating the integral of the current over this given period, the control unit can determine the quantity of energy stored in A.h corresponding to the difference between the initial and final state of charge level. This amount of energy corresponding to the difference in the state of charge levels makes it possible to establish an estimate of the useful capacity of each cell and of the battery as a whole. The useful capacity parameter of each cell is updated periodically, for example every day, week or month of use. Other alternative methods of estimating the useful capacity of a cell are conceivable and applicable depending on the application and the conditions of use of the battery. A protocol specifically provided for this purpose can be ordered periodically, for example by a constant current discharge method between a full state of charge and a minimum state of charge.

The state of charge parameter of each cell is calculated at each instant by measuring the current and calculating the amount of energy charged or discharged over a given period of time. The state of charge parameter can be determined at any instant by the control unit 2.

Furthermore, the control unit 2 comprises means for determining at each instant the remaining discharge quantity of a cell corresponding to the quantity of energy that a cell can deliver between its instantaneous state of charge and the minimum functional charge limit, as well as the remaining charge quantity corresponding to the quantity that a cell can deliver between its instantaneous state of charge and the maximum functional charge limit. For this purpose, the control unit may comprise in memory maps or vectors recording this information and consultable at any instant.

In , a diagram representing an embodiment of the balancing method according to the invention is illustrated. The balancing method is periodically executed by the battery control unit. The method aims to optimize the useful capacity of a battery by ensuring that the cell with the lowest capacity has the highest state of charge when the battery is charged, and that it has the lowest state of charge when the battery is discharged. More specifically, the method comprises determining balancing needs for the set of cells of the battery according to remaining discharge and charge capacities of each of the cells relative to the remaining discharge and charge capacity of the cell having the lowest useful capacity.

The control unit is provided with an integrated circuit calculator and electronic memories, the calculator and the memories being configured to execute the method. But this is not mandatory. Indeed, the calculator could be external to the control unit, while being coupled to the latter. In the latter case, it can itself be arranged in the form of a dedicated calculator comprising a possible dedicated program, for example. Consequently, the control unit, according to the invention, can be produced in the form of software modules (or computer modules (or even "software")), or electronic circuits (or "hardware"), or even a combination of electronic circuits and software modules.

More specifically, the method comprises a first step E1 of determination by the control unit of the useful capacity of each cell of a set of cells of the battery and of identification of a first cell of the set having the lowest useful capacity among the cells of the set. This cell is identified from a map, matrix or vector recorded and updated in the memory of the control unit.

The method further comprises a first sequence SQ1 for determining a balancing requirement of the cell having the lowest capacity based on the remaining discharge capacities of the cells in the set. It further comprises a second sequence SQ2 for determining balancing requirements of second cells in the set based on the remaining charge capacities of each cell in the set relative to the first cell. The second cells are the other cells in the set that have a useful capacity greater than the first cell with the lowest useful capacity in the set.

For this purpose, for the first sequence SQ1, the method comprises a second step E2 of determining the remaining discharge capacity up to the minimum functional charge limit for this first cell having the lowest useful capacity and for the second cells of said set of cells.

More specifically for the first sequence SQ1, the method comprises a third step E3 of evaluating the first balancing need Q1 of the first cell as a function of its remaining discharge capacity and the remaining discharge capacity of each second cell. The first balancing need Q1 is detected when at least one of said second cells has a remaining discharge capacity lower than the first cell. If this need is detected, the method comprises a fourth step E4 of calculating the first balancing need which corresponds to the quantity of energy Q1 to be discharged from the first cell. This quantity of energy Q1 is equal to the difference between the remaining discharge capacity of said first cell and the remaining discharge capacity of the cell among the other second cells of the set whose remaining discharge capacity is the lowest.

In , the steps of the first sequence are illustrated more precisely for the case of a set of four cells as a non-limiting example. It is assumed, as an example, that for each cell the useful charge state range is between the minimum discharge limit of value 1.3% state of charge and the maximum charge limit of value 98% state of charge.

As illustrated in Figures 31 and 32, cell CE1 has a useful capacity of 120 Ah and a state of charge of 50%, cell CE2 has a useful capacity of 123 Ah and a state of charge of 48%, cell CE3 has a useful capacity of 121 Ah and a state of charge of 51.5%, and cell CE4 has a useful capacity of 120.5 Ah and a state of charge of 50.7%. Cell CE1 is the cell with the lowest useful capacity.

As illustrated by graph 33, for cell CE1, the method calculates a remaining discharge equal to 58.4 A.h, for cell CE2 equal to 57.4 A.h, for cell CE3 equal to 60.7 and for cell CE4 equal to 59.5. In this example case, during step E2, the method detects the need to discharge cell CE1 by an amount of energy Q1 equal to 1 A.h.

Furthermore, for the second sequence SQ2, the method comprises a fifth step E5 of determining remaining charge capacities up to a maximum functional limit for the first cell and for the second cells of said set of cells. It further comprises a sixth step E6 of evaluating second balancing needs for each of the second cells as a function of the difference between the remaining charge capacity of each second cell and the remaining charge capacity of the first cell.

More specifically, a second balancing need is detected when at least one of the second cells in the set has a remaining charge capacity less than the first cell. Multiple second balancing needs may be detected. A second balancing need is a second amount of energy to be discharged from one or more of the second cells in the set.

If a second balancing need is detected, the method comprises a seventh step E7 of calculating each second quantity of energy. Each second quantity of energy is equal to the difference between the remaining charge capacity of the first cell and the remaining charge capacity of the second cell whose remaining charge capacity is lower than that of the first cell.

In , the second sequence E2 is illustrated for the same non-limiting example case of the . Graphs 41 and 41 illustrate the same situation as for graphs 31 and 32. As illustrated by graph 43, for cell CE1, the method calculates a remaining charge equal to 57.6 Ah, for cell CE2 equal to 61.5 Ah, for cell CE3 equal to 56.3 and for cell CE4 equal to 57. In this example case, during step E3, the method detects the need to discharge cells CE3 and CE4 by an energy quantity Q2 equal to 1.3 Ah and an energy quantity Q3 equal to 0.6 Ah respectively.

It should be noted that the first sequence SQ1 and the second sequence SQ2 can be executed alternately, or simultaneously. These two sequences of steps aim to target the cells that need to be balanced to optimize the useful capacity.

The balancing method further comprises an eighth balancing step E8 depending on the balancing needs evaluated in the preceding steps. In a so-called passive balancing configuration mode using exclusively discharge circuits, each balancing need of a cell Ci determines a discharge duration to be controlled in the discharge circuit CDi. The duration is calculated from a predetermined discharge current value dependent on the discharge circuit and the amount of energy to be discharged for said cell corresponding to the need. For each detected balancing need associated with a specific cell, the method controls during step E8 the actuation of the discharge circuit during the calculated discharge duration. The actuation is performed by closing a switch of the circuit.

It is contemplated that the balancing needs detected in accordance with the method according to the invention may be implemented in an alternative balancing configuration mode, for example an active balancing method. Each balancing need may correspond to an amount of energy to be transferred. For example, for the first sequence, the method may in an alternative embodiment control the recharging of the cell having the lowest remaining discharge capacity and which is lower than the cell having the lowest useful storage capacity.

The invention is described in the foregoing by way of example. It is understood that the person skilled in the art is able to produce different variant embodiments of the invention by associating, for example, the different characteristics above taken alone or in combination, without departing from the scope of the invention.

Claims (9)

Method for balancing a set of electrochemical cells (Cn) of an energy storage system (1), characterized in that it comprises the following steps:
- determining (E1) a first cell (CE1) having the minimum useful capacity among the set of cells (Cn),
- determining in a first sequence (SQ1) a first balancing requirement (Q1) for said first cell (CE1) and in a second sequence (SQ2) second balancing requirements (Q2, Q3) for second cells (CE2, CE3, CE4) of the set, the first balancing requirement (Q1) and each second balancing requirement (Q2, Q3) being calculated from remaining discharge and charge capacities of each of said second cells relative to a remaining discharge and charge capacity of the first cell (CE1). Balancing method according to claim 1 wherein:
- the first sequence (SQ1) comprises the determination (E2) of remaining discharge capacities up to a minimum functional limit for the first cell (CE1) and for the second cells (CE2, CE3, CE4) of said set of cells (Cn), and the evaluation (E3) of the first balancing need (Q1) of said first cell as a function of the remaining discharge capacity of said first cell and the remaining discharge capacity of each second cell, the first balancing need (Q1) being detected when at least one of said second cells (CE2) has a remaining discharge capacity lower than the first cell (CE1),
- the second sequence (SQ2) comprises determining (E5) remaining charge capacities up to a maximum functional limit for the first cell (CE1) and for the second cells (CE2, CE3, CE4) of said set of cells, and evaluating (E6) the second balancing needs (Q2, Q3) for each of the second cells (CE2, CE3, CE4) as a function of the difference between the remaining charge capacity of each second cell (CE2, CE3, CE4) and the remaining charge capacity of the first cell (CE1), a second balancing need (Q2, Q3) being detected when at least one of said second cells (CE3, CE4) has a remaining charge capacity lower than the first cell (CE1). Balancing method according to claim 2, wherein the first sequence (SQ1) further comprises the calculation (E4) of the first balancing requirement (Q1) corresponding to a first quantity of energy to be discharged from the first cell, the first quantity of energy being equal to the difference between the remaining discharge capacity of said first cell (CE1) and the remaining discharge capacity of said cell (CE2) among said second cells (CE2, CE3, CE4) whose remaining discharge capacity is the lowest. Balancing method according to claim 2 or 3, in which the second sequence (SQ2) further comprises the calculation (E7) of the second balancing requirement (Q2; Q3) corresponding to a second quantity of energy to be discharged from a second cell (CE3; CE4), the second quantity of energy (Q2; Q3) being equal to the difference between the remaining charge capacity of the first cell (CE1) and the remaining charge capacity of the second cell (CE3; CE4) whose remaining charge capacity is lower than that of the first cell (CE1). Balancing method according to any one of claims 1 to 4, wherein each balancing need of a cell (Ci) determines a discharge duration to be controlled in a discharge circuit (CDi) of said cell (Ci), said duration being calculated from a predetermined discharge current value dependent on the discharge circuit (CDi) and the quantity of energy to be discharged for said cell (Ci). Balancing method according to claim 5, further comprising a step of controlling discharge (E8) of the cells of said set for which a balancing need is detected, and in which a discharge command comprises the actuation of a discharge circuit during the calculated discharge duration. Energy storage system (1) comprising a set of electrochemical cells (Cn) and a control unit (2), characterized in that the control unit (2) comprises means for implementing the balancing method according to any one of claims 1 to 6. Electrified vehicle comprising an energy storage system (1) according to claim 7. Computer program product comprising instructions which, when the program is executed by a control unit (2) of an energy storage system (1) comprising a set of electrochemical cells, cause the latter to implement the balancing method according to any one of claims 1 to 6.