Method for estimating the state of health of an electric or hybrid vehicle battery

Abstract

The invention relates to a method for estimating the state of health of an electric or hybrid vehicle battery that can be carried out while the vehicle is running, comprising: (a) a full charge step; (b) a first relaxation step; (c) a step of discharging to a state of charge SOC≤SOCtarget carried out in k vehicle driving cycles, for each of which the capacity Qn discharged by the battery is determined; (d) a second relaxation step at the end of the cycle k; (e) a step of determining a state of charge SOCOCV as a function of the open circuit voltage; (f) a step of determining an initial discharge capacity Qi of the new battery from SOCOCV; and (g) a step of estimating the state of health SOH of the battery from the total capacity discharged by the battery during step (c) and from Qi.

Description

[0001] TITLE: METHOD FOR ESTIMATING THE STATE OF HEALTH OF A BATTERY IN AN ELECTRIC OR HYBRID VEHICLE

[0002] Field of invention

[0003] The present invention relates to a method for estimating the state of health of a battery. It relates in particular to batteries intended to power electric or hybrid motor vehicles.

[0004] Prior art

[0005] Determining the state of health, or aging, of a battery is an essential criterion for characterizing the condition of a battery, in order to optimize the energy discharge / charge, as well as the lifespan of the battery accumulators.

[0006] The state of health of a battery, or SOH (from the English "State of Health") can be defined by the ratio of the capacity of the battery to store energy at a given time t, compared to the nominal capacity of the battery to store energy (i.e. the capacity of the new battery, before any use). It is therefore an indicator of the state of health of a battery reflecting the level of degradation of the autonomy of the battery, in particular due to the age of the battery and the conditions of use of the latter.

[0007] Knowing the state of health of a battery, and especially vehicle traction batteries, is crucial for technical, economic and legal reasons. The state of health of a battery must be known precisely in order to avoid the risk of untimely failure, or unexpected degradation of the performance of the system powered by said battery. In particular, the electronic system for managing and controlling the various parameters of a battery, called BMS (Battery Management System), takes into account, for example, an estimated SOH to calculate the maximum available power, or to estimate the state of charge or SOC (State Of Charge) of the battery. Furthermore, the economic value of the battery is linked to its SOH which makes it possible to estimate the remaining lifespan of the battery.The economic value of an electric or hybrid vehicle is therefore also linked to the SOH of its traction battery. Finally, the regulations for electric and hybrid vehicles are evolving and it is planned to require that the state of health of a traction battery estimated by the BMS be determined with an accuracy of around 5%. Currently, the state of charge of a traction battery is estimated by the BMS from physical or empirical models. Some models use electrochemical properties and / or performances that can be measured during battery use. This is referred to as "online" estimation of the SOH. For example, document CN113296010 describes a method for online assessment of the state of health of a battery based on differential voltage analysis, which includes the establishment of a curve representing the voltage variation as a function of the capacity variation under given discharge conditions.The method described then establishes a model in which the slope of a tangent to this curve is correlated with the SOH. This model is used to estimate the SOH of a discharged battery under the same conditions as those used to establish the curve.

[0008] The accuracy of SOH estimation can, however, vary depending on the type of model and the quality of its calibrations. For example, empirical battery degradation models can provide good accuracy in estimating SOH up to a given SOH, such as 70%. Beyond that, the estimation error is likely to increase because degradation is more complex to model.

[0009] An accurate determination of a battery's SOH can also be achieved by directly measuring the battery's available capacity. However, this type of measurement requires removing the battery from the vehicle, which can be complex and expensive.

[0010] There is therefore still a need to reliably and accurately estimate the SOH of a battery, particularly online, and especially when the vehicle is moving, without disrupting the operation of the vehicle.

[0011] Summary of the invention

[0012] To this end, the present invention proposes a method for estimating the state of health of a battery of an electric or hybrid vehicle, comprising:

[0013] (a) a battery charging step until the battery reaches a maximum state of charge SOCmax,

[0014] (b) a first relaxation stage during which the battery is not used for a first duration,

[0015] (c) a step of discharging the battery until the battery reaches a state of charge less than or equal to a non-zero target state of charge SOCcibie, this discharging step being carried out in a number k of driving cycles of the vehicle, k being a non-zero integer, and, for each driving cycle n where 1 <n<k, on détermine la capacité Q n discharged by the battery, (d) at the end of the driving cycle k, a second relaxation stage during which the battery is not used for a second duration,

[0016] (e) a step of determining a state of charge SOCocv of the battery as a function of the open circuit voltage, the battery not being used,

[0017] (f) a step of determining an initial discharge capacity Q' of the battery in a new state from the state of charge determined in step (e) and from a correlation previously established for the battery in a new state and correlating the initial discharge capacity to the state of charge of the battery, and

[0018] (g) a step of estimating the state of health SOH of the battery from the total capacity discharged by the battery during the discharging step (c) and the initial discharge capacity determined in step (f).

[0019] The method according to the invention allows an accurate estimation of the state of health of the vehicle without having to remove the battery from the vehicle and without the driver having to modify the type of driving of his vehicle.

[0020] The method according to the invention may further comprise one or more of the following features: during the estimation step (g), the state of health SOH of the battery may be estimated using the following equation:

[0021] [Math 1 ] 100 (Eq 1 ) where Q nrepresents the capacity discharged by the battery during a driving cycle n, n being a non-zero integer ranging from 1 to k, and Q' represents the initial discharge capacity, between the first relaxation step (b) and the discharge step (c), a verification step may be provided, during which it is verified that the state of charge of the battery in the relaxed state has reached the maximum state of charge, during the discharge step (c), for each driving cycle, the capacity discharged by the battery may take into account a charge capacity received by the battery, during the discharge step (c), the capacity discharged by the battery may be determined (i) by coulometric counting of the ampere-hours leaving the battery, or (ii) by coulometric counting of the ampere-hours leaving and entering the battery and then by subtracting the incoming ampere-hours from the outgoing ampere-hours, for each driving cycle n of step (c) of dump,a value of the state of charge SOCn of the battery can be estimated (i) by measuring the battery voltage during driving or (ii) by coulometric counting, then the value of the state of charge SOCn is compared to the target state of charge value SOCcibie and it is determined that step (c) is completed when SOCn is less than or equal to SOCcibie, the correlation used in step (f) having been established for a defined temperature interval, during each driving cycle of the discharge step (c), (i) the battery can be maintained at a temperature included in this defined temperature interval, or (ii) it can be applied to the capacity Q, n discharged by the battery a corrective factor depending on the battery temperature.

[0022] The invention also relates to a computer program comprising the instructions for executing the steps of the estimation method according to the invention, when said instructions are executed by one or more processors.

[0023] The invention also relates to a computer-readable medium on which the computer program of the invention is stored.

[0024] "Computer-readable medium" means any memory, storage device, storage mechanism, and other storage and signaling mechanism, including interfaces and devices such as network interface cards and buffers therein, as well as any communication device and any received and transmitted signals, and any other current and evolving technology that a computer system can interpret, receive, and / or transmit. This concept includes not only a computer-readable medium such as a hard disk connected to a central processing unit and with which the recorded program is directly executed, but also a computer-readable medium such as a CD-ROM that records a program to be executed after installing it on a hard disk. A program here includes not only a program that can be executed directly, but also a program in source format, a compressed program, and an encrypted program.

[0025] The estimation method according to the invention can in particular be implemented by means of the management system of the invention described below.

[0026] The invention thus also relates to a battery management system configured to estimate the state of charge of a battery by implementing the steps of the estimation method according to the invention.

[0027] The invention finally relates to a motor vehicle equipped with a traction battery comprising a battery management system according to the invention. Description of the figures

[0028] Figure 1 represents a flowchart of the estimation method according to one embodiment of the invention.

[0029] Detailed description of the invention

[0030] The present invention is based on estimating the state of health SOH of a battery from a capacity.

[0031] The first stage (a) is a full battery charge stage, i.e. until the battery reaches a maximum state of charge noted SOCmax.

[0032] Thus, during this charging step (a), the state of charge of the battery can be monitored. It is thus possible to (i) charge the battery, (ii) monitor the state of charge of the battery during this charging and (iii) continue charging until this state of charge has reached a maximum state of charge. During this monitoring, the value of the state of charge can be determined, for example from a measurement of the battery voltage, and compared to the value of a maximum state of charge.

[0033] For example, we can proceed as follows:

[0034] (i) the battery is charged,

[0035] (ii) the value of the voltage at the terminals of the battery is measured,

[0036] (iii) the measured voltage value is compared to a threshold voltage value corresponding to a maximum state of charge of the battery,

[0037] (iv) if the measured voltage value reaches said threshold voltage value, the battery is stopped charging, it is determined that the battery is in a maximum state of charge and the charging is stopped, otherwise, the battery is continued charging. The frequency of reiteration may be determined by a person skilled in the art depending on the type of battery. The value of the voltage measured at the terminals of the battery corresponds here to the open circuit voltage, with the vehicle stationary. The threshold voltage value and the corresponding maximum state of charge may be determined by a person skilled in the art depending on the type of battery from curves of the state of charge as a function of the open circuit voltage. These threshold voltage and maximum state of charge values ​​may be recorded in a management system, for example in the form of maps or curves.

[0038] This charging step (a) is followed by a first relaxation step (b) during which the battery is not used for a first duration t_relax_1. This pause time makes it possible in particular to ensure temperature stabilization of the battery at the end of charging and thus to improve the accuracy of the estimation of its state of charge. This first duration of the relaxation step can be previously determined by a person skilled in the art. It is for example from 15 minutes to 6 hours, typically from 30 minutes to 1 hour.

[0039] In order to improve the accuracy of the determination of the state of charge, the method according to the invention may comprise, after the first relaxation step (b) and before the discharge step (c), a verification step during which it is verified that the state of charge of the battery in the relaxed state has reached the maximum state of charge SOCmax. If this is the case, the method proceeds to step (c), otherwise, the method may be interrupted, or steps (a) and (b) repeated until the verification step confirms that the battery in the relaxed state has reached the maximum state of charge.

[0040] During this verification step, we can, for example:

[0041] (i) determining the value of a state of charge of the battery in the relaxed state SOCreiax, for example by measuring the open voltage at the terminals of the battery,

[0042] (ii) compare the value of the battery state of charge in the relaxed state SOCreiax with the value of the maximum state of charge SOCmax,

[0043] (iii) determine that the charge is complete and proceed to step (c) if the value of SOCreiax is equal to the value of SOCmax, otherwise, repeat steps (a) and (b).

[0044] We can still proceed as follows:

[0045] (i) measure the value of the voltage across the battery terminals in the relaxed state,

[0046] (ii) compared the measured voltage value to a threshold voltage value corresponding to a maximum state of charge of the battery,

[0047] (iii) determine that the charge is complete and proceed to step (c) if the measured voltage value is equal to the threshold voltage value, otherwise, repeat steps (a) and (b).

[0048] Then, during the discharge step (c), the driver uses the vehicle in the usual manner. This discharge step lasts until the battery has reached a state of charge equal to or lower than a target state of charge SOCcibie, which can occur in one or more driving cycles. It should be noted that when this discharge step (c) is carried out in several driving cycles, the battery is not recharged. In other words, this step is implemented without charging the battery between two consecutive driving cycles.

[0049] This target state of charge SOCcibie is a non-zero state of charge, typically higher than a minimum state of charge necessary for vehicle operation during a driving cycle. This target state of charge SOCcibie can, for example, correspond to a state of charge of 10 to 15% of the maximum state of charge SOCmax.

[0050] Step (c) is implemented until the battery reaches a state of charge less than or equal to this target state of charge SOCcibie. For this purpose, the value of the state of charge can, for example, be monitored.

[0051] For example, we can proceed as follows. For each driving cycle of the discharge step: we estimate the value of the state of charge SOCn (i) by measuring the voltage of the battery during driving or (ii) by coulometric counting, then we compare the value of the state of charge SOCn to the target state of charge value SOCcibie, and we determine that step c) is finished when SOCn is less than or equal to SOCcibie.

[0052] In the case of estimating the value of the state of charge SOCn by measuring the battery voltage, the measured voltage does not correspond to an open-circuit voltage at the battery terminals. It is then possible to determine the open-circuit voltage from the measured voltage, for example by Kalman filtering type modeling, and then to estimate the state of charge SOCn from the open-circuit voltage thus determined.

[0053] During this step (c), for each rolling cycle n (n non-zero integer), the capacity Q is determined n discharged by the battery, which can notably take into account a charge capacity received by the battery, for example during braking.

[0054] This ability Q ndischarged is for example determined (i) by coulometric counting of the ampere-hours leaving the battery, or (ii) by coulometric counting of the ampere-hours leaving and entering the battery and by subtracting the ampere-hours entering from the ampere-hours leaving.

[0055] This ability Q n discharged can thus be written:

[0056] [Math 2]

[0057] Ahdéch-rouiage n is the capacity in ampere-hours discharged during the driving cycle n and is written:

[0058] [Math 3]

[0059] ,

[0060] ^"tear-rolling where I is the current flowing out of the battery in amperes, and t is time,

[0061] Ahch-rouiage n is the capacity in ampere-hours charged during the driving cycle n and is written:

[0062] [Math 4] where I is the current entering the battery in amperes, and t is the time.

[0063] In particular, it will be possible to record the value of the estimated state of charge SOC for each driving cycle. n and the value of the discharged capacity Q n .

[0064] At the end of step (c), namely at the end of the last driving cycle noted k, a second relaxation step (d) is implemented during which the battery is not used for a second duration t_relax_2. This pause time ensures that the open-circuit voltage of the battery returns to an equilibrium state. The longer this pause time, the closer the measurement of the open-circuit voltage is to reality. The duration of this second relaxation step can be determined beforehand by a person skilled in the art. It is, for example, from 15 minutes to 6 hours, typically from 30 minutes to 1 hour.

[0065] Then, during determination step (e), the state of charge SOCocv of the battery is determined as a function of the open circuit voltage, the battery not being used.

[0066] At the end of a discharge step, the SOC estimation can be associated with a fairly large error due to the increase in the internal resistance of the battery resulting from its aging. An accurate SOC estimation could be obtained when the battery is completely discharged (SOC of 0%). This solution would involve performing the discharge step until the vehicle cannot be driven, which is not practical for the driver. The second relaxation step allows the SOC to be estimated accurately without having to reach a minimum state of charge in which the vehicle can no longer be driven. Indeed, the SOC estimation (denoted SOCocv) from the measurement of the open circuit voltage (also denoted OCV) is not affected by the increase in the internal resistance of the battery due to its aging.

[0067] We can then determine (f) the initial discharge capacity Q' of the battery in a new state from the state of charge SOCocv determined in step (e) and from a correlation previously established for the battery in a new state and correlating the initial discharge capacity to the state of charge of the battery.

[0068] This initial discharge capacity Q' thus corresponds to the capacity which would have been discharged by the battery in its new state during the discharge step (c) between the maximum state of charge and the state of charge SOCocv determined in step (e).

[0069] We can then estimate (g) the state of health SOH of the battery from the total capacity discharged by the battery during the discharge step (c) and the initial discharge capacity determined in step (f).

[0070] In particular, the battery state of health SOH can be estimated using the equation Eq 1 cited above.

[0071] Capacity measurements depend on the battery temperature. Also, to estimate the SOH with greater accuracy, it is preferable that the capacities Qn and Q' are determined when the battery is in the same temperature range. Thus, preferably, when implementing the method according to the invention, when the correlation used in step (f) has been established for a defined temperature interval, then, during each driving cycle n of the discharge step (c), (i) the battery is maintained at a temperature within this defined temperature interval, or (ii) the capacity Q is applied n discharged by the battery a corrective factor depending on the battery temperature.

[0072] In case (i), the battery temperature can be maintained within this temperature range by appropriate control of a battery cooling / heating system. In case (ii), the battery temperature can be measured using a probe and a model expressing the corrective factor as a function of temperature can be used, this model being pre-established empirically or otherwise. The estimation method according to the invention is implemented by means of a management system. This management system typically comprises one or more processors (for example a microprocessor, a microcontroller or other), in particular programmed to implement the method according to the invention. It may also comprise means of communication, optionally bidirectional, with the batteries, and means for measuring the voltage and also the temperature of the battery.The processor(s) may include storage means which may be random access memory (RAM), electrically erasable programmable read-only memory (EEPROM), flash memory, external memory or other. These storage means may, among other things, store received data, a control model, one or more maps and one or more computer programs. The management system is for example part of, or forms, the vehicle's battery management system, also called BMS.

[0073] The estimation method according to the invention can be triggered at any time either by the driver of the battery or by a technician as part of a maintenance operation. This triggering can occur at any time during the life of the vehicle, for example in the event of resale of the vehicle, a range problem or simply for information.

[0074] It will be noted that if the relaxation steps are not respected, for example if they are interrupted before the end of the prescribed duration, and / or if the temperature of the battery during the discharge step is not within the defined temperature range, it may be provided to interrupt the estimation method, or to continue it in degraded mode. The state of health thus estimated in fact has a lower precision than if it had been determined under the conditions of prescribed duration of the relaxation steps and / or temperature of the battery during the discharge step.

[0075] When the evaluation method is in progress, we can provide:

[0076] (i) at each end of a rolling cycle n, the recording of the capacity Q n discharged during the driving cycle n and the state of charge SOCn for the driving cycle n,

[0077] (ii) at the next power-up of the management system, in other words at the next start of the vehicle, the comparison of the state of charge SOCn determined during the last driving cycle n with the target state of charge SOCcibie, and: a. if this state of charge SOCn is less than or equal to the target state of charge SOCcibie, and if the battery has not been used for the duration prescribed by the second relaxation step, then steps (e) to (g) are implemented, b. if this state of charge is greater than the target state of charge, the number of driving cycles is incremented by 1 (equal to n+1) and step (e) is continued, namely the discharged capacity for this driving cycle SOC is determined n +i then we return to step (i) for this cycle (n+1).

[0078] Furthermore, the management system can be configured (programmed) to perform tests each time it is powered up in order to check the status of the estimation method according to the invention. For example, it could perform the following tests:

[0079] Test T1: Is the battery fully charged (maximum state of charge reached)?

[0080] (i) If the answer is yes, then provision can be made for a reset of the recorded variables such as:

[0081] - zeroing of the discharge capacity Q n , the SOCn value, and the index n for counting the number of rolling cycles in discharge,

[0082] - zeroing of time quantities used to record the absolute start times of each of the rolling phases and initial SOC quantities at the start of each rolling, these quantities make it possible to check the influence of the number of rollings to go from the maximum state of charge (100%) to a state of charge lower than the SOCcibie.

[0083] The system is then ready to start the estimation method according to the invention.

[0084] (ii) If the answer is no, we move on to test T2.

[0085] Test T2: if the vehicle starts to roll and SOC > SOCcibie and the determination method according to the invention is in progress, then we increment the index n by 1, we determine Q n and the state of charge SOCn. If not, we move on to test T3.

[0086] Test T3: if the battery is in a relaxed state and SOC <SOCcibie (ce qui correspond à la fin de l’étape (d) de la méthode selon l’invention), alors on détermine l’état de charge de la batterie SOCocv (étape (e) de la méthode), la capacité de décharge initiale Q' (étape (f) de la méthode) et l’état de santé SOH (étape (g)). Si ce n’est pas le cas, on passe au test T4.

[0087] Test T4: Is the battery charging? If so, the estimation method according to the invention is interrupted.

Claims

DEMANDS 1. Method for estimating the health status of a battery in an electric or hybrid vehicle, comprising: (a) a battery charging step until the battery reaches a maximum state of charge SOCmax, (b) an initial relaxation phase during which the battery is not used for a period of time, (c) a battery discharge step until the battery reaches a state of charge less than or equal to a non-zero target state of charge SOCcibie, this discharge step being carried out in a number k of vehicle driving cycles, k being a non-zero integer, and, for each driving cycle n where 1 <n<k, on détermine la capacité Q n discharged by the battery, (d) at the end of the driving cycle k, a second relaxation stage during which the battery is not used for a second period, (e) a step of determining a state of charge SOCocv of the battery as a function of the open-circuit voltage, with the battery not under load, (f) a step of determining an initial discharge capacity Q' of the battery in a new state from the state of charge determined in step (e) and a previously established correlation for the battery in a new state and correlating the initial discharge capacity with the state of charge of the battery, and (g) a step of estimating the SOH of the battery from the total capacity discharged by the battery during the discharge step (c) and the initial discharge capacity determined in step (f).

2. Estimation method according to claim 1, wherein, during the estimation step (g), the SOH state of health of the battery is estimated by means of the following equation: [Math 1] 100 where Q nrepresents the capacity discharged by the battery during a cycle of rolling n, n a non-zero integer from 1 to k, and Q' represents the initial discharge capacity.

3. Estimation method according to claim 1 or 2, comprising, between the first relaxation step (b) and the discharge step (c), a verification step in which it is verified that the state of charge of the battery in the relaxed state has reached the maximum state of charge.

4. Estimation method according to any one of claims 1 to 3, wherein during the discharge step (c), for each driving cycle, the capacity discharged by the battery takes into account a charge capacity received by the battery.

5. Estimation method according to any one of claims 1 to 4, wherein during the discharge step (c), the capacity discharged by the battery is determined (i) by coulometric counting of the ampere-hours leaving the battery, or (ii) by coulometric counting of the ampere-hours leaving and entering the battery and by subtracting the ampere-hours entering from the ampere-hours leaving.

6. Estimation method according to any one of claims 1 to 5, wherein, for each driving cycle n of the discharge stage (c), a value of the state of charge SOCn of the battery is estimated (i) by a measurement of the battery voltage during driving or (ii) by coulometric counting, then the value of the state of charge SOCn is compared to the target state of charge value SOCcibie and it is determined that the stage (c) is complete when SOCn is less than or equal to SOCcibie.

7. Estimation method according to any one of claims 1 to 6, wherein the correlation used in step (f) has been established for a defined temperature range, and, during each rolling cycle of the discharge step (c), (i) the battery is maintained at a temperature within this defined temperature range, or (ii) the capacity Q is applied n discharged by the battery a correction factor depending on the battery temperature.

8. Computer program comprising instructions to execute the steps of the estimation method according to any one of claims 1 to 7, when said instructions are executed by one or more processors.

9. Battery management system configured to estimate the state of charge of a battery by implementing the steps of the estimation method according to any one of claims 1 to 7.

10. Motor vehicle equipped with a traction battery comprising a battery management system according to claim 9.

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