Method for charging a storage battery

Abstract

This method for charging a storage battery comprising a set of cells comprises the following steps: - identifying (step E1) the state of charge and the temperature of the battery; - applying a charging current (step E2) to the battery in accordance with a first map; - determining a maximum voltage applicable across the cells of the battery based on a second map (step E3); - measuring the effective voltage (step E4) of the cell having the highest voltage out of the set of cells of the battery; - comparing the measured effective voltage with the applicable maximum voltage (step E5); and - modifying the applied charging current (step E6) when the effective voltage becomes greater than or equal to the applicable maximum voltage in accordance with a third map.

Description

[0001] DESCRIPTION

[0002] TITLE: Battery Charging Method Technical Field

[0003] The present invention relates to the charging of an electrical energy storage device.

[0004] In particular, the present invention relates to the optimization of the charging speed of an electric accumulator battery, in particular a lithium ion accumulator battery of the LFP (lithium, iron, phosphate) type.

[0005] These batteries are used, for example, to power the motorization system of an electric motor vehicle.

[0006] Previous techniques

[0007] An electrical accumulator battery is generally made up of a set of cells, called elementary cells, connected in series. The cells have a cell voltage, denoted U, which must not exceed a certain threshold, denoted Umax, otherwise they will be damaged.

[0008] Furthermore, a battery has a computer, called BMS (for "Battery Management System"), which allows, among other things, the balancing of the charge states of the different cells and the knowledge of the different parameters of intensity and voltage circulating in the battery.

[0009] Charging, and in particular the charging speed of a battery, is a major issue in all sectors, especially the automotive sector.

[0010] Indeed, the charging speed directly impacts the number of kilometers of vehicle range in a given time, representing an important performance and sales argument.

[0011] However, an exaggeration of the charging power, for example inducing a voltage U that is too high, can impact the durability of the battery and can lead to its sudden destruction.

[0012] In the case of NMC (nickel, manganese, cobalt) type lithium ion batteries, the maximum voltage threshold Umax not to be exceeded is substantially constant for all battery state of charge levels.

[0013] It is worth recalling here that the state of charge represents a percentage, between 0% and 100%, of the battery's or cell's capacity. This capacity is expressed in Ah (amp-hours), and is determined by the BMS (Battery Management System) using a method known to the user, typically a coulometric counting method. The state of charge is also referred to as SOC (state of charge).

[0014] The battery cell manufacturer typically recommends a charging current based on the battery's state of charge. The induced voltage is also indicated.

[0015] Figure 1 shows an example of the charging current I to be applied to a battery cell according to the manufacturer's recommendations, along with the associated voltage U. Note that the times on the x-axis are not proportional to the exact times.

[0016] In a first PI phase, up to a given charge state threshold, the applied current I is substantially constant and the voltage U increases until it reaches a maximum value which will be maintained at a substantially constant voltage value thereafter.

[0017] In a second, shorter phase P2, until a practically complete state of charge, the recommended charging current I decreases so as not to further increase the voltage value U.

[0018] Finally, there is a third phase P3 corresponding to the very last percentages of state of charge before full charge, where the intensity I is reduced. This phase P3 is not the subject of the present invention.

[0019] To better understand why the current I must decrease, the cell can be modeled by an equivalent electrical circuit consisting of a voltage source equivalent to an open-circuit voltage, denoted OCV (for "open circuit voltage"), which depends on the state of charge SOC, in series with an internal resistance, denoted Rceii. The cell voltage, denoted U, then follows the following equation: U = OCV(SOC) + Rceii • I, where: I is the current flowing through the cell, positive during charging and negative during discharging.

[0020] To ensure durability and avoid the risk of cell deterioration, the cell voltage U must not exceed a maximum threshold Umax.

[0021] It is therefore necessary that the current I be decreased as the state of charge of the battery increases since the open circuit voltage is an increasing function of the state of charge and the cell voltage must remain below Umax.

[0022] For new lithium-ion battery chemistries, typically the so-called lithium, iron, phosphate chemistry, the problem is similar, with an increase in open-circuit voltage as a function of the state of charge, although said open-circuit voltage has a different behavior with a wide range of state of charge, typically from 30% to 98%, for which the open-circuit voltage of the cell is almost constant.

[0023] Figure 2 shows, for comparison purposes, an open circuit voltage OCVNMC of an NMC cell as a function of its state of charge SOC, and an open circuit voltage OCVLFP of an LFP cell as a function of its state of charge SOC.

[0024] The constant open-circuit voltage OCVLFP of the LFP cell results in a lower need for current decay I as the state of charge SOC increases, all other things being equal.

[0025] Furthermore, it has been observed that LFP cells are more sensitive to voltage than NMC cells when the state of charge (SOC) increases. In other words, the maximum voltage threshold (Umax) decreases as the SOC increases.

[0026] For this LFP chemistry, the cell manufacturer recommends applying a charging current I as a function of the state of charge (SOC), as illustrated in Figure 3, taking into account the decrease in Umax as the SOC increases. In practice, when following the cell manufacturer's recommendations, the resulting curve of current I and voltage U as a function of the SOC is shown in Figure 4.

[0027] The first phase, P1, is similar to that shown in Figure 1 but significantly shorter. In this phase, a constant current (I) is applied, and the voltage (U) increases until it reaches a maximum voltage for a specific state of charge (SOC1). A second phase, P2, then continues almost to full charge, at which point the maximum permissible voltage (Umax) decreases. The current must therefore be reduced accordingly. However, this second phase, P2, is considerably longer than for NMC-type batteries.

[0028] As with NMC batteries, there is also a very short third phase P3 at the end of charging.

[0029] Implementing a battery charge based on these curves recommended by the cell manufacturer does not, however, allow for optimal battery charging speed given the constraint of not exceeding the maximum voltage threshold Umax.

[0030] On the other hand, it should be noted that the position of the different points of the open-circuit voltage curve as a function of the state of charge also depends on the temperature and the state of aging of the battery, also called SOH ("State of Health" in Anglo-Saxon terms), with the exception of the 100% charge point.

[0031] It is recalled here that the aging of the SOH battery represents the percentage of battery capacity (in Ah) that the battery retains compared to a battery in new condition, noted SOHQ.

[0032] Alternatively, it can also be the percentage of residual energy (in kWh) that it can deliver compared to a new battery, noted SOHE.

[0033] For these reasons, in the state of the art, the charging current intensity I is determined as a function of the state of charge SOC and the temperature, for example from curves conforming to figure 1 or 3 corresponding to a given temperature.

[0034] Then, the intensity I is adjusted once again to account for SOH aging. More precisely, the intensity is decreased as aging increases because the resistance R ce ii of the cells increases.

[0035] However, the state of charge (SOC) and state of health (SOH) of a cell are not measured but estimated using numerical models. These models must consider margins of error to avoid exceeding the maximum voltage threshold (Umax). These margins of error lead to a decrease in charging power and, consequently, a reduction in the charging speed of an electric vehicle.

[0036] Description of the invention

[0037] The present invention therefore aims to overcome the aforementioned disadvantages and to provide a method for charging a battery cell that is as fast as possible without reducing the reliability of said cell, in particular by reducing the margins of error described above.

[0038] The present invention relates to a method for charging a battery of accumulators comprising a set of cells, the method comprising the following steps:

[0039] - Identification of the battery's state of charge and temperature;

[0040] - Application to the battery of a charging current intensity obtained from an initial mapping of a maximum applicable charging current intensity based on the identified state of charge and temperature of the battery;

[0041] - Determination of a maximum voltage applicable to the battery cells from a second mapping of the maximum applicable voltage as a function of the applied charging current intensity and the identified temperature;

[0042] - Measurement of the effective voltage of the cell with the highest voltage among all the cells in the battery;

[0043] - Comparison of the measured effective voltage with the maximum applicable voltage; and - Modification of the applied charging current intensity when the effective voltage becomes greater than or equal to the maximum applicable voltage, the value of the modified charging current intensity being obtained from a third mapping of the charging current intensity as a function of the maximum applicable voltage and the identified temperature.

[0044] Thus, battery charging is no longer achieved by applying a charging current intensity based on the state of charge, but rather by applying a charging current intensity based on the maximum voltage applicable to the corresponding charging current level. This is particularly relevant for LFP batteries, for which the maximum cell voltage thresholds vary during the second, and longest, charging phase. Furthermore, measuring the actual voltage allows the effects of temperature and aging to be taken into account directly in the charging process without the need for a margin. This results in optimized battery charging.

[0045] Advantageously, the steps of determining a maximum applicable voltage, measuring the effective voltage, comparing the effective voltage with the maximum applicable voltage, and modifying the intensity of the charging current are repeated one after the other until the state of charge of the battery reaches a predefined state, preferably repeated every 100 ms.

[0046] In a particular implementation mode, the applied charging current intensity is increased during the charging current intensity modification step if the battery temperature increases by more than a predefined value.

[0047] In one implementation mode, the first mapping is a mapping of a charging current intensity to be applied to the cells recommended for charging the cells by a manufacturer of said cells.

[0048] Advantageously, the first map includes steps in the value of the charging current. Advantageously, the maximum applicable voltage of the second map is obtained beforehand and empirically by recording in said second map local voltage maxima obtained for each step in the value of the charging current of the first map applied to a cell of a battery, and for different temperatures.

[0049] Advantageously, the third map includes the load current intensity value steps of the first map as a function of the local voltage maximum values ​​recorded in the second map and the temperature.

[0050] In one implementation mode, the battery is a lithium iron phosphate battery.

[0051] Advantageously, the present process is implemented by a battery control system.

[0052] The invention also relates to a battery of accumulators comprising a temperature sensor and a battery control system configured to implement the process as defined above.

[0053] Brief description of the drawings

[0054] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0055] [Fig 1]

[0056] which has already been mentioned, is a representation of the voltage and current intensity of the charging of an NMC battery cell as a function of its state of charge;

[0057] [Fig 2]

[0058] as already mentioned, is a graph representing an example of the open-circuit voltage of an NMC cell as a function of its state of charge and an open-circuit voltage of an LFP cell as a function of its state of charge; [Fig 3]

[0059] as already mentioned, is a graph provided by an LFP cell manufacturer representing the intensity of the charging current to be applied to an LFP battery cell, and the associated voltage, depending on its state of charge;

[0060] [Fig 4]

[0061] which has already been mentioned, is a graph of the intensity of the charging current and the associated voltage applied to a real LFP battery cell according to the recommendations of the graph according to figure 3;

[0062] [Fig 5]

[0063] is a schematic view of a battery according to the invention;

[0064] [Fig 6]

[0065] is a schematic representation of the different stages of the process according to the invention; and

[0066] [Fig 7]

[0067] is a zoomed-in view of a second phase of a graph of charging current intensity and associated voltage applied to an LFP battery cell as a function of its state of charge.

[0068] Detailed description of at least one embodiment

[0069] Figure 5 schematically represents a battery 1 of accumulators comprising a set of cells 3 connected in series.

[0070] Battery 1 is preferably a lithium iron phosphate battery.

[0071] An example of battery 1 is, for instance, a battery 1 comprising between 50 and 150 cells 3, each having a charge capacity of 150 to 200 Ah. The maximum permissible charge of battery 1, denoted Qmax, is the sum of the charge capacities of each cell 3.

[0072] Battery 1 also includes a battery 1 control system 5, mentioned previously and otherwise known as BMS.

[0073] This battery 1 control system 5 is adapted to measure the voltage across each cell 3, otherwise called the voltage of each cell 3, as well as to measure the intensity of the current through the battery 1. In addition, the battery 1 control system 5 is configured to implement the method according to the invention, described below.

[0074] Advantageously, battery 1 also includes a temperature sensor 7, or is connected to an external temperature sensor.

[0075] Figure 6 shows the different stages of the battery charging process 1 according to the invention.

[0076] We first perform a step El to identify the state of charge and temperature of the battery.

[0077] For this, the state of charge is identified by the BMS while the temperature sensor allows the battery temperature to be measured.

[0078] Then, we perform a step E2 of application to the battery of an intensity I of charging current obtained from a first mapping of a maximum applicable charging current intensity depending on the state of charge and the temperature of the battery identified.

[0079] This step E2 is implemented in particular when the battery has just been connected: this step E2 allows to start applying an intensity I of the charging current according to a first classic map such as can be found in the state of the art.

[0080] In particular, the first mapping is a mapping of the charging current intensity to be applied to the cells, the mapping being recommended for charging the cells by the cell manufacturer. Examples of this first mapping are illustrated in Figures 1 and 3, especially Figure 3 with regard to LFP batteries.

[0081] The first mapping includes, particularly during the second phase P2 mentioned earlier, steps in the charging current intensity values. The charging current intensity I thus decreases with the state of charge SOC of battery 1 in steps, thereby delimiting zones.

[0082] We have represented three of these zones in figure 7, said figure 7 illustrating a portion of the second phase P2 with the intensity I of the current decreasing with the state of charge according to three intensity levels II, 12 and 13. Then, we carry out a step E3 of determination of a maximum voltage applicable to the cells of the battery from a second mapping.

[0083] This second map represents the maximum voltage applicable to the 3 cells of the battery as a function of the intensity I of the charging current applied and the identified temperature.

[0084] This second map is preferably created prior to the implementation of the present process and is obtained empirically.

[0085] To create this second map, the first map is necessary, and the current intensity recorded in the first map is applied to a battery, for example, on a test bench. For each charging current intensity level, also identified as zones, the local voltage peaks obtained are measured and recorded in the second map.

[0086] These local voltage maxima are noted Vlmax, V2max and V3 max in the example of figure 7. These local maxima correspond to maximum voltages applicable for each level of intensity of the load current.

[0087] Thus, the second mapping allows us to reinterpret the first mapping and obtain the maximum voltage applicable to the cells 3 of the battery 1 as a function of the intensity I of the charging current applied and the temperature identified.

[0088] Advantageously, an E4 step is performed to measure the effective voltage of the cell having the highest voltage among the set of cells in the battery.

[0089] This measurement step E4 is implemented, for example, by the BMS, which measures the effective voltages across the cells 3 during the charging of battery 1. The effective voltage of the cell with the highest voltage is measured in particular because it ensures that no other cell 3 has a higher effective voltage and is at risk of being damaged. A step E5 is then performed to compare the measured effective voltage with the previously determined maximum applicable voltage.

[0090] If the effective voltage is less than the maximum applicable voltage, the intensity I of the charging current remains unchanged.

[0091] Conversely, when the effective voltage becomes greater than or equal to the maximum applicable voltage, a step E6 is performed to modify the intensity of the applied load current.

[0092] The value of the modified charging current intensity is obtained from a third mapping of the charging current intensity I as a function of the maximum applicable voltage and the identified temperature.

[0093] In other words, the third map includes the load intensity value steps of the first map as a function of the local maximum values ​​recorded in the second map and the temperature.

[0094] Thus, in practice, when the intensity I of the charging current is modified, it is generally reduced to the next level of charging current intensity of the first mapping.

[0095] In a particular implementation, a temperature measurement step is performed simultaneously with step E4, which measures the effective voltage. If the measured temperature has increased by more than a predefined value, for example, more than 1°C, compared to the previous measured temperature, the applied load current is increased during the load current adjustment step. The predefined value prevents interference from insignificant temperature fluctuations.

[0096] Indeed, an increase in the temperature of battery 1 makes it less sensitive to voltage, its maximum permissible voltage threshold Umax being higher.

[0097] Advantageously, steps E3, E4, E5 and E6 of determining a maximum applicable voltage, measuring the effective voltage, comparing the effective voltage with the maximum applicable voltage and modifying the intensity of the charging current are repeated one after the other until the state of charge of the battery reaches a predefined state, for example a state of charge between 95% and 100%.

[0098] Advantageously, steps E3 to E6 are repeated every 100 ms.

[0099] Advantageously, the battery control system 5 includes each map saved in internal memory.

[0100] The present invention finally allows, in particular for LFP batteries and during the second phase P2, the application of a charging current intensity I which is adapted according to a measurement of the effective voltage of the cells (3) of the battery 1.

[0101] Indeed, since the cell voltage is known, it will no longer be necessary to take margins of error into account when calculating the charging current intensity.

[0102] Furthermore, when a cell (3) ages, its internal resistance R ce ii increases, causing an increase in the effective voltage of cell (3), but this will be taken into account automatically during the measurement in step E4.

[0103] As a result, the margins taken when using the state-of-the-art solution will no longer be necessary, and the charging speed will be improved.

Claims

DEMANDS 1. A method for charging a battery (1) of accumulators comprising a set of cells (3), characterized in that it comprises the following steps: Identification (step El) of the state of charge and temperature of the battery; Application to the battery (1) of a charging current intensity (I) (step E2) obtained from a first mapping of a maximum applicable charging current intensity (I) as a function of the state of charge (SOC) and the temperature of the battery (1) identified; Determination of a maximum applicable voltage (Vlmax; V2max; V3max) to the cells (3) of the battery (1) from a second mapping of the maximum applicable voltage as a function of the intensity (I) of the applied charging current and the identified temperature (step E3); Measurement of the effective voltage (step E4) of the cell (3) having the highest voltage among the set of cells (3) of the battery (1); Comparison of the measured effective voltage with the maximum applicable voltage (step E5); and Modification of the intensity (I) of the applied charging current (step E6) when the effective voltage becomes greater than or equal to the maximum applicable voltage, the value of the intensity (I) of the modified charging current being obtained from a third mapping of the intensity (I) of the charging current as a function of the maximum applicable voltage and the identified temperature.

2. Method according to claim 1, wherein the steps (E3; E4; E5; E6) of determining a maximum applicable voltage, measuring the effective voltage, comparing the effective voltage with the maximum applicable voltage and modifying the intensity (I) of the charging current are repeated one after the other until the state of charge of the battery (1) reaches a predefined state, preferably repeated every 100 ms.

3. A method according to any one of claims 1 and 2, wherein the intensity (I) of the applied charging current is increased during the step (E6) of modifying the intensity (I) of the charging current if the temperature of the battery (1) increases by more than a predefined value.

4. A method according to any one of claims 1 to 3, wherein the first mapping is a mapping of a charging current intensity (I) to be applied to the cells (5) recommended for charging the cells by a manufacturer of said cells.

5. Method according to any one of claims 1 to 4, wherein the first mapping includes steps of charge intensity values ​​(11; 12; 13).

6. Method according to claim 5, wherein the maximum applicable voltage (Vlmax; V2max; V3max) of the second map is obtained beforehand and empirically by recording in said second map local voltage maxima (Vlmax; V2max; V3max) obtained for each step of the value of the charging current intensity (11; 12; 13) of the first map applied to a cell (3) of a battery (1), and for different temperatures.

7. Method according to claim 6, wherein the third mapping comprises the load current intensity value steps (11; 12; 13) of the first mapping as a function of the local voltage maximum values ​​(V1max; V2max; V3max) recorded in the second mapping and the temperature.

8. A method according to any one of claims 1 to 7, wherein the battery (1) is a lithium iron phosphate battery.

9. Method according to any one of claims 1 to 8 implemented by a control system (5) of the battery (1).

10. Battery (1) of accumulators comprising a temperature sensor (7) and a control system (5) of the battery (1) configured to implement the method according to any one of claims 1 to 9.

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