Method for determining the temperature of an electrochemical cell of a storage battery

Abstract

The invention relates to a method for determining a temperature datum of an electrochemical cell of a storage battery, comprising steps of: - applying a sinusoidal or square-wave voltage at a predetermined frequency across the terminals of the electrochemical cell, - measuring the resulting voltage and current across the terminals of the electrochemical cell, - calculating a value (PR) relating to the impedance of the electrochemical cell, - determining the temperature datum on the basis of the calculated value.

Description

DESCRIPTION TITLE OF THE INVENTION: METHOD FOR DETERMINING THE ELECTROCHEMICAL CELL TEMPERATURE OF A BATTERY CELL TECHNICAL FIELD OF THE INVENTION

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

[0002] It relates more specifically to a method for determining a temperature value of an electrochemical cell in a battery.

[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 vehicles generally have 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 heavily on the temperature of the electrochemical cells, and therefore on the ambient temperature. This capacity is particularly degraded at low temperatures, which can be especially problematic for vehicles used in the coldest geographical areas.

[0008] It is therefore essential to determine the lowest and highest temperatures within the electrochemical cell pack 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 warmer 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 top face of one of the central cells of each module, where the temperature is likely to be 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 some distance from the temperature sensor. More generally, it does not allow for optimal control of the electrical power supplied or stored, since its limited accuracy 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 specifically, 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 afterwards), - calculation of a value relative to the impedance of said electrochemical cell, - determination of the temperature data based on 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 it is able to provide a temperature at the heart of the electrochemical cell.

[0018] It is also quick to implement, making it possible to determine the individual temperatures of the cells.

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

[0020] It also allows the detection of 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 ​​denoted 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 casing 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 different features, variants and embodiments of the invention can be combined with each other in various ways as long as they are not incompatible or mutually exclusive. DETAILED DESCRIPTION OF THE INVENTION

[0025] The description that follows, 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] Regarding the attached drawings:

[0027] Figure 1 is a schematic perspective view of electrochemical cells of a battery of accumulators according to the invention;

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

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

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

[0031] Figure 5 is a graph illustrating the temperature gradients of 96 electrochemical cells in a battery pack.

[0032] Figure 1 shows a very schematic representation of part of a battery of accumulators, hereafter referred to as traction battery 1.

[0033] Here, and preferentially, this traction battery 1 is intended for use within a motor vehicle.

[0034] This motor vehicle could be of any type (truck, bus). Preferably, it will be a land vehicle, in this case a car, which typically includes 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 casing which houses all of its components, including 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, these electrochemical cells (10) number around one hundred, so that the electric motor 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 (around 200, for example) or less.

[0040] In Figure 1, not all the electrochemical cells are visible. However, they are arranged in several groups of cells called "modules 2." For example, there are eight modules 2 of twelve electrochemical cells, which are electrically connected to each other in series to form a pack of modules 2. Each module 2 has a frame (not shown) that holds the cells stacked against each other. This frame is open on its underside.

[0041] The traction battery housing 1 includes 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, passing through this support plate 20, and comprising a pump to circulate the coolant and a heat exchanger to reduce its temperature.

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

[0043] Of course, as an alternative, the support plate could be placed above the cells or on the side.

[0044] Preferably, a layer of thermal paste is placed between this support plate 20 and each of the electrochemical cells 10, in order to optimize 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 liquid cooling.

[0046] Additional 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 positioned 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, includes a processor (CPU), memory, and various input and output interfaces. Its memory stores data used in the process described below. Specifically, 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 fulfills several functions, including: - 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 provides 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 demanding at high or low temperature.

[0051] To achieve this, the control 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. However, those located at the two ends of the stack will be cooled more than the others.

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

[0055] It is also understood that each electrochemical cell 10 will exhibit a non-uniform temperature, lower on its lower face against the support plate 20 than on its upper face. Each electrochemical cell 10 will therefore be said to exhibit a temperature gradient, illustrated in Figure 1 by the arrows F1. 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 Figure 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 do this, an electrochemical impedance spectroscopy measurement, better known by its acronym EIS (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 for the examination of the complex internal chemistry of electrochemical cells. It consists of: - applying a sinusoidal voltage signal ll(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 allows us to plot on the Nyquist diagram in Figure 2 the variation of the impedance Z as a function of frequency, in the complex plane.

[0060] It is worth recalling in this regard that the relationship between impedance Z, resistance R (corresponding to the real part of the impedance represented by the x-axis) and 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 Figure 2, this operation can be repeated with other temperatures. It is therefore possible to plot several curves, each associated with an internal cell temperature. To facilitate the reading of this figure, only three curves for temperatures of 10°C, 0°C, and 20°C have been shown here.

[0064] Figure 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] We can then describe two embodiments of the invention, 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, the impedance Z of an electrochemical cell 10 is characterized 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 f1. This frequency f1 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 Figure 2, which illustrates a detail of the one on the left side, it is the real part PR of the impedance Z that is considered here, since it allows for more precise results.

[0070] Figure 2 shows the frequency responses of a test electrochemical cell 10 obtained in the laboratory, for frequencies ranging from 0.1 Hz to 5000 Hz and for three different internal temperatures: Tcora = -10°C, Tcor2 = 0°C, Tcon = 20°C.

[0071] It is understood here that it will not be necessary to plot these frequency responses, since only a frequency f1 will need to be considered.

[0072] It is also understood that it will be possible, and even preferable, to determine at this frequency f1 the real part PRTCOH of the impedance Z of this cell for a larger number of different Tcon temperatures, 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 maintains a uniform temperature.

[0074] Therefore, based on the responses obtained at different Tcon temperatures, it will be possible to create a map or calculate a mathematical function. allowing, from a measured real part PR, to obtain an approximation of the internal temperature T cor of the electrochemical cell.

[0075] Here, the plan is to construct a mathematical interpolation function Fi that associates each real part PRTCOH with an internal temperature T cor Preferably, it is a polynomial function, preferably of order 2. This function is illustrated in Figure 3. We observe that it passes very close to the points obtained during the tests carried out in the laboratory, 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 load (charging and discharging) and the management system monitors the temperatures of the electrochemical cells, it can measure the impedance Z of each electrochemical cell 10 of the battery at a frequency f1, deduce the value of the real part PR of this impedance Z, and then, using the interpolation function, deduce the internal temperature T cor of the electrochemical cell.

[0078] It should be noted here that the internal temperature T cor The temperature obtained is determined based on 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 1.

[0081] We can then describe a second embodiment allowing us to obtain the minimum temperature and the maximum temperature 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] We will also consider 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 AT deviation of temperatures 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 stacked 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 Figure 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 f1, 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 f1 and a coolant temperature measurement, and deduce the temperature difference AT using the data characterizing the FIAT interpolation curves.

[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 accumulator 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 accuracy (on the order of 2°C), it can control the charging or discharging of the battery to its best potential.

[0093] The temperatures obtained also allow for the detection of internal battery defects.

[0094] Typically, the management system can issue an alert if the AT temperature difference between the upper and lower edges of one of the electrochemical cells 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 other cells.

[0095] Typically, in Figure 5, we have represented a case of thermal runaway of one of the cells (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] This figure clearly shows 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 from 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, 3, 4, or 5 successive times) to avoid false detections. It follows that problem detection will be faster the higher the frequency at which the temperatures of the electrochemical cells are calculated. Here, since the AT deviation calculation is repeated every 3 seconds, it will be possible to perform this detection in less than 15 seconds, or even less than 10 seconds.

[0099] Of course, the two embodiments can be combined, so that it is possible to determine not only this difference AT but also the internal temperature T cor 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. Method for determining a temperature value (T cor , AT) of an electrochemical cell (10) of a rechargeable battery, comprising the following steps: - application across the terminals of said electrochemical cell (10) of a sinusoidal or rectangular voltage (U(t)) at a predetermined frequency (f1), - measurement across the terminals of said electrochemical cell (10) of the resulting electrical voltage (V) and current (I), - calculation of a value (PR) relative to an impedance (Z) of said electrochemical cell (10), - determination of the temperature data (T cor , AT) depending on the calculated value (PR).

2. A method for determining according to claim 1, wherein, at the determination step, the temperature data (T cor, AT) is determined by comparing the calculated value (PR) with predetermined values ​​(PRTCOH) each associated with a temperature data point (T cor , AT).

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

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

5. Method of determination according to claims 3 and 4, wherein several interpolation functions (FIAT) are provided which associate predetermined values ​​(PRTCOH) 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. Method of determination 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. A method for determining 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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