WO2010103216A1

WO2010103216A1 - Method for determining the state of charge of an electrochemical source for electric vehicle traction

Info

Publication number: WO2010103216A1
Application number: PCT/FR2010/050361
Authority: WO
Inventors: Denis Porcellato; Eric Gimet
Original assignee: Peugeot Citroën Automobiles SA
Priority date: 2009-03-09
Filing date: 2010-03-04
Publication date: 2010-09-16
Also published as: FR2942882A1; EP2406647A1

Abstract

The invention relates to a method for estimating the equilibrium electromotive force (Femeq) of a battery, based on data relating to the evolution over time of the no-load voltage at the terminals of such a battery during relaxation. The data can be obtained from preliminary tests on such a battery under different temperature conditions. The invention also relates to a method for estimating the state of charge (SOC) of a battery based on the equilibrium electromotive force. The state of charge estimation is based on data relating to the correspondence between a defined state of charge and a defined equilibrium electromotive force at a given temperature. The data can be obtained by preliminary tests on such a battery under different temperature conditions.

Description

Method for determining the state of charge of an electrochemical source for the electric traction of vehicles

The present invention relates to a method for evaluating the electromotive force at equilibrium of an electrochemical energy source, more particularly a battery. Such an electrochemical energy source is for example intended for electric traction. This invention relates in particular to the field of the automobile.

The present invention relates more specifically to the field of vehicles comprising electric traction means. They may be electric vehicles or hybrid vehicles, that is, vehicles that use several different energy sources to travel.

The present invention can relate to all types of electrochemical energy storage. The energy source may in particular be a storage battery, for example of the lithium-ion, Ni-MH or Ni-Zn type.

The current energy context has favored the industrial growth of electric and hybrid vehicles. The electrochemical source of energy, such as the battery or the supercapacitor, represents an expensive element of this type of vehicle.

The state of charge is an essential parameter in the electronic management of a battery. The state of charge, or SOC for State of Charge, represents the ratio of the amount of energy contained in the battery and the amount of energy in such a fully charged battery. It is usually expressed in percentages. For example, for a fully charged battery, SOC = 100%. For a fully discharged battery, SOC = 0%. The more precisely the state of charge is estimated, the more the battery is exploited to the best of its performance while maximizing its lifetime. There are different methods of determining the state of charge of a battery during operation. A known method is to perform, as a function of time, the integration of the intensity of the current flowing through the battery. It can be a charging current or a discharge current.

The reliability of such a method is, however, decreasing as a function of time, due in particular to inaccuracies in current measurement. We therefore notes a drift over time between the estimated state of charge and its actual value.

In most battery technologies, the periods of charge and / or discharge of the battery are separated by periods of relaxation, during which the battery is not crossed by a current.

After a certain period of relaxation, the battery reaches a state of equilibrium. It is then possible to measure the empty voltage of the battery, which corresponds to its electromotive force at equilibrium. However, each electromotive force at equilibrium corresponds to a precise state of charge, for each type of battery.

The measurement of the electromotive force at equilibrium, during the periods of relaxation, makes it possible to recalibrate regularly the estimate of the state of charge by integration of the current as a function of time.

When the battery is traversed by a current, its electrodes undergo a polarization. Depending on the type of battery, a relaxation time of several minutes to several hours is necessary to reach the state of equilibrium.

The relaxation time required to reach equilibrium state depends on the degree of polarization of the electrodes. This degree of polarization itself depends in particular on the intensity of the current having previously passed through the battery, as well as the duration of application of this current. Any determination of electromotive force out of equilibrium affects the accuracy of the state of charge estimation.

In addition, especially in the case of hybrid vehicles, the periods of relaxation of the battery may be too short to reach the state of equilibrium. It is therefore not possible to recalibrate the estimation of the state of charge during periods of relaxation.

The object of the present invention is to solve these problems by means of a method for estimating the electromotive force at the equilibrium of a battery, without said battery having reached a state of equilibrium. This estimation is made in particular from measurements of the voltage at the terminals of the battery, during periods of relaxation.

Such an estimate of the electromotive force at equilibrium allows a reliable estimation of the state of charge of the battery during periods of relaxation, even if said periods are too short for the battery to reach a state of equilibrium.

The subject of the present invention is therefore a method for estimating the equilibrium electromotive force of a battery, based on data relating to the evolution over time of the vacuum voltage at the terminals of such a battery. relaxation classes. These data can be obtained by preliminary tests on such a battery under different temperature conditions.

The present invention also relates to a method for estimating the state of charge of a battery, as a function of the electromotive force at equilibrium. This estimate of the state of charge is based on data relating to the correspondence of a defined state of charge and a defined equilibrium electromotive force for a given temperature. These data can be obtained by preliminary tests on such a battery under different temperature conditions.

Such a method is easy to apply to most vehicles having a battery as a source of electrical energy. This method is particularly advantageous in the case of hybrid vehicles, since it makes it possible to solve problems preventing the application of known methods for estimating the state of charge.

The present invention thus relates to a method for evaluating the equilibrium electromotive force of a battery for electric traction, comprising the following steps:

measuring the intensity (l (t _j )) of a current flowing through the battery (3), at several successive instants (t _j );

- if the intensity (l (t,)) measured at a time t, is zero, measuring the voltage (U (ti)) empty at the terminals of the battery (3) at a time ti such that ti≥t ,;

measuring the voltage (U (t ₂ )) at the terminals of the battery (3) at a time t ₂ such that the intensity (l (t)) is zero between t ₁ and t ₂ ;

- measuring the temperature (T (t,)) of the battery (3);

calculating a value p (ti, t ₂ ) as a function of (U (ti)), (U (t ₂ )), t ₁ and t ₂ ;

- determination of an equilibrium electromotive force (FenrieqCt,)) of the battery (3) for a relaxation period, as a function of p (ti, t ₂ ) and U (t _j ) with ti <t _j <t ₂ , said electromotive force at equilibrium being estimated at the temperature (T (t)).

According to a preferred form of the invention, the electromotive force at equilibrium (FenrieqCt,)) is estimated by taking into account the sign of the intensity (l (t, 1)), measured at a time t, _i such that the intensity (l (t)) goes from a non-zero value to a zero value between t, _i and t ,.

The present invention also relates to a method further comprising a step of determining a state of charge (SOC (t,)) of the battery (3), as a function of the electromotive force at equilibrium (FenrieqCt,) ), said state of charge being estimated at the temperature (T (t,)).

The present invention also relates to a method such that, when the intensity (l (t _j )) is non-zero, the state of charge (SOC (t _j )) is evaluated by integration of the current, from the state of charge (SOC (t,)) estimated at the last relaxation period observed. The present invention also relates to a device for managing an electrochemical energy source for electric traction, comprising means for implementing a method as described above.

The present invention finally relates to a vehicle equipped with such a device. According to a preferred form of the invention, said vehicle is a hybrid vehicle, that is to say that it comprises a source of electrical energy as well as a source of energy of a different nature, for example a hydrocarbon fuel.

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. These are given as an indication and in no way limit the invention. The figures show:

Figure 1: a schematic representation of a device comprising means for implementing a method according to a form of the invention. - Figure 2: evolution curves of the no-load voltage at the terminals of a battery, according to a logarithmic time scale. 3: curves representing a difference in electromotive forces as a function of the slopes of the curves represented in FIG. FIG. 1 represents a device comprising means for implementing a method according to a form of the invention. The device 1 comprises in particular a battery management system 2, or BMS (Battery Management System). The BMS 2 is connected to a battery 3. Via an interface 12, the BMS 2 is connected in particular to means 4 for measuring the temperature of the battery 3, to means 5 for measuring current flowing through the battery 3 and means 6 for measuring a voltage across the battery 3. Such measuring means 4, 5 and 6 are present in systems known from the state of the art. The BMS 2 includes a module 7 which gives a time base. At a time t _j , the BMS 2 stores a family of values [l (t _j ), U (t _j ), T (t _j )] respectively representing the intensity crossing the battery 3, the voltage at the terminals of the battery 3 and the temperature of the battery 3 at time t _j .

The BMS 2 comprises a module 8, connected to a memory 9 of data. The memory 9 stores the family of values [l (t _j ), U (t _j ), T (t _j )] for a certain number of consecutive instants t _j . As long as the intensity l (t _j ) is non-zero, the module 8 can calculate the state of charge of the battery 3 by integrating the current, for example according to the following formula (I):

(I): SOC (tj) = SOC ₀ - [(Jldc / C dt) * 100)] + [(JpWC dt) * 100)]

such as:

SOC (t _j ) = state of charge at time t _j SOCo = state of charge at an initial moment t ₀ - C = quantity of electricity in the fully charged battery

Jldch dt = quantity of electricity discharged by the battery between t ₀ and t, Jplch dt = quantity of electricity charged by the battery between t ₀ and t _j p = faradic efficiency of charge

The data memory 9 stores curves expressing the electromotive force at the equilibrium of batteries similar to the battery 3, as a function of the revolution over time of the electromotive force (or Fem) of said battery during the relaxation period. According to a form of the invention, distinct curves are established depending on whether the relaxation period follows a charging or discharging. A curve corresponds to a specific temperature of the battery 3.

These curves are obtained in particular by measurements during tests on battery banks similar to the battery 3. A method of establishing such curves will be detailed below.

When at a time t ,, the battery 3 is in the state of relaxation, the means 5 measures l (t,) = 0. The means 6 then measures the voltage across the battery 3 at two successive times t i and t ₂ , such that ti≥t, and that the intensity through the battery 3 remains zero between ti and t ₂ . When the battery 3 is in a state of relaxation, the voltage U at its terminals is equal to its electromotive force. From the values U (ti), U (t ₂ ), t i and t ₂ , the module 8 can calculate the slope p (t ₁ , t ₂ ) of a curve of evolution of the electromotive force of the battery 3 into function of a logarithmic time scale, according to the following formula (II):

(II): p (t ₁ , t ₂ ) = [U (t ₂ ) - U (t i)] / (log t ₂ - log t i)

To apply formula (II), the beginning of the relaxation period should be considered as initial moment. We can notably consider t, as initial moment, with for example ti = t, + 1 second and t ₂ = t, + 5 seconds.

If t, is considered as the initial moment for formula (II), it is advantageous to choose ti different from t ,, in order not to have log ti = log 0. More precisely, it is advantageous to choose ti> t , + 1 s (if the unit of time chosen is the second) to have log ti> 0.

The choice of t ₂ depends in particular on the duration of the relaxation periods on the type of battery 3 considered. The relaxation period must indeed be equal to or greater than the duration between t, and t ₂ .

The memory 9 stores curves ΔV = f (p), such that ΔV corresponds to the absolute value of the difference between the electromotive force at equilibrium (Fem _eq ) of a battery similar to the battery 3 and its Fem, or empty voltage, at a given moment at the beginning of relaxation. The variable p represents the slope of a curve described above, as calculated according to formula (II). For example, the curves ΔV = | Fem _e q - U (ti) | with ti = 1 s. Each curve ΔV = f (p) corresponds to a particular temperature T of the battery 3. Preferably, distinct curves are established depending on whether the relaxation period follows a charging or discharging pulse. Having calculated the value p from the values U (ti), U (t ₂ ), ti and t ₂ , the module 8 refers to the curve ΔV = f (p) appropriate according to the temperature T (t,) measured by the means 4. Preferably, the appropriate curve also depends on the sign of the intensity l (t, _i), t, _i being the instant preceding t, at which a non-zero value of I has been measured. More precisely, the appropriate curve is different depending on whether the relaxation period follows a charging pulse or a discharge pulse.

Having determined the appropriate curve, the module 8 determines Fem _eq (tι) as a function of p (t i, t ₂ ) and U (t i).

The module 8 can then determine the state of charge SOC (t,) as a function of FenrieqCt,) and of T (t,), as a function of curves Fem _eq = f (SOC) established at different temperatures T. These curves are in particular obtained by measurements during tests on battery bank similar to the battery 3, as detailed below.

Through an interface 13, the values SOC (t,) and / or FenrieqCt, can be communicated by the BMS 2 to a supervisor 10 of the battery 3. At any time, either of these values may be accessible to a user via a diagnostic socket 11.

At a time t _k , the means 5 measures a non-zero intensity l (t _k ), that is to say that the relaxation period has ended. The module 8 can then measure the new successive charge states by integration of the current, for example according to formula (I) above. Module 8 then considers SOCo = SOC (t,) in formula (I). The method of determining the state of charge during the relaxation period thus makes it possible to reset the calculation of the state of charge outside the relaxation period. FIG. 2 represents evolution curves of the no-load voltage at the terminals of a battery 3, as a function of a logarithmic time scale, ie curves Fem = f (log t). The measurements follow the application of discharge tips of different durations. The longer the pulses, the greater the polarization at the terminals of the battery 3. These curves have been established at the same temperature T of the battery 3, during bench tests. During these tests, for example, an initial state of charge of 50% is adopted.

During the first seconds of relaxation, it can be seen that the revolution of the vacuum voltage is almost linear on a logarithmic scale of time. The higher the polarization, the higher the slope p of the curve.

The curves evolve towards a value representing the electromotive force at equilibrium Fem _eq . This value is different according to the curve, that is to say according to the level of polarization of the terminals of the battery 3.

These curves make it possible to establish a curve ΔV = f (p), such that ΔV = I Fenrieq - Fem (ti) | with for example ti = 1 s. Such a curve, which corresponds to a given temperature T, is shown in FIG. 3. It can be seen that this curve is almost linear. Another curve ΔV = f (p), corresponding to the relaxation after a charging pulse at a given temperature T, can be established under similar conditions. Such a curve is also shown in FIG. 3. It can be seen that this curve is almost linear. Moreover, it can be seen that the curve ΔV = f (p) for a load tap is distinct from the curve ΔV = f (p) for a discharge tap.

The memorization of these curves in the data memory 9 allows FenrieqCt to be determined during the relaxation period of the battery 3, as previously described.

Preferably, the curves ΔV = f (p) are established for several temperatures T, distributed over a temperature range in which the temperature of the battery 3 is supposed to change. For example, the curves are established at intervals of 10 ^° C. a range [-30 ^° C .; 50 ⁰ C]. For each temperature T (t,) measured, such that T (t,) is situated between two reference temperatures, one can for example estimate FenrieqCt,) by performing a weighted average of the results given by two curves ΔV = f (p) .

In order to estimate the state of charge SOC (t,) with respect to Femeq (tι), it is necessary to establish curves Fem = f (SOC) at a given temperature T. It is possible to proceed as follows: The battery 3 is placed in a chamber regulated at the temperature T, at an initial state of charge of 100%. We note his Fem to balance. It is then applied a discharge nozzle to bring it to a SOC of 90%. We note his Fem when the equilibrium is reached. This is done in stages until a state of charge of 0%.

As for the equilibrium Fem, these curves can for example be established at intervals of 10 ^° C. over a range [-30 ^° C. 50 ⁰ C].

In some battery technologies, the Fem = f (SOC) curves are slightly different depending on whether charging pulses are applied from a discharged battery or discharge pulses from a charged battery. It is possible to memorize these two types of curve in the memory 9 and to use one or the other of these curves according to the sign of l (t, .i).

Claims

1. A method for evaluating the equilibrium electromotive force of a battery (3) for electric traction, comprising the following steps: measuring the intensity (l (t _j )) of a current flowing through the battery (3), several times (t _j ) successive;

- if the intensity (l (t,)) measured at a time t, is zero, measuring the voltage (U (ti)) empty at the terminals of the battery (3) at a time ti such that ti≥t ,; measuring the voltage (U (t ₂ )) at the terminals of the battery (3) at a time t ₂ such that the intensity (l (t)) is zero between t ₁ and t ₂ ;

- measuring the temperature (T (t,)) of the battery (3);

determination of an equilibrium electromotive force (FenrieqCt) of the battery (3) for a relaxation period, as a function of p (ti, t ₂ ) and

U (t _j ) with t _i <t _j ≤t ₂ , said equilibrium electromotive force being estimated at the temperature

(T (t)).

2. A process according to claim 1, characterized in that the electromotive force at equilibrium (FenrieqCt) is estimated by taking into account the sign of the intensity (l (t, -i)), measured at a given instant. t, _i such that the intensity (l (t)) changes from a non-zero value to a zero value between t, _i and t ,.

3. A process according to claim 1 or claim 2, characterized in that the value p (ti, t ₂ ) is calculated according to the following formula (II):

(II): p (ti, t ₂ ) = [U (t ₂ ) - U (ti)] / (log t ₂ - log ti) such that t, is considered as the initial moment and that ti> t,

4. Method according to one of the preceding claims, characterized in that it further comprises a step of determining a state of charge (SOC (t,)) of the battery (3), depending on the force equilibrium electromotive (FenrieqCt,)), said state of charge being estimated at the temperature (T (t,)).

5. - Method according to one of the preceding claims, characterized in that, when the intensity (l (t _j )) is non-zero, the state of charge (SOC (t _j )) is evaluated by integration of the current , from the state of charge (SOC (t,)) estimated at the last relaxation period observed.

6. A device for managing an electrochemical energy source for electric traction, comprising means for implementing a method according to one of the preceding claims.

7. Vehicle equipped with a device according to claim 6.

8. Vehicle according to claim 7, characterized in that it is a hybrid vehicle.