METHOD AND SYSTEM FOR ESTIMATING THE AVAILABLE CHARGE STATE OF A BATTERY AS A PERIOD OF THE INSTANT CALL VOLTAGE

Description

Domaine technique

[0001] The invention relates technically to electric batteries, and more particularly to the estimation of the state of charge of such batteries. Techniques antérieures

[0002] The state of charge (SOC) of a battery indicates the amount of charge present in the battery. It can be expressed as an absolute value (Ah) or relative to the maximum charge the battery can hold (%). The SOC is not a directly measurable quantity, so it must be estimated from available measurements (current, voltage, temperature).

[0003] The most common method for calculating the state of charge (SOC) is coulomb counting, which involves integrating the current over time. This is applicable when the initial state of charge (SOC) is known. The state of charge (SOC) at time t is therefore expressed (in Ah) by: SOC stored t = SOC stored , 0 + 1 3600 ∫ 0 t I t dt

[0004] If we want to express it as a percentage SOC stored_relative, we simply divide it by the maximum state of charge SOC max of the battery. SOC stored _ relative t = 100 SOC stored t SOC max

[0005] The state of charge (SOC) stored, estimated by coulomb counting, provides information about the battery's stored capacity. However, relying solely on this information is dangerous because some of this capacity may be unavailable, especially at low temperatures and / or high currents. These variations in available capacity result from variations in the battery's internal impedance. When a discharge current is passed through the battery, the voltage across its terminals decreases proportionally to the battery's impedance and the current flowing through its terminals.

[0006] There Figure 1 This graph shows the evolution of the battery voltage over time during a 1C discharge at two different temperatures, 10°C and 25°C. The thick curves are associated with 25°C, while the thin curves are associated with 10°C. The battery impedance increases as the temperature decreases, therefore the resulting voltage drop is greater. Similarly, if the impedance is the same, the voltage drop will be greater for a higher current. It is important to note that the battery discharge stops when the lowest permissible voltage is reached. However, for the same stored state of charge (SOC), this voltage is reached more quickly if the current is high or the temperature is low. Under these circumstances, the available state of charge (SOC) is lower.

[0007] In this context, it becomes crucial to differentiate between the stored state of charge (SOC) and the available state of charge (SOC). The available state of charge (SOC) is theoretically a function of the stored state of charge (SOC), the temperature (T), and the current (I): SOC available = f SOC stored T I

[0008] One technical problem to solve is determining the function between the state of charge available (SOC available), current, and temperature, or finding another way to take into account the effects of current and temperature on the state of charge available (SOC available).

[0009] Regarding temperature, it is possible to perform a series of charge and discharge tests on the battery at a sufficiently low current (so as not to heat the battery during the test) to obtain the limit values ​​of the charge stored in the battery. Figure 2 This illustrates the available battery capacity as a function of temperature and with respect to charging and discharging operations. Zone 1 corresponds to the unavailable capacity during charging as a function of temperature. Zone 3 corresponds to the unavailable capacity during discharging as a function of temperature. Zone 2 corresponds to the available capacity during both charging and discharging as a function of temperature. For example, at the end of a discharge at a temperature of 0°C, approximately 0.24 Ah remains stored in the battery and is not available for further discharge. Therefore, the available state of charge (SOC) at 0°C is less than the stored state of charge (SOC) by 0.24 Ah.

[0010] However, this approach cannot be adopted to study the dependence of available capacity on current, because high currents heat up the battery and the measured capacity value therefore cannot be associated with a specific operating point.

[0011] One approach that allows modeling the variation of available capacity with current is the kinetic model, illustrated in the Figure 3 The right-hand reservoir contains y1, which corresponds to the available charge / energy. Its width is c, less than 1. The level of this reservoir is hl, which represents the state of charge SOC / state of energy SOE. The left-hand reservoir contains y2, which is the constrained charge / energy. Its width is 1-c and its level is h2. When a load is applied (a current I if we are talking about a charge reservoir or a power P if we are talking about an energy reservoir), the level of the right-hand reservoir decreases. At the same time, the charge / energy can pass from the left-hand reservoir to the right-hand one via a valve k', which means that the flow rate at the inlet of the right-hand reservoir is always lower than that at the outlet. The greater the applied load, the faster the difference in level between the two reservoirs will increase. The unavailable charge / energy is therefore (1-c)*(h1-h2).However, as soon as the demand ceases, the level in the right-hand reservoir begins to rise; this is called charge / energy recovery. This behavior is analogous to the diffusion that occurs within a battery. Indeed, when a high demand is applied, the overvoltage (the difference between the cell voltage and its open-circuit voltage, OCV) can increase significantly, and if the battery voltage is already close to its lower limit, the discharge may be interrupted. However, this discharge could very well resume at a lower current / power, and one can even imagine that an infinitesimal current / power could completely discharge the battery.

[0012] The equations that govern this model are as follows: dy 1 dt = − I − k ′ h 1 − h 2 dy 2 dt = k ′ h 1 − h 2

[0013] To configure this model, it is necessary to generate profiles at different currents and temperatures from the battery's electrical model and use an optimization algorithm to find, at each temperature, a pair (k', c) that results in y1 = 0 at the end of the discharge at each current. These two parameters then allow us to obtain Ymax, which is the maximum capacity / energy that can be stored in the battery at the given temperature.

[0014] Unfortunately, this model has a high level of complexity, and its parameterization depends on the quality of the profiles obtained from the electrical model. Therefore, we have errors in both the electrical model and the parameterization of the kinetic model that accumulate.

[0015] In summary, the available charge in a battery differs from the stored charge and depends on temperature and electrical current. It is possible to estimate the available charge in a battery at different temperatures and at a low, constant current.

[0016] However, this cannot be done at high currents because they significantly increase the battery temperature, preventing measurement of the available charge at a given temperature for different currents. Battery voltage is directly influenced by both temperature and current. Furthermore, voltage is the primary criterion for stopping a discharge. This information can therefore be used to determine the available state of charge (SOC) from the stored SOC.

[0017] There is a need for a method of determining the available state of charge of a battery that takes into account the effect of temperature and current, even for high currents.

[0018] From the prior art, we know of document US2022 / 065934 disclosing a battery system comprising a battery and a device for estimating the battery state based on a model.

[0019] The state of the art does not meet the need identified above because the available models are either incomplete or difficult to parameterize. Exposé de l'invention

[0020] The invention relates to a method for estimating the state of charge of a battery according to independent claim 1 comprising the following steps: determination of the stored state of charge, determination of the instantaneous open-circuit voltage of the battery based on a predetermined map, the stored state of charge and the temperature, measurement of the instantaneous voltage at the battery terminals, determination of the available state of charge based on the stored state of charge, the instantaneous voltage, the instantaneous open-circuit voltage and a minimum permissible voltage, the minimum permissible voltage being a predetermined design value, the process being characterized in that it comprises the following step: To determine the available state of charge, we perform the product of the stored state of charge by the ratio between the difference between the instantaneous voltage and the minimum allowable voltage and the difference between the open circuit voltage and the minimum allowable voltage.

[0021] The determination of the stored state of charge can be carried out by performing the following steps: measurement of the open-circuit voltage of the battery at rest, determination of the initial stored state of charge according to a table linking the initial stored state of charge to the open-circuit voltage at rest of the battery and to the temperature, use of the battery in charge and / or discharge, periodic measurement of the instantaneous current, integration of the instantaneous current measured since the start of use after each measurement of the instantaneous current, and determination of the stored state of charge as the sum of the result of the integration and the value of the initial stored state of charge.

[0022] The determination of the stored state of charge can also be performed using a Kalman observer with a state vector defined by the stored state of charge and a diffusion voltage, the output vector being the instantaneous battery voltage

[0023] The future available state of charge of a battery can be determined to be equal to the available state of charge determined when the instantaneous voltage of the battery is a future voltage.

[0024] The invention also relates to a system for estimating the state of charge of a battery according to independent claim 5 comprising data processing means, at least one memory, at least one means for measuring the voltage across the terminals of the battery, and at least one means for measuring the current flowing between the terminals of said battery, the data processing means being configured to carry out the determination process steps as described above. Brève description des dessins

[0025] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example and made with reference to the accompanying drawings in which: there figure 1 illustrates an example of discharging a 1C battery at two different temperatures, the figure 2 illustrates the stored capacity at the end of charging and discharging as a function of temperature, the figure 3 illustrates the kinetic discharge model of a battery, the figure 4 illustrates the comparison of the evolution of the state of charge as a function of time for a continuous discharge at 3C and 0°C according to several determination methods. Description détaillée

[0026] As we saw in the introduction, battery voltage is directly influenced by current and temperature. The higher the current, the greater the voltage variation. The contribution of temperature is inverse, so the lower the temperature, the higher the electrical resistance and the greater the voltage variation. Therefore, we can use voltage to calculate the state of charge available (SOC). In this case, as soon as the voltage reaches a limiting value that stops the discharge, the state of charge available (SOC) must be zero. This is satisfied by equation [Eq. 5] below.

[0027] The charge state estimation method according to the invention is based on this observation.

[0028] The initial stored state of charge SOC stored, 0 is determined based on the open-circuit voltage (OCV) and temperature when the battery is at rest and relaxed. A battery at rest and relaxed is defined as one whose use has been interrupted for a period long enough for the battery voltage to reach the OCV and the battery temperature to reach ambient temperature.

[0029] During discharge, the current is measured periodically and then integrated after each current measurement since the start of the discharge in order to determine the stored state of charge. SOC stored , by coulombmetric counting by application of equation [Eq. 1].

[0030] Alternatively, the stored state of charge SOC stored is determined by a per-observer Kalman estimation.

[0031] The Kalman observer estimates the value of the state variables at each instant based on their value at the previous instant and the system inputs. Then, the system output is calculated based on these state variables and compared with a measured value. Depending on the observed difference and the filter settings, a correction factor of varying magnitude is applied to the state variables. The goal is therefore to adjust these non-measurable variables so that the output, which depends on them, is consistent with a measurable variable. Le vecteur d ′ état x est x = SOC stored U diff

[0032] With SOC: the battery's state of charge, and Udiff: internal voltage that reflects dynamic diffusion phenomena, which is an integral part of the battery voltage such as: Udiff + OCV + R S ∗ I = U t

[0033] With Rs*I internal voltage related to static phenomena and Rs the associated resistance.

[0034] The control vector u is u = {I} With I: the charging or discharging current, the equation of state is then as follows: x k + 1 = A . x k + B . u k With A and B: two matrices of coefficients, the output vector is y = { U ( t With U(t): the overall battery voltage at time t, the system output is then as follows: y k + 1 = C . x k + D . u k With C and D: two matrices of coefficients

[0035] The Kalman filter is initialized using the covariance matrix method, with a covariance matrix R for process noise, a covariance matrix Q for observation noise, and a covariance matrix P for state error. Matrices R and Q can remain constant throughout the observation, while matrix P is recalculated at each time step.

[0036] Based on the determination of the stored state of charge SOC stored and regardless of the method used to estimate the stored state of charge SOC stored used, we calculate the available charge state SOC available by applying the following equation: SOC available t = SOC stored t ⋅ U t − U min OCV t − U min

[0037] With : SOC stored : the stored state of charge (SOC). U(t): the overall battery voltage at time t and Umin: the minimum permissible voltage, which defines the end of discharge. OCV(t): the open-circuit voltage at time t

[0038] Note that the open-circuit voltage at time t, OCV(t), varies with each instant. Recall that the open-circuit voltage OCV is defined as the battery voltage when it is not connected to any load or power source. It follows that the open-circuit voltage OCV cannot be determined when the battery is in use. Therefore, we use a mapping of the open-circuit voltage OCV(t) as a function of the stored state of charge. SOC stored ( t) and the battery temperature T. The mapping is determined by testing, measuring the OCV voltage for different stored charge states. SOC stored and for different temperatures T. An example of such a mapping is illustrated with dashed lines in the figure Fig. 1 .

[0039] The minimum voltage U min is a manufacturer's specification related to the battery.

[0040] With equation [Eq. 5], it is possible to take into account both the effects of current and temperature. In this case, we assume that the available charge state SOC available is equal to the stored state of charge SOC stored When the battery is at rest (U=OCV), regardless of the temperature. As soon as the battery is under load, the resulting overvoltage U(t)-OCV(t) (and therefore the reduction in available SOC) is greater when the resistance is high and thus the temperature is low. The overvoltage is also greater when the current is high.

[0041] This estimation method makes the state of available charge SOC available decreases and increases following the same pattern as the voltage. This method therefore allows for very effective prediction of discharge interruptions, because the available state of charge SOC available is always zero when the voltage reaches its limit value, even if the stored state of charge SOC stored was incorrectly initialized.

[0042] The method for estimating the state of charge according to the invention thus comprises the following steps: In a first step, the stored state of charge SOC (t) is determined and the instantaneous open-circuit voltage OCV(t) of the battery is determined as a function of the stored state of charge. SOC stored ( t ) and the battery temperature T.

[0043] In a second step, the instantaneous voltage U(t) across the battery terminals is measured.

[0044] In a third step, the available state of charge SOC available (t) is determined as a function of the stored state of charge SOC stored (t), the instantaneous voltage U(t), the instantaneous open-circuit voltage OCV(t) and the minimum allowable voltage U min by applying equation [Eq. 5].

[0045] The minimum permissible voltage U min is a predetermined manufacturer's specification.

[0046] There Figure 4 shows results of the state of charge estimation method according to the invention for a continuous discharge at 3C and 0°C. The curve with a thick line and dashes corresponds to the stored state of charge (SOC stored), the curve with a light line and dashes corresponds to the available state of charge (SOC available) obtained with a method that takes into account the temperature but not the current (cf. Figure 2 The solid line curve represents the state of charge (SOC) obtained using the method according to the invention. This method, unlike others, clearly indicates that the state of charge (SOC) at the end of the discharge is 0 Ah. Furthermore, this method allows us to estimate from the beginning of the profile that the state of charge (SOC) decreases drastically due to the high current.

[0047] In another embodiment, instead of using the measured voltage U(t) to determine the available charge state SOC, a simulated voltage corresponding to a future voltage can be used. The available charge state SOC is then equal to the available charge state SOC at that future instant for a given current or power profile corresponding to the future voltage.

[0048] This method, presented for estimating the available state of charge (SOC), can also be applied to the state of energy (SOE). The SOC is preferred for determining the battery charge when a current is applied. The state of energy, which is related to the available power and indirectly to system losses, particularly thermal losses, is preferred when power is applied to the battery.

[0049] The state of charge estimator therefore makes it possible to accurately estimate the available state of charge SOC by taking into account the effects of current and temperature.

[0050] The method for estimating the available state of charge of a battery was described above in relation to battery discharge. However, those skilled in the art will readily understand that the described method can be applied equally to a battery that is charging, discharging, or undergoing a combination of charging and discharging.

Claims

1. Method for estimating the state of charge of a battery comprising the following steps: - determining the stored charge state (SOCstored). - determining the instantaneous open-circuit voltage (OCV) of the battery according to a predetermined mapping, the stored charge state (SOCstored), and the temperature, - measurement of the instantaneous voltage (U(t)) at the battery terminals, - determining the available state of charge (SOCavailable) according to the stored state of charge (SOCstored), the instantaneous voltage (U(t)), the instantaneous open-circuit voltage (OCV), and an allowable minimum voltage (Umin), - the allowable minimum voltage (Umin) is a predetermined design parameter, the method being characterized in that it comprises the following step: - to determine the available state of charge (SOCavailable ), the product of the stored state of charge (SOCstored ) is obtained by the ratio between the difference between the instantaneous voltage (U(t)) and the allowable minimum voltage (Umin ) and the difference between the open-circuit voltage (OCV) and the allowable minimum voltage (Umin).

2. Estimation method according to claim 1, wherein the determination of the stored state of charge is made by performing the following steps: - measuring the open-circuit voltage (OCV) of the battery at rest - determining the initial stored state of charge (SOCstored,0) according to a table relating the initial stored state of charge (SOCstored,0) to the open-circuit voltage (OCV) of the battery at rest and the temperature, - using the battery when charging and / or discharging, - periodically measuring the instantaneous current, - integrating the instantaneous current measured from the start of use after each instantaneous-current measurement, and - determining the stored state of charge (SOCstored) as the sum of the result of the integration and the value of the initial stored state of charge (SOCstored,0).

3. Estimation method according to claim 1, wherein the determination of the stored state of charge (SOCstored) is performed by a Kalman observer with a state vector defined by the stored state of charge (SOCstored) and a diffusion voltage and an output vector defined as the instantaneous voltage (U(t)) of the battery.

4. Estimation method according to any one of claims 1 to 3, wherein the future available state of charge of a battery is determined as being equal to the available state of charge (SOCavailable) determined when the instantaneous voltage (U(t)) of the battery is a future voltage.

5. System for estimating the state of charge of a battery comprising data processing means, at least one memory, at least one means for measuring the voltage at the terminals of the battery, and at least one means for measuring the current circulating between the terminals of said battery, the data processing means being configured to perform the determination method steps as claimed in claims 1 to 4.