Method for charging a vehicle battery

Abstract

The invention relates to a method for charging at least one electrochemical cell (4) of a vehicle battery (2) (1) from an initial charge level, comprising the steps: i) charging said electrochemical cell to a first charging current intensity, then ii) as soon as a voltage measured across said electrochemical cell reaches a voltage threshold, decreasing the charging current to another predetermined intensity, and then iii) charging said electrochemical cell to said other charging current intensity, steps ii) and iii) being repeated with increasingly higher voltage thresholds and increasingly lower current intensities. According to the invention, at least one of said voltage thresholds is determined as a function of the initial charge level of the cell. Figure for the abstract: Fig. 1

Description

Title of the invention: Method for charging a vehicle battery Technical field of the invention

[0001] The present invention relates generally to electric or hybrid motor vehicles.

[0002] It relates more particularly to a method of charging at least one electrochemical cell.

[0003] The invention finds a particularly advantageous application in the rapid recharging of accumulator batteries of motor vehicles.

[0004] It also relates to a motor vehicle equipped with a battery and a computer adapted to implement such a process. Prior art

[0005] A hybrid or electric powered car ordinarily comprises an electric motor and a high-capacity battery.

[0006] Such a battery typically comprises a large number of electrochemical cells connected together to deliver a desired electrical voltage, ranging from a few tens of volts for some to several hundred volts for others. These electrochemical cells must be recharged regularly. One solution is to connect the car's battery to a public charging station capable of delivering high charging currents. This is known as fast charging.

[0007] A major problem affecting such a battery is the phenomenon of "lithium plating," which refers to the electrodeposition of lithium on the anode of each electrochemical cell of the battery during rapid charging. This phenomenon of lithium electrodeposition on the anode of an electrochemical cell irreversibly degrades the performance of that cell and increases the risk of internal short circuits, potentially leading to various problems such as thermal runaway.

[0008] To date, various approaches have been explored to solve this problem. One of them proposes to limit the intensity of the charging current of the electrochemical cells of the battery.

[0009] The idea is to initially use high currents to ensure rapid charging. The electrical potential at the anode then gradually decreases until it reaches a threshold known as "lithium plating," that is, a threshold below which lithium precipitation begins. This threshold is practically 0V.

[0010] Once this threshold is reached, the battery charging current is then modified to decrease it, thereby reducing the ohmic contribution to the anode polarization. In other words, this decrease in the charging current causes an increase in the electrical potential at the anode, which allows the battery to continue charging without risk of lithium precipitation, but at a slower charging rate.

[0011] This decrease in charging current intensity is limited so as not to excessively increase the battery charging time. As a result, the electrical potential at the anode decreases again and once again reaches the "lithium plating" threshold.

[0012] So, each time this potential reaches this threshold, the intensity of the charging current is further decreased and the process is repeated until the electrochemical battery cells are fully charged.

[0013] In practice, it is not easy to measure the electrical potential at the anode of each electrochemical cell. Therefore, the switching from one charging current intensity to a lower one is based on the voltage measured across the electrochemical cells. It is thus planned to switch from one charging current intensity to another as soon as the measured voltage reaches one of several predetermined levels.

[0014] This solution proves effective in that it prevents the phenomenon of "lithium plating" from occurring.

[0015] The drawback is that the charging time is not as fast as one might expect, given the electrical power that the charging station is theoretically capable of delivering and that the battery is theoretically capable of receiving. However, charging time is a key factor in the public's acceptance of electric vehicles. Presentation of the invention

[0016] In order to remedy the aforementioned drawback of the prior art, the present invention proposes to modulate the intensity of the charging current according to the initial charge level of the electrochemical cell of the battery, that determined at the time of the start of charging.

[0017] More specifically, the invention proposes a method for charging at least one electrochemical cell of a motor vehicle battery, comprising the following steps: - acquisition of the initial charge level of the electrochemical cell, i) charging of said electrochemical cell to a first charging current intensity, then ii) as soon as a voltage measured across said electrochemical cell reaches a voltage threshold, the charging current drops to another determined intensity,

[0018] iii) charging said electrochemical cell to said other charging current intensity, steps ii) and iii) being repeated with increasingly higher voltage thresholds and increasingly lower intensities (preferably until the end of charging), at least one of said voltage thresholds being determined as a function of the initial charging level.

[0019] As explained above, the system is designed to switch from one charging current intensity to another as soon as the measured voltage reaches one of several predetermined voltage thresholds. In the prior art, these voltage thresholds are chosen to ensure the fastest possible charging from a low charge level (depending on the manufacturer: 0% or 15%) to a high charge level (depending on the manufacturer: 80% or 100%).

[0020] Thus, according to the prior art, for a complete charge from a low charge level, a high charging current is initially applied. This initial charging current is maintained for a relatively long time since the electrochemical cell is initially in a relaxed (non-polarized) state. However, as soon as the voltage reaches the first predetermined threshold, the current drops and is maintained for only a short time: the drops in charging current occur more rapidly. Indeed, with each drop, the electrochemical cell does not return to its initial relaxed (non-polarized) state.

[0021] According to the prior art, when the electrochemical cell is initially at an intermediate charge level (between 20% and 80%), the initial charging current to be applied is determined, for example, solely based on the voltage measured across the cell. Then, during charging, as the measured voltage initially increases rapidly and quickly reaches a threshold, the charging current must be rapidly reduced.

[0022] However, when the electrochemical cell is initially at an intermediate charge level, it is theoretically possible to wait longer before triggering the first decrease in charging current intensity. Indeed, the electrical potential at the anode of this electrochemical cell is, at the beginning of charging, always quite high.

[0023] Thus, thanks to the invention, it is planned to take into account the initial charge level in order to best adjust the voltage thresholds at which the intensity of the charging current must be reduced.

[0024] In other words, starting at an intermediate initial charge level from a relaxed (non-polarized) state of the cell will allow us, with respect to to the solution used in the prior art, to overcome the cumulative polarization resulting from the charging stages at SOC lower than that of the start.

[0025] Calculations show that this method makes it possible to halve the charging time from a charge level of 70% to a charge level of 100%.

[0026] Other advantageous and non-limiting features of the charging method according to the invention, taken individually or in all technically possible combinations, are as follows: - each of the said voltage thresholds is determined according to the initial load level; - in step i), said first intensity is independent of the initial charge level and of the voltage measured across said electrochemical cell; - said first intensity is not the same regardless of the initial charge level;

[0027] It is only in the example shown that it is always identical, regardless of the starting SOC; - each of the aforementioned other intensities is independent of the initial charge level; - from one repetition of steps ii) and iii) to the next, the difference between the tensions measured at the beginning and at the end of step iii) is increasingly greater; - said voltage thresholds are read in a map which associates with each initial load level a list of said voltage thresholds; - said mapping associates each voltage threshold with a different intensity; - it is planned to measure the temperature of said electrochemical cell; - at least one of the said voltage thresholds is determined as a function of the measured temperature; - said mapping associates with each pair of initial charge level and temperature a list of said voltage thresholds.

[0028] The invention also proposes a motor vehicle comprising: - at least one battery pack equipped with several electrochemical cells, - at least one electric motor adapted to be powered by said battery of accumulators, - at least one terminal block suitable for connection to a charging station to charge electrochemical cells, and - a computer programmed to implement a charging process as described above.

[0029] Of course, the different features, variants and embodiments of the invention can be combined with each other in various ways, provided that they are not incompatible or mutually exclusive. Detailed description of the invention

[0030] The following description, with reference 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 implemented.

[0031] On the attached drawings:

[0032] [Fig-1] is a schematic side view of a motor vehicle conforming to the invention;

[0033] [Fig.2] represents, on a first graph, the variations of the anode potential of a battery cell of the motor vehicle in [Fig.1] as a function of the charge level of the cell during a fast charge, on a second graph, an enlarged area of ​​the first graph, and on a third graph, the variations of a charging current rate applied to this battery cell during this fast charge;

[0034] [Fig.3] represents, on a first graph, the variations of a charge level of the battery cell as a function of the charging current rate applied to this battery cell throughout a fast charge from a zero charge level, on a second graph, the variations of a voltage across the terminals of the battery cell as a function of its charge level during this fast charge, and, on a third graph, the variations of the anode potential as a function of the charge level during this fast charge;

[0035] [Fig.4] represents, for several rapid charging cycles starting from levels of distinct initial charges, the variations in the charge level of the battery cell as a function of the charging current rate applied to that battery cell.

[0036] In [Fig.1], a motor vehicle 1 is shown.

[0037] This motor vehicle could be of any type (truck, bus, ship, airplane...). Here, it is a car which conventionally includes wheels and a chassis which supports, in particular, a powertrain and bodywork elements.

[0038] This motor vehicle 1 may be of the hybrid or electric type. Therefore, the powertrain includes at least one electric drivetrain.

[0039] This electric traction chain includes at least one battery of accumulators (called "traction battery 2" or battery pack), at least one electric machine 5 supplied with electric current by the traction battery 2, and at least one terminal block 6 for charging the traction battery 2.

[0040] The charging terminal 6 is, for example, in the form of a power outlet adapted to be connected to an electric charging station capable of delivering high electrical power. This power outlet is then adapted to deliver the electrical current from this charging station to the traction battery 2.

[0041] The traction battery 2 can be of any type. Here it is of the Lithium-Ion type. It comprises a housing 3 which contains one or more modules, each of these modules themselves comprising several electrochemical cells 4. These electrochemical cells 4 are preferably all identical.

[0042] The electrochemical cells 4 are, in the example considered here, of the NCM-Graphite type. Thus, each comprises a casing (here flexible) from which two electrical terminals emerge and which contains an electrolyte as well as a graphite anode and a cathode connected respectively to the two electrical terminals. The cathode is composed here of nickel (Ni), cobalt (Co), and manganese (Mn) in specific proportions (for example, 60%-20%-20%).

[0043] Each electrochemical cell 4, once charged, has a voltage across its terminals on the order of a few volts, here approximately 3 volts. The electrochemical cells 4 are then all electrically connected together, in particular in series, so that the traction battery 3 can have a high voltage, preferably greater than 40 volts and even more preferably greater than 100 volts.

[0044] Each electrochemical cell 4 of the traction battery 2 has a state of charge (SOC), which is defined here as the ratio between the instantaneous capacity of that cell and its nominal capacity. This state of charge is expressed as a percentage.

[0045] This traction battery also includes an electronic control unit BMS (Battery Management System), hereinafter referred to as BMS 7. A single BMS 7 is provided here for the entire battery, but alternatively more could be provided (for example one per module).

[0046] This BMS 7 allows measurements of the characteristics of the electrochemical cells 4. It is thus adapted to measure in particular the voltage Ucen across the terminals of each of these cells, as well as the internal temperature of these cells.

[0047] The BMS 7 also allows the voltage Uceii applied to the electrochemical cells 4 to be regulated.

[0048] This BMS 7 comprises for this purpose a processor, a storage unit (hereinafter referred to as memory), and a communication interface. It thus forms a computer.

[0049] Thanks to its interface, it is suitable for communicating with the charging station.

[0050] Its memory records data used in the process described below.

[0051] It records in particular a computer application, consisting of computer programs comprising instructions whose execution by the processor allows the implementation by the BMS 7 of the process described below.

[0052] This method will be described with reference to only one of the electrochemical cells 4, but it will of course apply in the same way to all the electrochemical cells 4 of the traction battery 2.

[0053] In the following, the expression "charging current" will refer to the current transmitted by the charging station and supplying in particular the electrochemical cell 4 under consideration.

[0054] The expression "charge current rate" will refer to the ratio of the intensity of the charge current delivered to the electrochemical cell 4 under consideration to the intensity that would theoretically have to be delivered to the electrochemical cell 4 to recharge it over a complete cycle in one hour.

[0055] A complete cycle refers to the charging of the electrochemical cell 4 from a low charge level to a high charge level. The values ​​of these low and high charge levels will be chosen by the manufacturer. Typically, the manufacturer may consider a low charge level to be 0% or 15%, and a high charge level to be 80% or 100%.

[0056] The voltage across this cell will correspond to the potential difference between the cathode and the anode. It will be approximated that this voltage is identical across all the electrochemical cells 4. It will also be approximated that the charge level is the same for all the electrochemical cells 4.

[0057] The process described below explains how this electrochemical cell 4 can be charged rapidly.

[0058] It comprises several steps implemented successively.

[0059] In a preliminary step, the traction battery 2 is connected to the charging station, via its housing 6, and the user selects a charging cycle. The selected charging cycle is preferably "fast".

[0060] Fast charging is defined as a charging current delivered to the traction battery 2 exceeding 100 Amperes. It can thus reach 500 Amperes, for example.

[0061] Therefore, the BMS 7 measures the voltage Ucen across the terminals of the electrochemical cell 4 under consideration. In practice, this measurement is carried out during the entire charging process (continuously or regularly, at short time intervals).

[0062] The first step then implemented by the BMS7 occurs at the beginning of the charging process, even before a charging current is delivered by the charging station.

[0063] This first step consists of determining the initial charge level SOC0 of the electrochemical cell 4. This determination is possible because it is carried out at a time when the cell is neither charging nor discharging.

[0064] This first step can be carried out in various ways. For example, the initial charge level SOC0 can be calculated solely based on the voltage Ucen across this cell, given a function that maps a charge level to each voltage. Alternatively, this calculation could be performed based on the state of charge if it is known. Alternatively still, a more precise, taking into account at least one other factor, such as temperature, age or the health status of the cell...

[0065] The second step consists of controlling the recharging of the electrochemical cell 4 to a first intensity i0 of charging current.

[0066] During charging, the voltage Uceii across said electrochemical cell 4 increases.

[0067] Then, as soon as the measured voltage Ucen reaches a first voltage threshold Sui, the BMS 7 commands a first reduction in the charging current intensity to another intensity h lower than the first intensity i0. This other intensity h is maintained until the measured voltage Uœn reaches another voltage threshold Su2 (strictly higher than the first). Then the current is reduced again. And the process is repeated in this way until the end of charging.

[0068] The idea is to reduce the charging current when the electrical potential of the anode is estimated to be equal to, or too close to, a threshold below which the phenomenon of "lithium plating" would occur. This threshold is 0V here, but a margin is provided to ensure that it is never reached.

[0069] Indeed, as shown in graph 2a of [Fig.2], during charging, the Vano electric potential of the anode decreases until it approaches the threshold.

[0070] As shown in detail in Figures 2b and 2c of [Fig. 2], the objective is that, when this potential reaches the predetermined threshold (here of 35 mV), the intensity of the charging current (represented here by the charging current rate Tich) is reduced, causing this potential to rise sharply. Then, as charging continues, the electrical potential at the anode decreases again, so that a further decrease in the charging current intensity must be commanded.

[0071] The charging cycle can then be generalized by considering that it is a repetition of two steps, one consisting of charging the electrochemical cell 4 to an intensity ij, and the other consisting, as soon as the voltage Uceii reaches a voltage threshold Suj, of ordering a decrease in the charging current to another intensity ij+1 (the index j being a natural number greater than or equal to 1).

[0072] As shown in Figure 3b of [Fig. 3], the voltage Ucen therefore increases in a sawtooth pattern since a slight decrease occurs after each reduction in charging current. Figure 3c illustrates the decrease in the electrical potential at the anode of the electrochemical cell 4 as a function of the charge level. It can be observed that this decrease is initially large, in that the cell is initially in a relaxed (unpolarized) state and its value is initially high. Subsequently, it remains essentially constant until the end of charging, varying in a sawtooth pattern above the 35 mV threshold.

[0073] It should be noted here that the end of charging corresponds to the moment when the traction battery has reached a desired charge level, typically around 80%, or when the user wishes to end the charging.

[0074] Fig. 3 illustrates, on its first graph 3a, the intensity (here represented by the charge current rate Tich) that can be applied to the electrochemical cell as a function of the charge level SOC of this cell.

[0075] It is observed that the charging current rate Tich is, at the beginning of charging, 2, until the charging level SOC reaches 25% (which corresponds to the first voltage threshold), then it decreases in steps at each new voltage threshold, here until reaching the value of 0.5 at the end of charging.

[0076] According to a particularly advantageous feature of the invention, at least one of the voltage thresholds is determined as a function of the initial load level SOC0.

[0077] In practice, it can be foreseen that the first (lowest) voltage threshold is determined as a function of the initial load level SOC0. Preferably, all voltage thresholds will be determined as a function of the initial load level SOC0.

[0078] Indeed, the curve illustrated on the first graph 3a represents here the variation of the intensity that can be applied to the electrochemical cell as a function of the SOC charge level of this electrochemical cell, from a low charge level.

[0079] On [Fig.4], the curves shown represent, on the other hand, the variations in intensity that can be applied to the electrochemical cell 4 as a function of the charge level SOC of this electrochemical cell, from different initial charge levels SOC0.

[0080] It is noted here that the first intensity i0 applied at the beginning of charging is always the same regardless of the initial charge level SOC0 and the voltage across the terminals of the electrochemical cell: the initial charging current rate Tich is thus 2.

[0081] Thus it is possible to take advantage of the fact that the electrochemical cell 4 is initially non-polarized to apply a high charging current intensity to it, regardless of its initial charge level SOC0.

[0082] For this, it is understood that it is necessary that at least the first of the voltage thresholds be adapted to the initial load level SOC0.

[0083] In practice, here all voltage thresholds are determined as a function of the initial load level SOC0.

[0084] In other words, once the initial charge level SOC0 is known, the BMS 7 is expected to read from its memory the voltage thresholds that it will have to consider, depending on the initial charge level SOC0.

[0085] It is also noted on [Fig.4] that not only is the first intensity i0 to be applied at the start of charging independent of this initial charge level SOC0, but that the same is true of the other intensities ij to be applied subsequently.

[0086] Thus, even if the voltage thresholds are not the same depending on the initial load level SOCo, the currents i0, ii...ij successively applied upon reaching each voltage threshold are the same. Of course, these currents could alternatively differ depending on the initial load level SOC0.

[0087] It is also noted on graph 3b of [Fig.3] that the voltage thresholds are increasingly separated two by two as the voltage Ucen increases.

[0088] At this stage, we can describe in more detail how the BMS 7 proceeds to vary the charging intensity.

[0089] After determining the initial charge level SOC0 and the temperature of the electrochemical cell 4 considered, the BMS 7 reads from a map the voltage thresholds Suj that it will have to consider and the currents ij that it will have to apply once each of these thresholds is reached.

[0090] This mapping allows us to associate with each pair of initial charge level SOCo and electrochemical cell temperature, a list of voltage thresholds Suj and currents ij.

[0091] Then the BMS 7 applies the first intensity i0.

[0092] Then, as soon as the measured voltage Uceii reaches the first voltage threshold Su i, the BMS 7 reduces the charging current to an intensity ii (or it sends a request to the charging station to that effect).

[0093] It then repeats the same procedure until the end of charging, taking into account the voltage thresholds Su j lus and the corresponding current ij.

[0094] 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.

[0095] Typically, the value of the first applied current i0 could be chosen according to the initial charge level SOC0 and / or the voltage Ucen.

[0096] According to another embodiment of the invention, the mapping used could be simpler and associate with each initial charge level SOC0, a list of voltage thresholds Suj and currents ij. Thus, the temperature of the electrochemical cell 4 would not be taken into account.

[0097] Alternatively, the mapping used could be even simpler and associate each initial load level SOC0 with a list of voltage thresholds Suj. Since the currents ij to be applied do not depend on the initial load level, they could be stored elsewhere in the BMS 7 memory.

[0098] According to another variant, the mapping used could associate with each pair of initial charge level SOC0 and temperature, a list of voltage thresholds Suj. Here again, the currents ij to be applied would then be stored elsewhere in the memory of the BMS 7.

[0099] Alternatively, the electrochemical cell could be used alone. It could also be used in an application other than automotive, for example in a mobile phone.

Claims

Demands

1. A method for charging at least one electrochemical cell (4), comprising the steps: i) of charging said electrochemical cell (4) to a first charging current intensity (i0), then ii) as soon as a voltage (Uceii) measured across said electrochemical cell (4) reaches a voltage threshold (Suj), of lowering the charging current to another intensity (ij), iii) of charging said electrochemical cell (4) to said other charging current intensity (ij), steps ii) and iii) being repeated with increasingly higher voltage thresholds (S^) and said other intensities (ij) increasingly lower, characterized in that, prior to step i), there is a step for acquiring an initial charge level (SOC0) of the electrochemical cell (4), and in that at least one of said voltage thresholds (S^) is determined as a function of the initial charge level (SOC0).

2. A charging method according to claim 1, wherein each of said voltage thresholds (S^) is determined as a function of the initial charge level (SOC0).

3. A charging method according to any one of claims 1 and 2, wherein each of said other intensities (ij) is independent of the initial charge level (SOC0).

4. A charging method according to any one of claims 1 to 3, wherein, from one repetition of steps ii) and iii) to the next, the difference between the voltages (Uœn) measured at the beginning and at the end of step iii) is increasingly greater.

5. A charging method according to any one of claims 1 to 4, wherein said voltage thresholds (Suj) are read in a map which associates to each initial charge level (SOC0) a list of said voltage thresholds (Suj).

6. A charging method according to claim 5, wherein said mapping associates with each voltage threshold (Suj) another intensity (ij).

7. A charging method according to any one of claims 1 to 6, wherein it is provided to measure the temperature of said electrochemical cell (4) and wherein at least one of said voltage thresholds (Suj) is determined as a function of the measured temperature.

8. A charging method according to claims 5 and 7, wherein said mapping associates with each pair of initial charge level (SOCo) and temperature a list of said voltage thresholds (Suj).

9. Motor vehicle (1) comprising: - at least one accumulator battery (2) equipped with several electrochemical cells (4), - at least one electric motor (5) adapted to be supplied with current by said accumulator battery (2), - at least one casing (6) adapted to be connected to a charging terminal for charging the electrochemical cells (4), and - a computer (7) programmed to implement a method of charging the electrochemical cells (4) according to any one of claims 1 to 8.

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