Patent Application
20170016961
The invention relates to a method and a system for assessing the state of charge of a battery comprising a plurality of electrochemical cells connected in series.
This invention can be applied irrespective of the type of battery and extends, non-exclusively, to vehicles. In particular, the invention can be applied particularly in industrial sectors such as the automotive and computing sectors; the invention is applicable for any system, whether on-board or not.
In the non-limiting field of electric and hybrid vehicles, one of the main challenges of traction battery management systems is that of assessing the state of charge of the battery, also referred to as the SOC. This information is displayed on the instrument panel in the form of a “battery gauge” and allows the driver to know the remaining autonomy in kilometers. Because the autonomy of an electric vehicle is much lower than that of a combustion-powered vehicle, it is important to reassure the driver by providing him with the most reliable information possible. Errors in the assessment of the battery gauge can indeed result in the driver finding himself in unfavorable situations (empty fuel tank), or even dangerous situations (loss of power when overtaking).
Nowadays, the state of charge SOCpack of a battery comprising N electrochemical cells Ci (where i is an integer between 1 and N) connected in series is assessed conventionally on the basis of measurements relating to the battery considered as a whole. Thus, a first piece of equipment measures the total voltage UBAT delivered by the battery, measured at the terminals of the totality of the cells in series, and current and temperature sensors measure, respectively, the current IBAT passing through the battery and the temperature TBAT of the battery. On the basis of these three measurements a software unit calculates an assessment of the state of charge SOCpack using a conventional method, such as an ampere-hour counting method, or a modeling of the Kalman filtering type. An assessment of this type based on overall measurements thus corresponds roughly to an average of the state of charge of the cells.
The electrochemical cells forming the battery, on account of their construction, have characteristics that differ from one another in terms of distribution of their capacity and of their internal resistance, and in addition experience different temperature variations as a result of their placement in the battery. Consequently, these cells necessarily have states of charge which differ from one another, which is why the battery is said to be imbalanced. When this is the case, the range of use of the battery is set by the cell charged to the greatest extent and by the cell charged to the lowest extent. In this case, the assessment based on overall measurements is false.
Further envisaged assessment devices recommend assessing the state of charge of each cell individually so as to deduce therefrom a state of charge value for the battery by taking into consideration the imbalance of the cells. A device of this type ideally comprises a first piece of equipment measuring, simultaneously, the voltages U1 to UN at the terminals of each cell Ci forming the battery, a current sensor respectively measuring the current IBAT passing through the N cells of the battery, and temperature sensors providing the temperature Ti of each cell Ci forming the battery. On the basis of each measurement Ui, Ti and IBAT, N software units calculate an assessment of the state of charge SOCi of each cell Ci by using a conventional method such as an ampere-hour counting method, or a modeling of the Kalman filtering type. The state of charge SOCpack of the battery is then assessed by a calculation module on the basis of the N states of charge SOCi delivered by the software units. These devices are certainly more accurate, but are also more expensive and more complex in terms of software. They require voltage measurements at the terminals of each of the cells forming the battery and advanced models in order to describe the behavior of each cell (Kalman filtering in particular). In the case of a high-voltage battery, such as cells used for an electric vehicle, the large number of elementary cells (96 bi-cells in modern batteries) makes the cost of the device significant.
Lastly, in this field, a method is known for assessing a state of charge of a battery in which, on the basis of assessments relating to a maximum state of charge SOCmax of the cell charged to the greatest extent and relating to a minimum state of charge SOCmin of the cell charged to the lowest extent, it is possible to reconstruct the state of charge SOCpack of the battery; the value of the state of charge SOCpack tends toward 0 when the minimum state of charge SOCmin tends toward 0, and toward 1 when the minimum state of charge SOCmax tends toward 1. A method of this type is disclosed by the applicant in FR2990516. It has been noted that this method was not optimal, because it uses a minimum allowable state of charge value BSOCmin and a maximum allowable state of charge value BSOCmax which are fixed, which makes it impossible to hold the maximum amount of energy stored in the battery at a constant value, in particular regardless of the state of aging of the cells. For the user, the variability of the maximum amount of stored energy is detrimental because it can result in unfavorable situations, such as an empty fuel tank or a loss of power during overtaking: these situations would be caused by a poor assessment of the state of charge of the battery.
In this context, the object of the invention is to overcome the disadvantages of the prior art by proposing, at a lower cost, a method for accurately assessing a state of charge of a battery taking into consideration the imbalance of the cells. In particular, the object of the invention is to provide a method in which the maximum amount of energy stored is constant on the whole so as to prevent the user from finding himself in an uncomfortable situation preventing him from assessing whether the remaining autonomy of the vehicle is sufficient to complete his journey. A further objective targeted here is to adjust the range of use of state of charge of each cell by taking into consideration the state of health of the cell, in particular the state of aging thereof. Lastly, the present invention aims to propose a method for assessing a state of charge of a battery on the basis of assessments of state of charge of the cells or of the battery in order to limit the number of processors necessary for carrying out this method.
The proposed solution is that the method for assessing a state of charge of a battery comprising a plurality of electrochemical cells connected in series, each of the cells having a state of charge held between a minimum allowable state of charge value and a maximum allowable state of charge value, comprises the following steps:
This solution makes it possible overcome the aforementioned problems.
More precisely, the adjustment of the minimum allowable state of charge value and of said maximum allowable state of charge value of each cell depending on a physical quantity representative of a state of health of the cell makes it possible to take into consideration the state of health of each cell so as to sensibly choose a range of use of state of charge minimizing the uncertainties of assessment of the state of charge of the battery comprising said cells. This approach makes it possible to assess more reliably the remaining autonomy of the battery used conventionally in an electric or hybrid vehicle. The dependency of the ranges of use of state of charge of the cells on the respective states of health of said cells makes it possible to preserve a substantially constant maximum amount of stored energy of the battery. in addition, this method makes it possible to adjust the minimum and maximum states of charge of the battery depending on the state of health of each cell, moreover with use of minimal equipment. By way of example, a current sensor placed in series with the cells, a sensor for measuring the temperature of the battery, an electronic component able to measure solely the minimum cell voltage and maximum cell voltage, and a system for managing the state of charge of the battery collecting the current measurement taken by the current sensor, the temperature measurement taken by the temperature sensor, and the minimum cell voltage measurement and the maximum cell voltage measurement, make it possible to arrive at this result with few computing resources.
In one embodiment the method comprises at least one step including:
In another embodiment, when the maximum state of charge of the cell having the maximum cell voltage is less than or equal to the maximum allowable state of charge value of said cell and the minimum state of charge of the cell having the minimum cell voltage is greater than or equal to the minimum allowable state of charge value of said cell, the assessment method comprises a step including assessing the state of charge (SOCpack) of the battery, at a given moment k, by means of the relationship:
In one embodiment of the invention, when the maximum state of charge of the cell having the maximum cell voltage is strictly greater than the maximum allowable state of charge value of said cell and the minimum state of charge of the cell having the minimum cell voltage is strictly lower than the minimum allowable state of charge value of this cell, the method comprises a step including attributing the “unavailable” value to the state of charge of the battery.
In one embodiment said at least one physical quantity representative of a state of health of the cell is a voltage measured at the terminals of this cell and/or a current passing through the cell and/or a temperature associated with the cell.
In one embodiment the correspondence between the minimum and maximum allowable state of charge values and said at least one physical quantity representative of the state of health of the cell is predetermined, preferably in a value table.
In one embodiment:
A second subject of the invention is also targeted, in which a system for assessing a state of charge of a battery comprising a plurality of electrochemical cells connected in series, each of the cells having a state of charge to be held between a minimum allowable state of charge value and a maximum allowable state of charge value, comprises:
In one embodiment the system comprises a first module able to deliver directly to the electronic control unit solely the minimum cell voltage and the maximum cell voltage.
In accordance with a third subject, a vehicle comprising an assessment system according to any one of the above-mentioned embodiments is also targeted.
Hereinafter, a battery comprising N electrochemical cells C1 to CN connected in series will be considered. During operation, the same current IBAT thus passes through the N cells, and the voltage UBAT at the terminals of the battery corresponds at all times to the sum of the N voltages U1 to UN taken at the terminals of the N cells.
In accordance with the invention, the assessment of the state of charge of the battery is obtained on the basis of two particular values of the N cell voltages at a given moment, one corresponding to the minimum value over all the cell voltages, referred to as the minimum cell voltage, the other corresponding to the maximum value over all the cell voltages, referred to as the maximum cell voltage, these two values being denoted, respectively, as UCmin and UCmax. Each of the cells C1 to CN has a state of charge SOC within a range of use of state of charge comprising a minimum allowable state of charge value BSOCmin and a maximum allowable state of charge value BSOCmax; the operation of the cells in this range of use makes it possible to protect them against potential degradation.
It is indeed possible to define a physical quantity on which the state of charge SOCpack of the battery is directly or indirectly dependent, this physical quantity being dependent itself analytically, directly or indirectly, on the minimum state of charge SOCmin and the maximum state of charge SOCmax in accordance with an equation including weighting elements which assure that the weight associated with the maximum state of charge SOC increases when the state of charge of the associated cell increases, and the weight associated with the minimum state of charge SOCmin increases when the state of charge of the associated cell decreases. Thus, the minimum cell voltage UCmin and the maximum cell voltage UCmax are first determined, at a given moment, from the voltages at the terminals of the cells. A minimum state of charge SOCmin of the cell having the minimum cell voltage UCmin and a maximum state of charge SOCmax of the cell having the maximum cell voltage UCmax are then calculated, the state of charge SOCpack of the battery being between said minimum state of charge SOCmin and said maximum state of charge SOCmax.
The invention aims to ensure that the weight associated with the maximum state of charge SOCmax is maximum when this state of charge is in the vicinity of a predetermined maximum use threshold corresponding to the maximum allowable state of charge value BSOCmax of the associated cell, and the weight associated with the minimum state of charge SOCmin is maximum when this state of charge SOCmin is in the vicinity of a predetermined minimum use threshold corresponding to the minimum allowable state of charge value BSOCmax of the associated cell. Between the two, the variation of the physical quantity must be continuous and without sudden variations. After the step of calculation making it possible to determine the minimum state of charge SOCmin by means of the minimum cell voltage UCmin and the maximum state of charge SOCmax by means of the maximum cell voltage UCmax, it is then possible to adjust the state of charge of the battery pack SOCpack depending on the minimum and maximum states of charge SOCmin and SOCmax, said minimum allowable state of charge value BSOCmin and said maximum allowable state of charge value BSOCmax.
In accordance with the invention, said minimum allowable state of charge value BSOCmin and said maximum allowable state of charge value BSOCmax of each cell are variable. More precisely, these values BSOCmin and BSOCmax are adjusted depending on at least one physical quantity representative of a state of health of the cell and/or depending on the temperature TBAT of the battery. This state of health of the cell in fact defines the state of aging of the cell.
A basic diagram of an assessment system comprising means for carrying out the method according to the invention is shown in
The system also comprises a current sensor (not shown) able to provide a measurement IBAT of the current of the battery and one or more temperature sensors (not shown) able to provide one or more measurements TBAT of the temperature of the battery.
Typically, the electronic control unit ECU therefore collects the current measurement IBAT, the temperature measurement TBAT of the battery, and therefore the minimum cell voltage UCmin and the maximum cell voltage UCmax. The electronic control unit ECU calculates, by means of a second assessment module 20, the minimum state of charge SOCmin of the cell on the basis of the minimum cell voltage UCmin the current measurement IBAT and the temperature measurement TBAT of the battery. A third assessment module 30 calculates the maximum state of charge SOCmax of the cell on the basis of the maximum cell voltage UCmax, the current measurement IBAT and the temperature measurement TBAT of the battery. These second and third assessment modules 20, 30 calculate assessments of the state of charge of the cell SOCmin, SOCmax respectively, on the basis of the three values. The maximum state of charge SOC. and the minimum state of charge SOCmin are typically assessed by integration of the current IBAT of the battery, by Kalman filtering, or by any other method known to a person skilled in the art.
A fourth computing module 40, preferably in the electronic control unit ECU, receives information relating to the state of health of the cells, in particular the state of aging thereof. The physical quantities entering this fourth computing module 40 are the cell voltage, the current measurement IBAT, the temperature measurement TBAT of the battery, the discharge time of the cell, the maximum capacity of the battery pack, the assessment of the increase of internal resistance of the battery, or any other quantity characteristic of the aging of the cells and the battery pack. The calculation of the minimum allowable state of charge value BSOCmin and of said maximum allowable state of charge value BSOCmax by the fourth module 40 can be further refined by taking into account the temperature in the vicinity of the two identified cells, and by using the maximum capacity thereof.
On the basis of at least one of these physical quantities, this fourth module 40 adjusts the minimum allowable state of charge value BSOCmin and said maximum allowable state of charge value BSOCmax defining the range of use of the cells, which makes it possible to take into consideration the state of aging of the cell. An arrangement of this type helps to preserve a maximum quantity of usable energy of the battery at a substantially constant level.
In one embodiment the range of use defined between the minimum allowable state of charge value BSOCmin and the maximum allowable state of charge value BSOCmax is such that it becomes broader depending on the state of health of the cell and the progression of aging thereof. In another embodiment said range of use is such that it is limited when the temperature of the battery is relatively low and below a predetermined temperature threshold, depending on the characteristics relating to performance and/or aging of the electrochemical cells forming the battery in cold conditions, for example 0° C. It is the fourth module 40 which processes this information in order to modify the range of use
A fifth assessment module 50, in the electronic control unit ECU, receives, on the one hand, the assessments of the minimum state of charge SOCmin and of the maximum state of charge SOCmax provided by said second and third assessment modules 20, 30, and, on the other hand, the minimum allowable state of charge value BSOCmin and the maximum allowable state of charge value BSOCmax, and calculates an assessment of the state of charge of the battery SOCpack on the basis of these values. One function of this fifth assessment module 50 is to weight the values SOCmin, SOCmax depending on the signals BSOCmin and BSOCmax (which define the range of use in SOC of each of the cells) so as to give greater weight to the information SOCmax when a cell approaches the maximum value BSOCmax, and, in the reverse case, to give greater weight to the information SOCmin when a cell approaches the minimum value BSOCmin. Between these two extreme cases, the state of charge SOCpack of the battery must have continuous behavior, without sudden changes to its value, limited by the values SOCmin, and SOCmax of the cells. Beyond the nominal range of use, that is to say when the cell charged to the lowest extent reaches a state of charge SOC lower than BSOCmin or when the cell charged to the greatest extent reaches a state SOC greater than BSOCmax, the state of charge SOCpack of the battery must follow the variation of the most limiting cell (i.e. SOCmin or SOCmax respectively).
To arrive at this result, the fifth module 50 implements an algorithm.
In a number of cases:
in which SOCmin, SOCmax, BSOCmin and BSOCmax are, respectively, sampled values, at the discrete moment k, of the minimum state of charge, of the maximum state of charge, of the minimum allowable state of charge value BSOCmin, and of the maximum allowable state of charge value BSOCmax.
SOC
pack(k)=SOCmin(k)
SOC
pack(k)=SOCmax(k)
The use of an algorithm of this type for assessing the state of charge SOCpack of the battery makes it possible to obtain the behaviors described in
By comparing with prior-art methods, of which the results are shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 1451892 | Mar 2014 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2015/050566 | 3/9/2015 | WO | 00 |
