METHOD FOR OPTIMIZING THE LIFESPAN OF AN ELECTRICAL STORAGE BATTERY

Abstract

The invention relates to a method for optimizing the lifespan of an electrical storage battery (6), implemented in a computer (4) comprising memory means (8) storing an aging model (12) of the electrical storage battery (6), the electrical storage battery (6) being an active-balancing battery with power transfer or a reconfigurable battery and comprising a plurality of electrochemical cells or electrochemical cell modules connected in series and / or in parallel, the computer (4) further comprising a controller (10) configured to control the electrical state of charge of each electrochemical cell or each electrochemical cell module of the battery (6). The invention also relates to a computer program product for implementing such a method, as well as a motor vehicle (2) comprising a computer (4) storing such a computer program product.Figure from the summary: Fig. 1.

Description

Title of the invention: METHOD FOR OPTIMIZING THE LIFESPAN OF A BATTERY ELECTRICAL STORAGE

[0001] The present invention relates to optimizing the lifespan of an electrical storage battery. The electrical storage battery is typically a traction battery for an electric or hybrid motor vehicle, although this is not a limitation within the scope of the present invention. The electrical storage battery may also be, according to other applications envisaged for the invention, a stationary battery or an electrical storage battery for a non-motor vehicle such as, for example, a heavy goods vehicle, a boat, or an aircraft.

[0002] An electric or hybrid motor vehicle is typically equipped with at least one electrical storage battery used in the vehicle's powertrain. Such a battery is called a "traction battery." A traction battery is generally composed of several electrochemical cells connected in series and / or parallel to increase the battery's voltage, current, capacity, energy, and / or power. Furthermore, when the battery consists of at least several dozen electrochemical cells, battery manufacturers often choose to initially assemble modules composed of cells in series and parallel before assembling the battery from these various series- and / or parallel-connected modules.

[0003] Today, most large electric batteries are built with a fixed configuration of identical electrochemical cells in series and / or parallel. A balanced state among the cells is therefore preferred so that, during battery use, all the cells reach the fully charged and discharged states simultaneously to maximize the use of the onboard energy (storable energy). However, during the battery's lifetime, slightly different conditions between cells can lead to a dispersion between them, preventing them from reaching the fully charged and discharged states at the same time. Cell balancing methods are known and are used to compensate for this dispersion.

[0004] Among such balancing methods, the following two methods may be mentioned in particular: - Passive balancing, which consists of overcharging the batteries with the highest states of charge until all the batteries are fully charged. Indeed, the additional energy is dissipated as Joule loss or as a secondary chemical reaction within the battery; and - Dissipative balancing involves dissipating energy from the most charged cells through Joule losses in power electronics components until all cells are fully charged. This is the most commonly used method for lithium-ion batteries because the secondary chemical reactions that occur during the overcharging phase of passive balancing are destructive to lithium-ion cells.

[0005] However, neither passive balancing methods nor dissipative balancing methods allow for the simultaneous achievement of a fully discharged state for all the accumulators. Therefore, more complex methods and batteries are used to achieve this objective. This is particularly the case with the following two balancing technologies: - Active balancing with power or energy transfer between batteries, which involves using power electronics and energy storage components such as capacitors and inductors; and - Reconfigurable batteries that use power electronics to change the series or parallel configuration of the cells or modules in real time. Batteries with power converters at the module level can also be included in this latter category.

[0006] Hereafter, "actively balanced batteries" in the context of the present invention refers to both batteries implementing a power-transfer balancing method and reconfigurable batteries. With such actively balanced batteries, it is possible to simultaneously achieve the fully charged and fully discharged states of all the cells in the battery, thus allowing for the storage of more usable energy.

[0007] Prior art solutions for optimizing the lifespan of an actively balanced vehicle battery are known, enabling a reduction in battery capacity loss through improved temperature balancing and balancing of the battery cells' states of charge. For the purposes of this invention, "state of charge" refers to a percentage ranging from 100% to 0%, characterizing the remaining electrical capacity of an electric battery relative to its total electrical capacity. A state of charge of 100% indicates a fully charged battery, while 0% indicates a fully discharged battery. The criteria for 100% charged and 0% charged are defined by the manufacturers and vary depending on the chemistry of the electrochemical cells in the battery.

[0008] In particular, certain known solutions allow for minimizing the loss of electrical capacity in the battery with optimal control strategies on a battery model that includes an aging model. Such an aging model generally incorporates a function representing the capacity loss of the electrical storage battery as a function of the battery's energy state. For the purposes of this invention, "energy state" means a percentage that ranges from 100% to 0% and that characterizes a measure of the remaining energy in an electrochemical accumulator relative to the total energy in that accumulator. The total energy can be calculated by integrating the accumulator's power over a complete discharge. An energy state of 100% indicates a fully charged battery, while 0% indicates a fully discharged battery.The energy state of an electrochemical accumulator can also be defined as the energy state of the accumulator with its voltage equivalent to the open-circuit voltage with a state of health equal to 100% and a temperature equal to 25°C.

[0009] The results of optimal control show that a control favoring the use of a particular battery group leads to a significant reduction in aging. However, a drawback of these known solutions is that they only seek to optimize the lifespan of the actively balanced battery during battery cycling, and therefore do not take into account the vehicle's parking time. Yet, the parking time of electric or hybrid vehicles plays an important role in the loss of traction battery capacity in such vehicles.

[0010] There is therefore a need to have a method for optimizing the lifespan of an actively balanced battery, taking into account the time of inuse of the battery and allowing the battery's lifespan to be increased, and this for any type of chemistry relating to the electrochemical accumulators of the battery.

[0011] To this end, the invention relates to a method, implemented in a computer, for optimizing the lifespan of an electrical storage battery, the computer being connected to the electrical storage battery, the computer comprising memory means storing an aging model of the electrical storage battery, said aging model incorporating a function representing a capacity loss of the electrical storage battery as a function of the battery's energy state, the electrical storage battery being an active-balancing battery with power transfer or a reconfigurable battery and comprising a plurality of electrochemical cells or electrochemical cell modules connected in series and / or in parallel, the computer further comprising a controller configured to control the electrical state of charge of each electrochemical cell or each electrochemical cell module of the battery, the electrical states of charge of the electrochemical cells or electrochemical cell modules respecting at least one usage constraint, said method being implemented for a duration divided into constant sampling instants, each sampling instant corresponding to an energy state of the battery, the method comprising the steps of: - initial determination of whether the function representing the loss of capacity of the electrical storage battery as a function of the energy state of the battery exhibits a strict global convexity or a strict global concavity; - if said function exhibits strict global convexity, implementation, at each sampling instant, of a control for balancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules; - if said function has a strict global concavity, implementation, at each sampling instant, of a control for unbalancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules; - if said function exhibits neither strict global convexity nor strict global concavity, the following steps are implemented at each sampling instant: • determination of whether the function representing the loss of capacity of the electrical storage battery as a function of the battery's energy state exhibits a strict local convexity or concavity around the current energy state of the battery; • if said function has a strict local concavity around the current energy state of the battery, implementation of a control for unbalancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules; • if said function exhibits strict local convexity around the current energy state of the battery, determination of whether a balancing control of the electrical charge states of electrochemical accumulators or electrochemical accumulator modules corresponds to a global minimum of said function; • if a control for balancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules corresponds to an overall minimum of said function, implementation of a control for balancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules; • if a control for balancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules does not correspond to an overall minimum of said function, implementation of a control for unbalancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules.

[0012] The optimization method according to the invention takes advantage of the fact that a balanced state between accumulators or modules is sought when the battery approaches a fully charged or fully discharged state, in order to maximize the usable storable energy. However, for an actively balanced battery (as is the case in the method according to the invention), a balanced state is not necessarily required in an intermediate state of charge of the battery, which gives such a battery an additional degree of freedom. Indeed, electrical storage batteries have a high probability of being used within a wide range of state-of-charge due to what is called "user range anxiety" and the fact that they are often not charged to 100% state-of-charge due to a manufacturer's restriction or usage recommendations.

[0013] The principle of the present invention consists in using this additional degree of freedom of the battery between its fully charged and discharged states, in order to significantly increase the battery's lifespan. This is achieved by taking into account specific states for which an imbalance in the charge states of the battery cells is optimal (i.e., when the function representing battery aging has strict local or global concavity, or if the balancing control does not correspond to a global minimum of the function) and other states for which, on the contrary, a balancing of the charge states of the battery cells is optimal (i.e., when the function representing battery aging has strict global convexity, or strict local convexity, and the balancing control corresponds to a global minimum of the function).This allows for finer and more precise control of the state of charge of the batteries, compared to prior art optimal control strategies. These methods aim to utilize a specific group of batteries, which substantially increases battery life regardless of the battery chemistry. Furthermore, this optimization process takes into account battery inactivity time, which is advantageous from a control accuracy standpoint.

[0014] According to a preferred embodiment, the computer is embedded in an electric or hybrid motor vehicle, and the electrical storage battery is a battery of that vehicle. According to this preferred embodiment, the battery's inactivity time corresponds to the motor vehicle's parking time.

[0015] According to one embodiment, the computer further comprises an estimation module connected between an output of the electrical storage battery and an input of the controller. The estimation module is configured to receive at least one measurement of a physical parameter relating to the battery and to estimate, based on said at least one measurement, at least one parameter relating to the battery's state. Each command to balance or unbalance the electrical charge states of the electrochemical cells or electrochemical cell modules is carried out taking into account said at least one parameter relating to the battery's state. This makes it possible to consider, at the controller's input, the state(s) of charge, energy, or health of the battery, and thus further improve the control accuracy.

[0016] According to one embodiment, the estimation module is further configured to update the function representing the capacity loss of the electrical storage battery as a function of the battery's energy state, based on at least one parameter relating to the battery's state. Each sampling instant also corresponds to an instant at which the function representing the capacity loss of the electrical storage battery as a function of the battery's energy state is updated. This allows any variation in the battery's aging model over time to be taken into account in near real-time at the controller's input, thus further improving the control's accuracy.

[0017] According to one variant, each balancing or unbalancing command of the electrical charge states of electrochemical accumulators or electrochemical accumulator modules is carried out via the implementation of a control method belonging to the group consisting of: state feedback control; hierarchical control; Proportional, Integral, Derivative control; predictive control; open-loop control; reinforcement learning; on / off control and optimal control.

[0018] The invention also relates to a remarkable computer program product in that it comprises a set of program code instructions for execution steps of a process as described above, when said program is running on a computer or calculator.

[0019] The invention also relates to a motor vehicle comprising a computer storing a computer program product as described above.

[0020] The following are non-limiting examples of embodiments of the present invention, with reference to the accompanying figures in which: - [Fig. 1] schematically illustrates a vehicle according to the invention, the vehicle being equipped with an on-board computer; and - [Fig.2] is a flowchart representing a method for optimizing the lifespan of an electrical storage battery, implemented by the computer in [Fig.1], according to the present invention.

[0021] For the purposes of this invention, "total capacity" means the capacity of an electrochemical battery, generally expressed in Ah, when it is fully charged. From a chemical standpoint, the total capacity can be seen as the maximum amount of electrical charge (within the operating range specified by the manufacturer) that can flow between the anode and cathode of the battery. This total capacity decreases as the battery ages.

[0022] For the purposes of this invention, "capacity at the beginning of life" means the total capacity at the beginning of the life of an electrochemical accumulator, generally expressed in Ah.

[0023] For the purposes of this invention, "capacity loss" means the difference in capacity between the capacity at the beginning of life and the total capacity of an electrochemical accumulator, generally expressed in Ah.

[0024] For the purposes of this invention, "remaining capacity" means the amount of electrical charge that an electrochemical accumulator can still supply, relative to its total capacity. This remaining capacity is generally expressed in Ah and can also be defined as the difference between the initial capacity and the amount of electrical charge consumed.

[0025] For the purposes of this invention, "state of health" refers to a percentage ranging from 100% to 0% that characterizes a measure of the total capacity of an electric battery relative to its capacity at the beginning of its life. The state of health of an electric battery characterizes the level of deterioration of the battery, taking into account replacement and maintenance costs. A state of health of 100% indicates a battery in excellent condition, while 0% indicates a battery in very poor condition.

[0026] For the purposes of this invention, "open circuit voltage" means an electrical voltage of an electrochemical accumulator in an open circuit, dependent on the state of charge, temperature and condition of the accumulator.

[0027] With reference to [Fig.2], the present invention relates to a method, implemented in a computer 4, for optimizing the lifespan of an electrical storage battery 6. According to a preferred embodiment shown in [Fig.1], the computer 4 is embedded in an electric or hybrid motor vehicle 2, and the electrical storage battery 6 is a battery of the vehicle 2. In variants not shown, the electrical storage battery 6 can also be a stationary battery or an electrical storage battery of a non-motor electric vehicle such as, for example, a heavy goods vehicle, a boat or an airplane.

[0028] The electrical storage battery 6 is an actively balanced battery (i.e., either a power transfer battery or a reconfigurable battery), and is typically a traction battery for the vehicle 2, although this last point is not limiting within the scope of the present invention. The electrical storage battery 6 conventionally comprises several electrochemical cells or several electrochemical cell modules connected in series and / or in parallel, the latter not being shown in the figures for clarity. For such an actively balanced battery 6, it is (realistically) assumed that the energy in the battery 6 is the sum of the energies of each cell (due to their ability to rebalance at the end of charging or discharging).

[0029] The computer 4 is connected to the electrical storage battery 6 and includes memory means 8 and a controller 10 connected to the memory means 8. Preferably, as illustrated in [Fig. 1], the computer 4 further includes an estimation module 11. The computer 4 is typically part of an electrical battery management system 6 (such a system not being shown in the figures for reasons of clarity), also called a BMS (Battery Management System).

[0030] The memory means 8 store an aging model 12 of the electrical storage battery 6. The aging model 12 incorporates a function representing a capacity loss of the electrical storage battery 6 as a function of the energy state of the battery 6. The controller 10 is configured to control the electrical state of charge of each electrochemical cell or each module of electrochemical cells of the battery 6. More specifically, this control takes the form of sending an instruction to the power electronics of the battery 6, this instruction allowing to individually manage (with different constraints depending on the technology used) the rate of discharge, charge or energy transfer of each cell, which makes it possible to achieve specific states of charge for each cell.The electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of battery 6 comply with at least one usage constraint CL. Such a usage constraint Cl consists, for example, of a constraint taking into account the dynamics and balancing speed of the active balanced battery 6, or of a constraint taking into account the limit voltages of the accumulators, or of a constraint taking into account the type of active balanced battery 6 used, or of a constraint imposing a predefined amount of energy on the battery 6. This constraint may also take into account that the battery 6 must not be unbalanced too much in order to guarantee that it has time to rebalance the states of charge of the accumulators in case the battery 6 approaches the fully charged or discharged state in order to maximize the useful storable energy.

[0031] The estimation module 11 is connected between an output of the electrical storage battery 6 and an input of the controller 10, and is configured to receive at least one measurement of a physical parameter relating to the battery 6 and to estimate, based on the measurement(s), at least one parameter relating to the state of the battery 6 (typically the state of charge, the energy state, and / or the health state of the battery 6). Preferably, the estimation module 11 is further configured to update the function representing the capacity loss of the battery 6 as a function of its energy state, based on the parameter(s) relating to the state of the battery 6 estimated by the module 11.

[0032] With reference to [Fig.2], an embodiment of the method for optimizing the lifespan of an electrical storage battery 6 according to the invention, implemented by a computer 4 as described previously, will now be described.

[0033] It is assumed that initially the memory means 8 of the computer 4 store a Model 12 of the aging of the electrical storage battery 6. The process is implemented over a period divided into constant sampling instants, and includes steps that are iteratively looped at each new sampling instant. Each sampling instant corresponds to an energy state of the battery 6, the current sampling instant corresponding to the current energy state of the battery 6. When the estimation module 11 is configured to update the function representing the capacity loss of the battery 6 as a function of its energy state, each sampling instant also corresponds to an instant at which the function representing the capacity loss of the electrical storage battery 6 as a function of the energy state of the battery 6 (this function being incorporated in the aging model 12) is updated.

[0034] The method includes an initial step 20 during which the computer 4 determines whether the function representing the capacity loss of the electrical storage battery 6 as a function of the energy state of the battery 6 has or does not have a strict global convexity or a strict global concavity.

[0035] If this function has a strict global convexity, the method includes a subsequent step 22 during which the computer 4 implements, at each sampling instant, a balancing control of the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of the battery 6.

[0036] Otherwise, if the function has a strict global concavity, the method includes a subsequent step 24 during which the computer 4 implements, at each sampling instant, a control for unbalancing the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of the battery 6.

[0037] Otherwise, this means that the function representing the capacity loss of the electrical storage battery 6 as a function of the battery's energy state 6 exhibits neither strict global convexity nor strict global concavity. In this case, during a subsequent step 26, the computer 4 implements the following substeps at each sampling instant: • a substep 28 of determining whether the function representing the capacity loss of the electrical storage battery 6 as a function of the energy state of the battery 6 exhibits a strict local convexity or concavity around the current energy state of the battery 6 (this current energy state of the battery 6 corresponding to the current sampling instant); • if this function has a strict local concavity around the current energy state of battery 6, a subsequent substep 30 of implementation of a control of unbalancing the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of battery 6; • otherwise, if the function exhibits strict local convexity around the current energy state of battery 6, a subsequent substep 32 of determining whether a balancing control of the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of battery 6 corresponds to a global minimum of this function; then • if a control for balancing the states of electrical charge of the electrochemical accumulators or electrochemical accumulator modules of battery 6 corresponds to an overall minimum of the function, a subsequent substep 34 of implementing a control for balancing the states of electrical charge of the electrochemical accumulators or electrochemical accumulator modules of battery 6; or if a control for balancing the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of battery 6 does not correspond to an overall minimum of the function, a subsequent substep 36 of implementation of a control for unbalancing the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of battery 6.

[0038] It should be noted that, in this latter case (i.e. if a balancing control of the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of the battery 6 does not correspond to an overall minimum of the function), the unbalancing control of the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of the battery 6 implemented during substep 36 is chosen from a restricted set of unbalancing controls, because not all unbalancings are optimum in this specific case.

[0039] Preferably, each balancing or unbalancing command of the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of the battery 6 is carried out via the implementation of a control method belonging to the group consisting of: a state feedback control; a hierarchical control; a Proportional, Integral, Derivative control; a predictive control; an open loop control; a reinforcement learning; an on / off control and an optimal control.

[0040] When the computer 4 includes an estimation module 11, as is the case in the illustrative example of [Fig.1], each command to balance or unbalance the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules of the battery 6 is carried out taking into account the parameter(s) relating to the state of the battery 6 which is estimated by the estimation module 11.

[0041] The process can be repeated during vehicle use, if necessary.

[0042] Compared to prior art optimization methods, the method according to the invention takes into account the time of inuse of the battery 6 and makes it possible to increase the life of the battery 6, and this for any type of chemistry relating to the electrochemical accumulators of the battery 6.

[0043] The method according to the invention therefore allows finer and more precise control of the state of charge of the accumulators, compared to the optimal control strategies of the prior art which aim at the use of a particular group of accumulators, which makes it possible to substantially increase the battery life 6.

Claims

1. Demands Method, implemented in a computer (4), for optimizing the lifespan of an electrical storage battery (6), the computer (4) being connected to the electrical storage battery (6), the computer (4) comprising memory means (8) storing a model (12) of aging of the electrical storage battery (6), said aging model (12) incorporating a function representing a loss of capacity of the electrical storage battery (6) as a function of the energy state of the battery (6), the electrical storage battery (6) being an active balanced battery with power transfer or a reconfigurable battery and comprising a plurality of electrochemical accumulators or electrochemical accumulator modules connected in series and / or in parallel,the computer (4) further comprising a controller (10) configured to control the electrical state of charge of each electrochemical accumulator or each electrochemical accumulator module of the battery (6), the electrical states of charge of the electrochemical accumulators or electrochemical accumulator modules respecting at least one usage constraint (Cl), said method being implemented for a duration divided into constant sampling instants, each sampling instant corresponding to an energy state of the battery (6), the method being characterized in that it comprises the steps of:, - initial determination (20) of whether the function representing the capacity loss of the electrical storage battery (6) as a function of the energy state of the battery (6) exhibits or does not exhibit a strict global convexity or a strict global concavity; - if said function has a strict global convexity, implemented (22), at each sampling instant, by a control for balancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules; - if said function exhibits strict global concavity, implemented (24), at each sampling instant, of a control for unbalancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules; if said function does not exhibit strict global convexity or strict global concavity, implementation (26), at each sampling instant, of the following steps: • determination (28) of whether the function representing the capacity loss of the electrical storage battery (6) as a function of the energy state of the battery (6) exhibits strict local convexity or concavity around the current energy state of the battery (6); • if said function has a strict local concavity around the current energy state of the battery (6), implementation (30) of an unbalancing control of the electrical charge states of electrochemical accumulators or electrochemical accumulator modules; • if said function exhibits strict local convexity around the current energy state of the battery (6), determination (32) of whether a balancing control of the electrical charge states of electrochemical accumulators or electrochemical accumulator modules corresponds to a global minimum of said function; • if a control for balancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules corresponds to an overall minimum of said function, implementation (34) of a control for balancing the electrical charge states of electrochemical accumulators or electrochemical accumulator modules; • if a control for balancing the electrical charge states of the batteries electrochemical or electrochemical accumulator modules does not correspond to an overall minimum of said function, implementation (36) of an unbalancing control of the electrical charge states of electrochemical accumulators or modules electrochemical accumulators.

2. A method according to claim 1, wherein the computer (4) further comprises an estimation module (11) connected between an output of the electrical storage battery (6) and an input of the controller (10), the estimation module (11) being configured to receive at least one measurement of a physical parameter relating to the battery (6) and to estimate, based on said at least one measurement, at least one parameter relating to the state of the battery (6), and wherein each command to balance or unbalance the electrical charge states of the electrochemical accumulators or electrochemical accumulator modules is carried out taking into account said at least one parameter relating to the state of the battery (6).

3. A method according to claim 2, wherein the estimation module (11) is further configured to update the function representing the capacity loss of the electrical storage battery (6) as a function of the energy state of the battery (6), as a function of said at least one parameter relating to the state of the battery (6), each sampling instant further corresponding to an instant for which said function representing the capacity loss of the electrical storage battery (6) as a function of the energy state of the battery (6) is updated.

4. A method according to any one of claims 1 to 3, wherein each balancing or unbalancing of the electrical charge states of electrochemical accumulators or electrochemical accumulator modules is achieved via the implementation of a control method belonging to the group consisting of: state feedback control; hierarchical control; proportional, integral, derivative control; predictive control; open-loop control; a 15. Reinforcement learning; an all-or-nothing command and optimal control.

5. Computer program product comprising program code instructions for carrying out the steps of a process according to any one of claims 1 to 4, when said program is running on a computer or calculator (4).

6. Motor vehicle (2) comprising a computer (4) storing a computer program product according to claim 5.

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