EP3991233A1 - Method for optimizing the service life of a battery - Google Patents

Abstract

The invention relates to a method for optimizing the service life of a battery and to a device and a vehicle having means for implementing such a method. The method for managing the state of charge of a battery according to the invention comprises a step of determining a target state of charge as a function of the temperature of the battery, said target state of charge tending, in accordance with a reference degradation profile, to decrease when the temperature increases so as to control the degradation of the battery, and a step of charging the battery until the target state of charge is reached. Advantageously, since the reference degradation profile describes the relative increase in the internal resistance of the battery as a function of the time that has elapsed since the start of life of the battery at a reference state of charge and at a reference temperature, the target state of charge is effectively determined iteratively such that, at the end of a step of estimating the relative increase in the internal resistance, the estimated relative increase tends to remain lower than the relative increase extracted from the reference degradation profile. The device according to the invention is particularly suitable for the lithium batteries of combustion motor vehicles.

Description

Description

Title of the invention: Method for optimizing the life of a battery

The present invention relates to a method for optimizing the life of a battery, as well as a device and a vehicle comprising means for implementing such a method.

[0002] For a century, all motor vehicles have been equipped with a lead-acid service battery, in particular for starting the heat engine. Technological advances and regulatory constraints increasingly require the replacement of these lead-acid batteries by alternative systems. However, this replacement is not easy to make because lead batteries have specific technical characteristics and a limited cost. Among others, the two fundamental qualities of cold power and energy reserve, the latter compensating for standby currents (or "off current" in the English terminology) when the vehicle is in long-term parking. It goes without saying that this ability to start cold must be guaranteed after prolonged parking, that is to say after a partial discharge of the battery. Very conventionally, a lead-acid battery can perfectly start an engine at -200 or -300 after a parking period of 1 or 2 months. Its state of charge (or SOC according to the acronym for "State Of Charge") is still greater than 60% and 20 to 30% of its capacity has been discharged by the "off currents".

[0003] In addition, a lead battery is resistant to the difficult conditions of an engine under-hood where the temperature can reach 800 and where the battery can remain several hours a day at more than 600. This "hot" environment has a significant impact on its aging. However, its low cost makes it possible to change it regularly (every 5 years on average in Europe) like all wearing parts of the vehicle. In addition, the recyclability rate of a lead battery is very high (> 95%) and it can be converted back into new batteries. [0004] It is important to understand that this ability to withstand temperature makes it possible to place it close to the engine and therefore to the alternator and to the starter.

In fact, the cables are short and the resistive losses low.

[0005] Replacing the lead-acid battery with a lithium battery also supposes placing it close to the engine and therefore in a hot zone. However, there are three major drawbacks here.

First of all, the cost of a lithium battery (or LIB technology in the present application) has nothing to do with that of a lead-acid battery (or LAB technology in the present application), it there is at least a cost factor equal to 5 between these two technologies. It is therefore no longer a question of changing it as often, it would not be bearable by customers, its lifespan must be at least 10 years and ideally 15 years, which is the lifespan of an automobile. today. The difficulty and high cost of recycling LIBs, unlike lead, must also be taken into account. In addition, from an environmental protection point of view, it is interesting that these batteries last a long time.

[0007] In addition, the degradation of an LIB, whether in capacity or in power (internal resistance), is accelerated exponentially by the increase in its temperature. This wear is obviously very dependent on the anode technology used: the lithium titanate oxides (or LTO technology in the present application) will withstand much better than graphite (or GRA technology in the present application) for example. However, whatever technology is used, its lifespan will be directly related to its temperature.

[0008] The temperature of the battery cannot be controlled because it depends on two overriding factors, namely the climate of the country and the behavior of the customer. Even if by design, everything is done to thermally protect the battery from the heat of the engine, the simple fact of positioning it in the engine compartment, makes the temperature of the battery dependent on that of the engine. However, the temperature of the engine is directly linked to the behavior of the customer, because with each use, the engine will heat up to 950, then stay there for the duration of travel, then cool down after stopping. During each phase of this cycle, the engine will heat the battery, especially during the hours of cooling. We easily understand that depending on the customer's use, between urban and highway, or depending on the number of runs per day, the battery temperature will be very different.

[0009] It is therefore of great interest to find solutions allowing the lifetime of LIBs to be increased as much as possible with respect to temperature and this is the object of the proposed invention

[0010] Document US8937452 describes, in order to optimize the life of a battery of a vehicle, a method of controlling the state of charge comprising controlling the charge of the battery while remaining within a range of SOC determined according to the predicted battery temperature. However, this process offers limited effectiveness.

[001 1] The method, the battery and the vehicle according to the invention described in this

The application makes it possible to remedy the aforementioned drawbacks, in particular they aim to provide a simple and effective solution for adapting the management of the battery to its temperature in order to correct as much as possible the dispersions of climates and of driver behavior. The invention is based on the knowledge of the typical behavior of LIB technology vis-à-vis the state of charge - temperature combination and how the alternator can be driven. It can be applied equally to systems with one or two batteries in parallel.

To this end, the method of managing the charge of a battery according to the invention comprises a step of determining a target state of charge as a function of the temperature of the battery, said target state of charge tending, according to a reference degradation profile, to decrease as the temperature increases so as to control the degradation of the battery, as well as a step of charging the battery until reaching the target state of charge. Advantageously, the reference degradation profile describing the relative increase in the internal resistance of the battery as a function of the time spent since the start of the life of the battery at a reference state of charge and at a reference temperature, the target state of charge is efficiently determined iteratively such that, at the end of a step of estimating the relative increase in the internal resistance, the estimated relative increase tends to remain lower than the relative increase taken from the reference degradation profile.

[0013] According to particular embodiments described below with reference to the figures introduced below:

- the reference state of charge may be greater than 80%, in particular equal to 100%, and / or in that the reference temperature may be between 100 and 300, in particular equal to 200;

- if, at a given iteration ti, where i positive integer, the estimated relative increase in internal resistance is greater than the relative increase taken from the reference profile, then the target state of charge can be determined so that that, at the following iteration ti _{+ i} , the estimated relative increase tends towards the relative increase drawn from the reference profile;

- two iterations can always be separated by the same predefined time interval and said time interval can always correspond in the reference profile to the same relative increase in internal resistance.

- This same time interval may be between 30 minutes and 2 hours, in particular equal to 1 hour, and / or this same relative increase in internal resistance may be less than 5%, in particular equal to 2%.

- if the battery temperature measured between two iterations ti and ti _{+ i} is greater than the reference temperature, then the target state of charge can be determined at the iteration ti _{+ i} so as to be lower than the state of reference load and the degradation of the internal resistance due to the time interval between the iterations ti _{+ i} and to this lower state of charge may be considered, in the subsequent steps of estimating the relative degradation, as less than the reference degradation.

[0014] The accompanying drawings illustrate the invention:

[0015] [Fig.1] schematically illustrates a conventional layout with a single LIB and the corresponding electrical architecture.

[0016] [Fig.2] schematically illustrates an implementation with two LIBs in parallel and the corresponding electrical architecture.

[0017] [Fig.3] graphically illustrates an example of a degradation profile of an LIB as a function of temperature and SOC. [0018] [Fig.4] illustrates with a table an example of degradation of an LIB as a function of temperature and SOC.

[0019] [Fig.5] shows a table an example of expected life with an increase in internal resistance of 30%.

[0020] [Fig.6] graphically illustrates an "ideal" degradation profile at 200 and 100% SOC.

[0021] [Fig.7] illustrates with a table examples of degradation coefficients.

[0022] [Fig.8] graphically illustrates the operating principle of the invention when rolling.

[0023] [Fig.9] graphically illustrates the operating principle of the invention when stationary and in parking.

[0024] [Fig.10] graphically illustrates the principle of correction of the state of charge according to the invention.

[0025] [Fig.1 1] graphically illustrates an example of the evolution of the cumulative SOC% corrected according to the invention.

[0026] [Fig.12] graphically illustrates an example of the distribution of used batteries as a function of time.

There are two main families of lithium battery systems: with one or with two batteries. In both cases, it is a matter of placing the power source as close as possible to the motor so as to limit as much as possible the resistive losses in the cables, because these result in either large cable diameters and therefore copper, mass and cost, or by a

oversizing in power of the LIB and therefore of the cost.

[0028] Figure 1 shows a conventional layout according to the prior art with a single LIB. This is located close to the engine with the associated thermal protections. It thus provides the starting power and energy required for all phases of use, including micro-currents during parking phases.

[0029] Figure 2 shows a variant according to the prior art where two LIBs are

connected in parallel, each providing its share of functions: a small battery LTO technology, located close to the engine provides starting power and any other exchange at high current, while another LIB, rather with a graphite anode (GRA) and positioned in a cold environment, at the rear and which ensures all energy needs, including the supply of “off currents” in parking lots.

In the two configurations of Figure 1 and Figure 2, a computer that we will call "Energy Management System" (or "SGE"), receives the state of charge information (SOC in percentage), voltage in volts (V), temperature T in degrees celsius (Ό) and current I of the BMS ("Battery Management System" according to the classic English terminology) which manage the LIB (s). This computer belonging to the vehicle is able to control the voltage (Valt) of the alternator or the DCDC converter for certain hybrid or electric vehicles.

The invention applies equally to the two configurations of Figure 1 and Figure 2, as well as to all Lithium battery technologies because they all share the same behavior with respect to the state. load and temperature.

The invention uses a characteristic presented in the graph of FIG. 3, the curves of which provide an example of a typical behavior of LIBs, namely the exponential degradation of their performance in capacity and in internal resistance (here we present internal resistance) with respect to temperature and state of charge. For each type, size or chemistry of a lithium battery, it is possible to determine these curves by bench tests. These curves represent an intrinsic characteristic of each of them.

[0033] Apart from protecting the battery as much as possible from the thermal influence of

engine, the temperature of the battery cannot be controlled, which will be the result of the combined effects of its position in the vehicle, the climate and its own self-heating linked to the use of the vehicle by the customer. On the other hand, we can act on its state of charge. To better explain the proposed concept, we will take as an example an LIB whose table in figure 4 is the transcription into digital data of the graph in figure 3. In this For example, the maximum degradation tolerable by the vehicle of the internal resistance is an increase of 30% for a required service life of 15 years.

The table of FIG. 5 shows the consequence in terms of service life if we combine the aging coefficients of FIG. 4 and a maximum tolerable degradation of 30% of the internal resistance. All values above 15 years have been blocked at 15 years.

From the tables of Figures 4 and 5 we choose as conditions of

reference point 200 and 100% of SOC which presents a degradation of 2% per year and therefore which allows us to predict a lifespan of 15 years with an increase in internal resistance of 30% (15 x 2%).

The curve of Figure 6 therefore shows the so-called "ideal" evolution of the

internal resistance with respect to time with 200 and 100% SOC as parameters.

In principle, we will assign a coefficient for each combination of temperature and state of charge, taking as a basis the point 200 and 100% of SOC for which we assign the coefficient 1. This gives the table in figure 7, which is directly derived from that in figure 4. This table directly gives a "weight" for each configuration of

operation. It will allow us to assign each hour of calendar lifespan a degradation coefficient compared to the same hour spent at 200 and 100% SOC. According to the table in figure 7, one hour spent at 100% SOC and 300 is equivalent to 2 hours at 100% SOC and 200. On the other hand, at 50% SOC and 300 the aging is the same as with the reference values.

The diagram of Figure 8 shows the operation of the system when driving.

When the vehicle is running, the state of charge and the temperature of the battery are constantly changing. The BMS permanently sends this information to the SGE computer which determines per unit of time, for example the hour, the average state of charge and the average temperature for the unit of time considered. Here, for example, it is SOCmoyl and TOmoyl for the first hour of travel, then SOCmoy2 and TOmoy2 for the second, etc. The diagram of Figure 9 shows the operation of the parking system. Same thing as when driving, when the SGE computer wakes up periodically, it collects information from the BMS of the LIB and determines the averages, hour by hour, for temperature and state of charge. Thus at each hour of life (each corresponding to an iteration ti, ti + i,.., Ti + n where n integer in the present embodiment), the battery can be assigned a coefficient from the table of the figure 7 and these “corrected hours” are accumulated in the memory of the EMS. It is now easy for the EMS to compare the accumulated corrected hours with calendar time. When this becomes greater than the reference line, this means that the battery may degrade faster than expected. When this deviation reaches a limit threshold such as 5 or 10 hours, the EMS sets up corrective management by acting on the state of charge of the battery.

For this, when driving, the SGE will lower the voltage Valt by

alternator or DCDC depending on the state of charge of the LIB transmitted by the BMS until reaching a SOCMax target value between 95% and typically 50% of SOC. As a result, the calculated corrected aging hours will drop despite the same temperature window for the battery.

The diagram of Figure 10 shows the operation of the system. As soon as the BMS indicates to the SGE that the SOCMax is reached, it automatically reduces the regulation voltage Valt so that the load current tends towards 0 and the SOC level remains constant.

The graph of Figure 1 1 shows an example of the evolution of the cumulative corrected cumulative time relative to the ideal line. As we can see the first part of the curve tends to deviate from the ideal line. When this difference becomes significant and greater than a predefined threshold, the management system is activated and the state of charge of the battery is reduced, which immediately changes the direction of the curve which returns to the ideal line. When the cumulative value drops below the ideal line up to a hysteresis value, the SOC limitation is deactivated.

In conclusion, the invention provides a simple system which does not require any physical modification of the vehicle but which makes it possible to guarantee the service life. batteries by counting the time and by managing the state of charge of the battery for customers whose climate or type of use prematurely wears out the battery as shown in the diagram in figure 12, which illustrates how the battery life of a population of customers is determined. The

designer will make every effort to ensure that the vast majority of customers get the target lifespan. For example, the average is beyond the target value. However, there is still a population, hatched in Figure 12, which, due to its use of the vehicle and the climate in which it is driven, will see its battery reach its end of life before the target value. The invention described in this application can be activated only by this population and remain inactive for the vast majority of users.

[0044] By reducing the SOC of the battery, some might object that we are less likely to start cold. This must be put into perspective, because if we always choose the SOCMax value greater than the state of charge necessary to start at the ambient temperature considered and, if the device is activated, it means that the ambient temperature is certainly high and the motor has most likely to be able to be easy to start.

[0045] By reducing the SOC of the battery, others might object that the battery will have less capacity to supply the off current. This is only true for the single battery system. For the two-battery system, only the front battery is concerned with regulating its SOC, and the rear battery provides the off current. As for the case of a single battery, a hot battery denotes frequent use, so it is very unlikely that this vehicle will be abandoned for long periods of parking which would jeopardize its startability.

Claims

Claims [Claim 1] A method of managing the state of charge of a battery comprising a step of determining a target state of charge as a function of the temperature of the battery, said target state of charge tending, in accordance with a reference degradation profile, to decrease as the temperature increases so as to control the degradation of the battery battery and; a step of charging the battery until it reaches the target state of charge; the method being characterized in that, the reference degradation profile describing the relative increase in the internal resistance of the battery as a function of the time spent since the beginning of the battery's life at a reference state of charge and at a reference temperature, the target state of charge is determined iteratively so that, at the end of a step of estimating the relative increase in internal resistance, the estimated relative increase tends to remain less than the increase relative taken from the reference degradation profile. [Claim 2] Process according to Claim 1, characterized in that the reference state of charge is greater than 80%, in particular equal to 100%, and / or in that the reference temperature is between 10Ό and 30Ό, in particular equal to 20Ό. [Claim 3] A method according to claim 1 or 2, characterized in that if, at a given iteration ti, where i positive integer, the estimated relative increase in internal resistance is greater than the relative increase derived from the reference profile , then the target state of charge is determined so that, at the following iteration ti + 1, the estimated relative increase tends towards the relative increase drawn from the reference profile. [Claim 4] Method according to Claim 3, characterized in that two iterations are always separated by the same predefined time interval and said time interval always corresponds in the reference profile to the same relative increase in internal resistance. [Claim 5] A method according to claim 4, characterized in that the same time interval is between 30 minutes and 2 hours, in particular equal to 1 hour, and / or in that the same relative increase in internal resistance is less to 5%, in particular equal to 2%. [Claim 6] A method according to claim 4 or 5, characterized in that if the battery temperature measured between two iterations ti and ti _{+ i} is greater than the reference temperature, then the target state of charge is determined at the iteration ti _{+ i} so as to be lower than the reference state of charge and the degradation of the internal resistance due to the time interval between iterations ti _{+ i} and ti + 2 at this lower state of charge will be considered , in the subsequent steps of estimating the relative degradation, as lower than the reference degradation. [Claim 7] A battery comprising the hardware and software means for implementing the method according to any one of the preceding claims. [Claim 8] Vehicle comprising a battery according to the preceding claim.