Method for determining the temperature of an electrochemical cell in a rechargeable battery

Abstract

The invention relates to a method for determining the temperature of an electrochemical cell in a rechargeable battery, comprising the steps of: - applying a sinusoidal or rectangular voltage at a predetermined frequency across the terminals of said electrochemical cell, - measuring the resulting electrical voltage and current across the terminals of said electrochemical cell, - calculating a value (PR) relative to the impedance of said electrochemical cell, - determining the temperature value based on the calculated value. Figure for the abstract: Fig. 2

Description

Title of the invention: Method for determining the temperature of an electrochemical cell in a rechargeable battery Technical field of the invention

[0001] The present invention relates generally to accumulator batteries.

[0002] It relates more particularly to a method for determining a temperature data of an electrochemical cell of a battery accumulator.

[0003] It also relates to a battery of accumulators equipped with a system adapted to implement this process, and a motor vehicle equipped with such a battery.

[0004] The invention finds a particularly advantageous application in electric or hybrid powered motor vehicles. State of the art

[0005] Electric motor vehicles generally include an electric motor powered by a battery, commonly called a traction battery. Such a traction battery comprises a plurality of electrochemical cells, for example of the Lithium-ion type, connected together to deliver a high voltage.

[0006] These electrochemical cells heat up when they deliver or receive electrical energy. To prevent their temperatures from exceeding a threshold beyond which they could degrade, the traction battery is generally equipped with a cooling circuit through which a fluid circulates. This circuit typically includes a hollow plate within which the fluid circulates and above which the electrochemical cells are positioned.

[0007] It is also known that the capacity of a traction battery to deliver or store electrical energy depends strongly on the temperature of the electrochemical cells, and therefore on the ambient temperature. This capacity is particularly degraded at low temperatures, which can prove especially problematic for vehicles used in the coldest geographical areas.

[0008] It is therefore essential to determine the lowest and highest temperatures within the pack of electrochemical cells in order to adjust accordingly the electrical power that these cells can supply or store.

[0009] Currently, it is known to distribute electrochemical cells into several modules of several cells.

[0010] It is then observed that the central cells of each module heat up more than the others, since they are not in contact with the surrounding air (unlike the cells located at the periphery of the module).

[0011] It is also observed that each cell is hotter on the upper side than on the lower side which is in contact with the hollow plate in which the cooling fluid circulates.

[0012] This is why it is known to place a temperature sensor on the upper face of one of the central cells of each module, where the temperature is probably highest.

[0013] This solution, however, is not particularly precise. Typically, it does not allow for the rapid detection of thermal runaway in a cell located at a distance from the temperature sensor. More generally, it does not allow for optimal control of the electrical power supplied or stored, since its limited precision necessitates significant safety margins. Presentation of the invention

[0014] In order to remedy the aforementioned drawback of the prior art, the present invention proposes to determine the temperature of at least one of the electrochemical cells of the battery (and preferably of all the cells), by means of a measurement carried out according to a protocol inspired by the impedance measurement protocol by EIS (from the English "Electrochemical Impedance Spectroscopy", which means electrochemical impedance spectroscopy).

[0015] More particularly, the invention proposes a method for determining the temperature of an electrochemical cell in a rechargeable battery, comprising the following steps: - application across the terminals of said electrochemical cell of a sinusoidal or rectangular voltage at a predetermined frequency, - measurement across the terminals of said electrochemical cell of the resulting electrical voltage and current (i.e., which are measurable simultaneously during the application of the sinusoidal or rectangular voltage and / or directly after), - calculation of a value relative to an impedance of said electrochemical cell, - determination of the temperature data as a function of the calculated value.

[0016] Thus, thanks to the invention, it is planned to determine the impedance or a component of the impedance of an electrochemical cell, preferably at a single predetermined frequency, and to compare this value with data from tests in order to deduce the desired temperature data.

[0017] This method proves to be very precise. It is based on the electrochemical reaction of the cell, so that it is able to provide a temperature at the heart of the electrochemical cell.

[0018] It is also quick to implement, so that it allows the individual temperatures of the cells to be determined.

[0019] It therefore offers the advantage of precisely determining the highest temperature among the temperatures of all the cells in the battery, in order to control this battery in the best possible way, which makes it possible to increase the performance of the battery while ensuring its durability.

[0020] It also makes it possible to detect defects or problems within the battery, such as thermal runaway resulting from a short circuit or a manufacturing defect in one of the cells.

[0021] Other advantageous and non-limiting features of the process according to the invention, taken individually or in all technically possible combinations, are as follows: - at the determination stage, the temperature data includes two values ​​noted AT and Tcor; - at the determination stage, the temperature data is determined by comparing the calculated value with predetermined values, each associated with a temperature data point; - the temperature data is in one embodiment the internal temperature of the electrochemical cell; - the predetermined values ​​are each associated with a minimum temperature and a maximum temperature difference within the electrochemical cell; - the temperature data is in one embodiment the maximum deviation; - at the determination stage, the temperature data is determined using at least one interpolation function that associates predetermined values ​​with temperature data; - Several interpolation functions are planned which associate predetermined values ​​with temperature data, and which are each valid for a maximum temperature difference; - each interpolation function is polynomial in form, preferably of order 2; - said minimum temperature is a temperature of a coolant circulating in a circuit adapted to cool said electrochemical cell; - said value relating to an impedance of said electrochemical cell is a resistance value.

[0022] The invention also proposes a battery of accumulators comprising a case which houses at least one electrochemical cell and which includes a circuit adapted to apply a sinusoidal or rectangular voltage to the terminals of said electrochemical cell and a control system for said circuit, adapted to implement a process as described above.

[0023] The invention also proposes a motor vehicle comprising an electric machine and a battery of accumulators as above, which is adapted to supply current to said electric machine.

[0024] Of course, the various features, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. Detailed description of the invention

[0025] The following description with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0026] On the attached drawings:

[0027] [Fig-1] is a schematic perspective view of electrochemical cells of a battery of accumulators conforming to the invention;

[0028] [Fig.2] is a Nyquist digraph illustrating spectra in the complex plane impedance of one of the electrochemical cells of [Fig.1] for three different temperatures, the complete diagram being shown on the right and a detail of this diagram being illustrated on the left;

[0029] [Fig.3] is a graph illustrating a temperature interpolation curve of the electrochemical cell of [Fig.2] as a function of the real part of the impedance, for a given frequency;

[0030] [Fig.4] is a graph illustrating six interpolation curves representing the variations of an equivalent resistance of the electrochemical cell of [Fig.2] as a function of its minimum temperature, for six different temperature gradients;

[0031] [Fig.5] is a graph illustrating the temperature gradients of 96 cells electrochemicals of a battery of accumulators.

[0032] In [Fig.1], a part of a battery of accumulators, hereinafter referred to as traction battery 1, has been very schematically represented.

[0033] Here, and preferably, this traction battery 1 is intended to be used within a motor vehicle.

[0034] This motor vehicle could be of any type (truck, bus). It will preferably be a land vehicle, here a car which classically comprises a chassis, wheels of which at least two are driven, and a powertrain adapted to turn the driven wheels.

[0035] The powertrain is preferably purely electric, but could alternatively be hybrid. In all cases, it includes at least one electric motor powered by the traction battery 1.

[0036] The traction battery 1 includes a housing which contains all of its components, in particular electrochemical cells 10 (preferably all identical).

[0037] These could typically be lithium-ion type cells, but other variants would be conceivable.

[0038] Each electrochemical cell 10 here has a voltage across its terminals of the order of 3 to 5 V. These cells are then connected in series to reach the voltage level required by the application.

[0039] Here, there are approximately one hundred of these electrochemical cells 10, so that the electric machine can develop sufficient torque and power to propel the vehicle for a desired duration. Thus, the voltage at the outer terminals of the traction battery 1 is approximately 400V. In practice, 96 cells are used here. Of course, the number of cells could be greater (approximately 200, for example) or less.

[0040] In [Fig. 1], not all the electrochemical cells are visible. However, they can be seen distributed in several groups of cells called "modules 2". For example, eight modules 2 of twelve electrochemical cells are provided, which are electrically connected to each other in series, so as to form a pack of modules 2. Each module 2 has a frame (not shown) which allows the cells to be stacked against each other. This frame is open on its underside.

[0041] The traction battery housing 1 includes, in particular, a support plate 20 on which the modules 2 are mounted. This support plate 20 is preferably hollow so that a coolant can circulate through it. A cooling circuit is preferably provided which passes through this support plate 20 and which includes a pump for circulating the coolant and a heat exchanger for reducing its temperature.

[0042] In the following description, the term "lower" will be used to designate a side or an object turned towards the side of the support plate 20, while the term "upper" will be used to designate a side or an object turned towards the opposite side.

[0043] Of course, alternatively, the support plate could be arranged above the cells or on the side.

[0044] Preferably, a layer of thermal paste is interposed between this support plate 20 and each of the electrochemical cells 10, so as to optimize the heat exchange between these elements.

[0045] A temperature sensor 31 is placed in the cooling circuit, for example inside this support plate 20, to determine the temperature of the coolant.

[0046] Other temperature sensors 32 may optionally be placed above a portion of the electrochemical cells. Typically, one temperature sensor 32 may be provided per module 2, ideally placed near the center of the upper surface of module 2, against one of the electrochemical cells 10.

[0047] The traction battery 10 also includes a battery management system, better known by the acronym BMS (from the English "battery management system").

[0048] This management system, not shown in the figures, comprises a processor (CPU), memory, and various input and output interfaces. Its memory stores data used in the process described below. In particular, it stores a computer application consisting of computer programs containing instructions whose execution by the processor enables the implementation of the process described below.

[0049] The management system typically performs several functions, including those of: - continuously monitor the individual voltages of each electrochemical cell and the charge and discharge currents, to ensure that all these parameters remain within safe operating ranges, and - balance the voltages between the electrochemical cells.

[0050] Here, as will be described below, it also ensures thermal monitoring of the electrochemical cells 10. The objective of this monitoring is in particular to prevent the electrochemical cells 10 from being used in a way that is too restrictive at high or low temperature.

[0051] To achieve this, the management system is equipped with a specific circuit (not shown) that applies a sinusoidal voltage to the terminals of each electrochemical cell. This specific circuit may, for example, include MOSFET transistors and discharge resistors.

[0052] Before describing in detail how the management system implements this monitoring, we can describe on what physical bases the invention is based.

[0053] Thermally, it is understood that during their use, the electrochemical cells 10, which are stacked in modules, heat up in the same way regardless of their position within the stack. On the other hand, those located at the two ends of the stack will be cooled more than the others.

[0054] Furthermore, since the modules are distributed here in several rows and several columns (see [Fig. 1]), the best-cooled electrochemical cells 10 will be those located against the external casing of the traction battery 1. As shown in [Fig. 1], the electrochemical cells will therefore be found inside a cylinder 40. 10 of the hottest. There is therefore a temperature dispersion between the different electrochemical cells.

[0055] It is also understood that each electrochemical cell 10 will exhibit a non-uniform temperature, lower on the side of its lower face in contact with the support plate 20 than on the side of its upper face. Each electrochemical cell 10 will then be said to exhibit a temperature gradient, illustrated in [Fig. 1] by the arrows Fl. This temperature gradient will be characterized here by a temperature difference AT between the upper and lower faces of a cell.

[0056] As shown in [Fig.2], it is possible to construct in the laboratory, on an electrochemical cell 10 identical to those that will be used in the traction battery 1, a spectral response of the impedance of this cell.

[0057] To this end, an electrochemical impedance spectroscopy measurement, better known by the acronym EIS (from the English "Electrochemical Impedance Spectroscopy"), is performed. The method for performing such a measurement is described, for example, in document US20060170397.

[0058] In summary, electrochemical impedance spectroscopy is a sophisticated diagnostic method that allows examination of the complex internal chemistry of electrochemical cells. It consists of: - applying a sinusoidal voltage signal U(t) across the terminals of the electrochemical cell 10, which generates a sinusoidal intensity signal (of the same frequency but out of phase), and - Measure the resulting electrical behavior of the cell by dividing the measured voltage response V by the current response I. This calculation yields the cell's impedance Z. It takes into account the phase shift between voltage and current, providing a complex number with magnitude and phase components, revealing the resistive and reactive characteristics of the cell.

[0059] These two steps can be repeated for different frequencies, which makes it possible to plot on the Nyquist diagram of [Fig.2] the variation of the impedance Z as a function of frequency, in the complex plane.

[0060] It is recalled in this regard that the relationship between the impedance Z, the resistance R (corresponding to the real part of the impedance represented by the x-axis) and the reactance X (corresponding to the imaginary part of the impedance represented by the y-axis) is given by the mathematical formula:

[0061] Z=R+jX

[0062] This measurement operation is preferably carried out when the cell has a uniform internal temperature.

[0063] As shown in [Fig. 2], this operation can be repeated with other temperatures. It is thus possible to plot several curves, each associated with a Internal cell temperature. To facilitate reading this figure, only three curves for temperatures of 10°C, 0°C and 20°C have been shown here.

[0064] Fig. 2 then clearly shows that the amplitude of the impedance is a function of the internal temperature of the electrochemical cell 10.

[0065] It is then understood that it is possible to determine the internal temperature of any electrochemical cell 10 by measuring its impedance (provided that tests have been carried out on an electrochemical cell of the same type in order to characterize its impedance for different temperatures).

[0066] Two embodiments of the invention can then be described, allowing the temperatures of the electrochemical cells of the traction battery 1 to be determined.

[0067] We can begin by describing a first simplified embodiment of the invention.

[0068] In this first mode, it is proposed to characterize the impedance Z of an electrochemical cell 10 by the value of its real part PR (i.e., its resistance R) or its imaginary part Pim (i.e., its reactance X) when subjected to an EIS measurement for a given frequency fl. This frequency fl is between 0.1 Hz and 5000 Hz. For example, it is equal to 60 Hz.

[0069] As shown in the graph on the right side of [Fig.2], which illustrates a detail of that on the left side, it is the real part PR of the impedance Z that is considered here, since it allows us to obtain more precise results.

[0070] On this [Fig.2], the frequency responses of an electrochemical test cell 10 obtained in the laboratory have been plotted, for frequencies varying between 0.1Hz and 5000HZ and for three different internal temperatures: Tcor3 = -10°C, Tcor2 = 0°C, Tcorj = 20°C.

[0071] It is understood here that it will not be necessary to plot these frequency responses, since only one frequency fl will have to be considered.

[0072] It is also understood that it will be possible, and even preferable, to determine at this frequency fl the real part PRTcori of the impedance Z of this cell for a larger number of different temperatures Tcor, in order to better appreciate how this real part varies with temperature.

[0073] This laboratory determination will be carried out here for internal temperatures ranging from -20°C to 35°C, for example in increments of 5 or 10°C. During these laboratory tests, care will be taken to ensure that the cell has a homogeneous temperature.

[0074] Then, taking into account the responses obtained at the different temperatures Tcor;, it will be possible to establish a map or calculate a mathematical function allowing, from a measured real part PR, to obtain an approximation of the internal temperature Tcor of the electrochemical cell.

[0075] Here, it is planned to construct a mathematical interpolation function Fi that associates each real part PRTcori with an internal temperature Tcor. Preferably, this is a polynomial function, preferably of order 2. This function is illustrated in [Fig. 3]. It can be seen that it passes very close to the points obtained during the laboratory tests, so that the margin of error remains less than 2°C.

[0076] This interpolation function (or alternatively the mapping) is stored in the memory of the traction battery management system 1.

[0077] Therefore, when the battery is under stress (charging and discharging) and the management system is looking for the temperatures of the electrochemical cells, it can perform an impedance measurement Z of each electrochemical cell 10 of the battery at a frequency fl, deduce the value of the real part PR of this impedance Z and then, using the interpolation function, deduce the internal temperature Tcor of the electrochemical cell.

[0078] It should be noted here that the internal temperature Tcor obtained is determined taking into account the electrochemical reactions of the electrochemical cell 10, since the EIS test characterizes these reactions. This temperature is therefore a temperature at the core of the cell, and not a temperature outside the cell such as those that can be measured by standard temperature sensors.

[0079] The internal temperatures of the different electrochemical cells 10 of the accumulator battery 1 could thus be determined, so as to be able to then control the charging or discharging of the accumulator battery in order to guarantee its durability.

[0080] However, the internal temperatures thus obtained do not allow us to know exactly what the lowest and highest temperatures are within the battery of accumulators 1.

[0081] A second embodiment can then be described allowing the minimum and maximum temperatures to be obtained within each of the electrochemical cells 10.

[0082] For this, we will first consider that the minimum temperatures Tmin of the electrochemical cells 10 are all equal to the temperature of the coolant, since the cells are in contact with the support plate 20. We recall that this temperature is known since it is measured by an ad hoc sensor.

[0083] It will also be considered that the cells have a maximum temperature at their upper edges (opposite the support plate 20).

[0084] In this second embodiment, the objective will be to determine the temperature difference AT between the upper and lower edges of each cell. It will thus be possible, taking into account the temperature of the coolant, to determine the maximum temperature Tmax of each cell.

[0085] It has been shown above that the real part PR of the impedance Z of an electrochemical cell 10 varies with the internal temperature Tcor of this electrochemical cell. Theoretically, one could imagine discretizing this electrochemical cell 10 into several elementary sub-cells superimposed along a vertical axis. The lowest sub-cell would be at the minimum temperature Tmin, the highest cell at the maximum temperature Tmax, and each of the other sub-cells at a uniform temperature between these two extremes. It is therefore understandable that the temperature difference AT will also be a function of the real part PR of the impedance Z of an electrochemical cell.

[0086] As shown in [Fig.4], it is then possible to construct in the laboratory, using tests, new FiAT interpolation curves relating the real part PR of a test electrochemical cell 10 (i.e. its resistance R) to its minimum temperature Tmin (on its lower edge), for different temperature differences AT.

[0087] These tests are then carried out by EIS measurement at the frequency fl, ensuring that the temperature of the electrochemical test cell 10 has a temperature gradient that is as linear as possible along the vertical axis.

[0088] Here, these tests are carried out with temperature differences AT between 0°C and 10°C, with a step of 2°C.

[0089] Therefore, when the battery is under stress (charging and discharging) and the management system is looking for the temperatures of the electrochemical cells 10, it can perform on each cell an impedance measurement at a frequency fl and a coolant temperature measurement, and deduce the temperature difference AT from the data characterizing the interpolation curves FiAT.

[0090] Thus, at the end of this operation, the management system can determine the minimum and maximum temperature of each of the electrochemical cells 10 of the battery of accumulators 1.

[0091] He is therefore able to deduce the highest temperature among all the highest temperatures of the electrochemical cells 10.

[0092] Knowing this data with high precision (on the order of 2°C), it can control the charging or discharging of the battery at its best potential.

[0093] The temperatures obtained also make it possible to detect internal defects in the battery.

[0094] Typically, the management system can issue an alert if the AT temperature difference between the upper and lower edges of one of the cells The electrochemical value of 10 is higher than a threshold. This threshold can be predetermined (for example, equal to 10°C) or determined based on the AT differences observed on the other cells.

[0095] Typically, in [Fig.5], a case of thermal runaway of one of the cells has been represented (each cell being referenced by an index k ranging from 1 to 96, the one affected by this runaway has the index k = 46).

[0096] It can be clearly seen in this figure that one of the cells exhibits a much stronger temperature gradient than the other cells.

[0097] It is then possible to detect this runaway very early, and to electrically isolate the cell of the accumulator battery very quickly.

[0098] To achieve this, the management system compares the difference between the largest AT deviation and the average of the AT deviations with a predetermined threshold, at several distinct times (for example, at 3, 4, or 5 successive times) to avoid any false detection. It is therefore understood that the detection of the problem will be all the faster the higher the frequency at which the temperatures of the electrochemical cells 10 are calculated. Here, since the calculation of the AT deviations is repeated every 3 seconds, it will be possible to perform this detection in less than 15 seconds, or even in less than 10 seconds.

[0099] Of course, the two embodiments can be combined, so as to be able to determine not only this AT deviation but also the internal temperature Tcor of the electrochemical cell.

[0100] The present invention is in no way limited to the embodiment described and represented, but a person skilled in the art will be able to make any variation in accordance with the invention.

Claims

Demands

1. A method for determining a temperature data (Tcor, AT) of an electrochemical cell (10) of a rechargeable battery, comprising the steps of: - applying a sinusoidal or rectangular voltage (U(t)) at a predetermined frequency (fl) across the terminals of said electrochemical cell (10), - measuring the resulting electrical voltage (V) and current (I) across the terminals of said electrochemical cell (10), - calculating a value (PR) relative to an impedance (Z) of said electrochemical cell (10), - determining the temperature data (Tcor, AT) as a function of the calculated value (PR).

2. Method of determination according to claim 1, wherein, in the determination step, the temperature data (Tcor, AT) is determined by comparing the calculated value (PR) with predetermined values ​​(PRTcori) each associated with a temperature data (Tcor, AT).

3. Method of determination according to claim 2, wherein the predetermined values ​​(PRTcOri) are each associated with a minimum temperature (Tinf) and a maximum temperature deviation (AT) within the electrochemical cell (10).

4. A method of determination according to any one of claims 2 and 3, wherein, at the determination step, the temperature data (Tcor, AT) is determined using at least one interpolation function (Fi) which associates predetermined values ​​(PRTcori) with temperature data (Tcor, AT).

5. Method of determination according to claims 3 and 4, wherein several interpolation functions (FiAT) are provided which associate the predetermined values ​​(PRTcOri) with temperature data, and which are each valid for a maximum temperature deviation (AT).

6. A method of determination according to any one of claims 4 and 5, wherein each interpolation function (Fi, FiAT) is of polynomial form, preferably of order 2.

7. A method for determining according to any one of claims 3 to 6, wherein said minimum temperature (Tinf) is a temperature of a coolant circulating in a circuit adapted to cool said electrochemical cell (10).

8. Method of determination according to any one of claims 1 to 7, wherein said value (PR) relating to an impedance (Z) of said electrochemical cell (10) is a resistance value.

9. Accumulator battery (1) comprising a casing which houses at least one electrochemical cell (10) and which comprises a circuit adapted to apply a sinusoidal or rectangular voltage across the terminals of said electrochemical cell and a control system for said circuit, adapted to implement a method according to any one of claims 1 to 8.

10. Motor vehicle comprising an electric machine and a battery of accumulators (1) which is adapted to supply current to said electric machine and which conforms to claim 9.

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