Fast charging method

Abstract

The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel The invention relates to a method for rapid charging of a battery system from

Description

TECHNICAL FIELD

[0001] The present application relates to a method for fast charging of a lithium-ion battery cell or of a battery system having a plurality of lithium-ion battery cells with the aid of impedance measurement or impedance spectroscopy. BACKGROUND

[0002] For battery systems used in automotive applications, in particular for vehicles operated purely electrically, fast charging capability constitutes a particular challenge. In practical terms it is desirable for the charging of the battery system to not last significantly longer than a refueling process in a vehicle operated with an internal combustion engine.

[0003] For this a high charging current in the region of, for example, 2 C or more is required. Such a charging current can however lead to strong self-heating and thus to a gradual degradation of the electrolyte and accelerated aging of the battery. Furthermore, there is a risk at high currents that lithium is deposited on the anode in addition to intercalation, which in turn can lead to internal short circuits.

[0004] It is furthermore difficult that suitable fast charging conditions are typically dependent on the state of health (SOH) of the battery cell. It can thus occur that certain fast charging conditions optimized with new battery cells lead to problems in battery cells with poor SOH.

[0005] Fast charging strategies for automotive applications up to 350 kW charging power are currently being developed / researched at OEMs and battery cell manufacturers. Based on the lack of information about the influence of fast charging on aging and the lack of field data on the use of this with up to 350 kW charging power, the charging strategies can only be designed very conservatively with large buffers in order to also work when the battery cell continues to age.

[0006] Task

[0007] In the current fast charging methods of the prior art, the charging conditions are typically adapted with the aid of the SOC, which in turn is determined from the battery cell voltage (no-load voltage). It is possible, for example, to initially charge at a constant charging current (CC) at low SOC, the CC charging continues at a lower charging current when a limit value is exceeded, and the charging continues at a constant voltage (CV) when another limit value is exceeded until a defined target SOC (i.e. a defined target voltage) is reached. The battery cell voltage is not determined solely by the SOC, however, but can also be dependent on the temperature and the state of health, i.e. not necessarily only the voltage is a reliable measure for the SOC.

[0008] It is also desirable to specify the fast-charging condition as a function of temperature, since in combination with high charging currents, at high temperatures, electrolyte degradation is promoted, while at low temperatures, lithium deposition occurs. Here, however, there is the difficulty that the ambient temperature, which is measured, for example, by means of a sensor mounted on the housing of the battery system or the battery cell, can deviate from the temperature inside the battery cell. Finally, the influence of the state of health (SOH) should also be taken into account as a limiting factor for the maximum charging current or the maximum charging rate, in particular.

[0009] In summary, the ideal fast-charging condition for a lithium-ion battery cell depends, inter alia, on the temperature, the SOC or the battery cell voltage, and the SOH. In view of this problem, it is therefore the task to develop a fast-charging method which takes these correlations into account and thereby, on the one hand, enables a charging time as short as possible to be achieved, and on the other hand, can avoid premature aging or damage to the battery cell.

[0010] Summary of the invention

[0011] The invention is proposed in relation to the above-mentioned task and provides a method for fast charging a battery system, in which method an optimized fast-charging condition is determined using impedance measurement or impedance spectroscopy (EIS) as a function of at least one of the battery cell temperature T, the SOC, and the SOH.

[0012] The invention relates in particular to a method for fast charging a battery system from an initial state of charge SOC0to a predetermined target state of charge SOC ziel , the battery system having a plurality of lithium-ion battery cells, wherein cell units composed of individual battery cells or of stacks of parallel-connected battery cells are connected in series, and further having means for measuring the voltage and at least one component of the impedance of these battery cell units,

[0013] wherein the method comprises:

[0014] - continuous or intermittent determination of the battery cell voltage and the impedance value of the battery cell units, wherein the impedance value has one or more components of the impedance at one or more frequencies;

[0015] - determination of the state of charge SOC of the battery system from the battery cell voltage and optionally from the determined impedance value;

[0016] - determination of the temperature T 1…N of the individual battery cell units from the determined impedance value;

[0017] - determination of the state of health SOH 1…N of the individual battery cell units, the state of health having at least a capacity-related state of health SOH_C 1…Nand preferably also has a state of health SOH_R associated with the internal resistance determined by a determined impedance value 1…N ;

[0018] - charging the battery system with a first charging profile P1 selected by means of a value for SOC0 and for T 1…N and SOH 1…N determined by the detection until a first state of charge limit value SOC1 is reached or until a predetermined maximum temperature T max，1 or a minimum temperature T min，1 is exceeded in one of the battery cells,

[0019] - charging the battery system with one or more further charging profiles P 2...M selected by means of a value for SOC and for T 1…N and SOH 1…N determined by the respective detection until a respective state of charge limit value SOC 1...N is reached for the respective charging profile or until a predetermined maximum temperature T max，2...M or a minimum temperature T min，2...M is exceeded in one of the battery cells,

[0020] until a target state of charge SOC ziel is reached or the charging process is interrupted.

[0021] A further aspect of the application relates to a battery system which is designed for carrying out the described fast charging method.

[0022] Detailed description

[0023] Battery system

[0024] The fast charging method according to the application is used for charging a battery system having a plurality of lithium-ion battery cells. The battery cells are connected in series in chains, either individually or in the form of stacks of parallel-connected battery cells, in order to provide the total voltage of 200 to 500 volts typically required for use in electrically operated vehicles or (plug-in) hybrid electric vehicles. The stacks composed of parallel-connected single battery cells act electrically as single battery cells with a correspondingly larger capacity. The single battery cells connected in series or the parallel stacks in the battery system are then collectively referred to as battery cell units.

[0025] For each battery cell unit, means are provided for monitoring the voltage and for measuring at least one component of the impedance, wherein the implementation of the means is not particularly restricted. In one possible embodiment, each battery cell unit can be provided with a controller for battery cell supervision (Cell Supervision Circuit, CSC), which is designed at least for measuring the voltage. The CSC is again connected with a central controller for battery management (Battery management unit, BMU). Advantageously, the measured voltage data are used at the same time for determining the impedance, wherein the impedance calculation can optionally take place in the CSC or in the BMU. In order to avoid an excessive load of the communication channel with voltage data, a calculation by the CSC is preferred.

[0026] It is also possible to use CSCs which monitor a plurality of battery cell units at the same time, or the monitoring function of all battery cell units can be integrated into the BMU as the only controller.

[0027] The control of the fast charging method is typically carried out by the BMU taking into account the voltage and impedance data of the individual battery cell units. For this purpose, the BMU is connected with the charger by means of a suitable data connection, for example a CAN bus, so that the charging current provided or the voltage applied can be adjusted accordingly.

[0028] The charger which provides the charging current can be fixedly integrated into the battery system or into the vehicle in which the battery system is installed, or an external charger can be used which is connected with the battery system only for carrying out the charging process.

[0029] Impedance measurement

[0030] In the fast charging method according to the application, the impedance measurement or the impedance spectroscopy serves, inter alia, for one or more of the following purposes:

[0031] - determination of the battery cell temperature T; by means of the impedance, the temperature inside the battery cell can be determined directly at the respective point in time; temporal inertial effects or spatial averaging over a plurality of battery cells as in conventional temperature sensors can be avoided;

[0032] - improvement of the determination of the SOC; conventionally, the SOC is determined by means of the no-load voltage, but the no-load voltage can also be dependent on the state of aging and thus possibly not give the SOC again sufficiently;

[0033] - determination of the SOH; the impedance spectroscopy enables the determination of the electrolyte conductivity and allows conclusions to be drawn about the kinetics of the lithium insertion / extraction at the electrodes; the state of aging of the electrolyte and the electrodes can thus be assessed again; - determination of the lithium precipitation limit; an optimized temperature limit value can thus be determined below which the charging current should be reduced or the charging interrupted.

[0034] Impedance can generally be measured by applying an oscillating current signal (I(t), constant current) or a voltage signal (U(t), constant potential) to a single cell as an excitation signal and measuring the corresponding response signal U(t) or I(t). The impedance can then be calculated as U(t) / I(t) and is typically a complex number.

[0035] Advantageously, in the method according to the invention, a current signal is used as the excitation signal, which can, for example, be modulated. The charging current is measured, and the mechanism for voltage measurement provided for each individual battery cell is also used to detect the response signal.

[0036] The excitation signal can have a single frequency or a superposition of multiple frequencies, and it can be applied to the battery cell continuously or pulsedly. The frequency is not particularly limited and can be, for example, in the range of 10 Hz to 10 kHz, advantageously in the range of 100 Hz to 5 kHz. In principle, a single excitation frequency is sufficient. Alternatively, two or more excitation frequencies can be used alternately or superimposed, or a predetermined bandwidth of the excitation frequencies can be used to obtain the spectrum. As another possibility, the excitation can be pulsed, for example in the form of pulses constituting a superposition of many frequencies, and the measured signal can be analyzed by means of a Fourier transform. The spectrum thus obtained then interacts with the spectrum of the excitation pulses to obtain the impedance spectrum in the same way.

[0037] Generally, the frequency affects the processes within the battery cell, and this effect contributes to the response signal. At high frequencies (e.g., 1 kHz), impedance is primarily achieved through the impedance components of ions and electrons in the electrolyte and in the electrodes and outlet, while at low frequencies, other contributions are added, which can be attributed to processes with relatively slow time scales, such as solid-state diffusion or charge-through reactions.

[0038] Furthermore, at low frequencies, the correlation with other factors, such as especially the state of charge (SOC) and state of aging (SOH) of individual cells, also increases. At higher frequencies, conversely, the influence of electrolyte impedance is primarily considered, which is largely dependent on temperature and aging condition.

[0039] Based on the different frequency correlations of the effects of temperature, SOC, and SOH on impedance (where the effects can also be distinguished as real or imaginary), temperature, SOC, and SOH can be determined conversely by impedance measurements at multiple different frequencies.

[0040] Suitable methods for determining T, SOC and SOH by means of impedance are known in principle in the prior art and can be used in the method according to the application. Thus DE 102013103921, for example, describes cell temperature measurement and degradation measurement in a lithium cell system of an electrically operated vehicle by determining the cell impedance on the basis of an alternating voltage signal predetermined by the inverter. The method is based on the observation that the recorded curve of the impedance with respect to the signal frequency is correlated to the temperature.

[0041] The detection of the lithium precipitation limit can be carried out, for example, by assessing the anode overvoltage when measuring the internal resistance in order to determine SOH_R.

[0042] In one possible embodiment, reference data can be determined in such a way that the cell is placed at predetermined temperature (T) and SOC values and the impedance is measured at a plurality of frequencies f in order to obtain the impedance as a function of T, SOC and f. From the data, a Lookup table can then be constructed, for example. In the implementation of the fast charging method according to the application, the table can then be used to read or interpolate the respective values for T and SOC from the input of the measured impedance values for different measurement frequencies. Additionally, the data can be investigated for changes in dependence on the number of cell cycles and / or the lifetime in order to determine the influence of SOH.

[0043] It is preferably possible here to additionally take other parameters into account, such as, in particular, the cell voltage and the housing temperature. It is possible, for example, to take the cell voltage into account as an additional input parameter for SOC, whereby the degrees of freedom can be reduced and the accuracy in determining the remaining parameters, such as T and SOH, can be improved. The housing temperature can be taken into account, for example, for the test result plausibility, for example, deviations can also be a sign of an anomaly, for example, the beginning of a short circuit, which can make further measures, such as interrupting the charging process or outputting an alarm, necessary.

[0044] In another embodiment, the cell can be modelled by means of an equivalent circuit diagram comprising a series impedance R s and at least one RC element, which is supplemented by a Warburg element if necessary, wherein R represents the penetration impedance and C represents the capacitance of the charge double layer. The parameters of the equivalent circuit are subsequently determined from the impedance measurement values and interact with T as well as with SOC and SOH.

[0045] Rs is thus essentially dependent on temperature and the state of aging of the electrolyte. R and C, in contrast, are dependent on SOC, T and perhaps also on the state of aging of the electrodes, the temperature dependency, however, distinguishing from that of Rs and approximately having the Arrhenius characteristic. The SOC-, SOH- and T- dependencies of the parameters of the equivalent circuit can again build up reference data from which the SOC, SOH and T are then determined, if necessary in consideration of the cell voltage and the ambient temperature, when implementing the method according to the application.

[0046] Charging method

[0047] The method according to the application serves to quickly charge a battery system from an initial state of charge SOC0 to a predetermined target state of charge SOC ziel .

[0048] In general, the external supply required distinguishes between alternating current charging (AC charging) and direct current charging (DC charging). In AC charging, the battery system is provided with a charger integrated into the vehicle (typically < 11 kW), which is connected to an alternating network in order to provide the direct current required for charging the battery system. In DC charging, in contrast, an external charger is used which provides the charging current (> 50 kW, up to 350 kW). For high charging currents as required for fast charging, DC charging is currently generally used. The method according to the application can be used not only in connection with AC charging but also in connection with DC charging.

[0049] The initial SOC, i.e. SOC0, is not particularly limited. In practice, however, fast charging is considered in particular in the case where the battery system has been discharged to a large extent and should be charged again as much as possible in a short time, for example at a "tank stop" in a journey with an electrically operated vehicle, which must be put on a charging column, and the journey should then continue. SOC0 is thus typically less than 50%, for example approximately 10 to 30%, of the total capacity.

[0050] In order to avoid hasty aging, the target state of charge SOC ziel is preferably less than 100% of the total capacity and, for example, 60 to 80%. Here it can be a predetermined maximum SOC, which is specified for fast charging by the battery system. Alternatively, a desired lower target SOC can be predetermined depending on the use case, which is selected, for example, in consideration of the distance to be traveled with the electrically operated vehicle. As a further alternative, the charging time available can be predetermined, and the target SOC achievable in this time is calculated by the battery management system.

[0051] The current determination of the SOC is at least with the aid of the no-load voltage (cell voltage) which is monitored during charging for each cell. The correlation between the SOC and the cell voltage is known in advance, for example by means of a reception characteristic curve, and is stored in the form of reference data in the battery management system, so that from the measured cell voltage the SOC can be derived.

[0052] The cell voltage can however also be dependent on other influencing factors, for example the temperature (T) and the capacity-related state of health (SOH_C). In the method according to the application, these additional influences are preferably also taken into account, for example by means of an additional determination of the SOC with the aid of an impedance measurement and, if necessary, correction of the SOC value determined from the cell voltage. Furthermore, the SOC reference data can also contain T- or SOH- dependencies. With the aid of the impedance measurement used in the method according to the application, T and SOH can be determined and taken into account in the determination of the SOC. The determination of the SOH is here carried out, if necessary, taking into account other SOH-related parameters, such as in particular the ageing of the cells, the number of charging cycles and / or the energy taken out or charged overall, which are recorded in the battery management system.

[0053] The charging curves P1...P N The charging curves can be in particular constant current (CC) or constant voltage (CV) charging curves. In CC charging, the current remains constant and the voltage increases as the SOC rises, while in CV charging the voltage remains constant and the current decreases as the SOC rises. Charging curves with constant power are also possible, in which the product of current and voltage remains constant. Pulse charging is also taken into account, in which current pulses, for example as rectangular pulses, are delivered, followed by pauses. The pulses can again have a constant current amplitude or a constant voltage.

[0054] In the method according to the application, a CC charging curve is preferably used as the first charging curve P1 and a CV charging curve is used as the last charging curve P2 or P N The CV charging curve is used. Between this, it is possible to change the charging curve, for example to another CC charging curve with a reduced charging current, when a determined SOC threshold value SOC1...SOC N-1 is reached.

[0055] The selected charging current in the charging profile typically decreases with increasing SOC, i.e. the current is usually largest in the first charging profile P1, wherein the selected value depends at least on the initial SOC and, if necessary, on the temperature and SOH. The charging or discharging current of the battery system is generally given relative to the capacity of the battery system as a so-called C-rate, which is defined as the quotient of the maximum current and the (nominal) capacity. A C-rate of 1, for example, represents a charging or discharging with a current of 1 A over 1 h in a battery system having a nominal capacity of 1 Ah. At fast charging, a charging time of less than 30 minutes, for example approximately 10 to 15 minutes, is desirable, which corresponds to a theoretical charging current of approximately 2.0 to 6.0 C in this case. The initial SOC is typically, however, greater than 0% and the target SOC is less than 100%, i.e. the charge to be delivered is less than the nominal capacity, so that small charging currents are also taken into account. On the other hand, the charging current is typically selected in dependence on the SOC and can be initially higher and decrease as the SOC rises. The charging current can thus be, for example, 2.0 to 10.0 C, preferably 2.5 to 5.0 C, in the initial SOC region of approximately 10-30%. As the SOC increases, it is then possible to switch to a smaller charging current, for example 1.0 to 5.0 C, preferably 1.5 to 3.0 C, for an SOC of 30-50, and the current can then be further reduced or replaced by a charging profile with constant power or constant voltage.

[0056] It can be necessary, if necessary, to first select a charging profile with a smaller current for P1, for example in order to prevent the risk of lithium precipitation at low temperatures. The battery cells heat up when charging, so that a switch to a charging profile with a larger current can be made when a certain temperature limit is reached.

[0057] The battery cell temperature is determined in the method according to the application from the impedance data for the individual battery cells in order to adapt the charging profile to the temperature. At high temperatures, for example above 50°C, there is a risk of premature ageing, while at too low temperatures, for example below 10°C, lithium precipitation can occur, especially in combination with a large charging current.

[0058] If the battery cell temperature exceeds or falls below a certain temperature limit value T max or T min , a corresponding adapted charging profile with a reduced charging current can thus be switched to, or the fast charging can be interrupted in order to first cool or heat the battery cells to the theoretical temperature. A plurality of temperature limit values T max，1...N or T min，1...N may also be selected, wherein the charging current is first continuously reduced and finally the charging process is interrupted when exceeded or fallen below, respectively.

[0059] The SOH again gives the state of aging of the battery cells. With increasing aging of the battery cells not only in time but also with regard to the number of cycles and the overall converted energy, irreversible degradation processes can occur, such as in particular electrolyte decomposition, lithium loss, active material degradation or corrosion effects. The degradation processes lead to an increase in internal resistance and a loss of usable capacity compared to the initially rated capacity. In correspondence therewith, a distinction is made between a capacity-related SOH (SOH_C) and a resistance-related SOH (SOH_R).

[0060] The SOH_C can be expressed by a capacity loss, for example as the ratio of the usable capacity to the initially rated capacity. The usable capacity can be determined from the SOC data determined by the battery management system in combination with the amount of charge drawn off or delivered upon charging, and is stored in the memory of the battery management system for each battery cell unit and is continuously updated during operation.

[0061] The SOH_R again gives the increase in internal resistance through electrolyte aging and can be determined from impedance data. In the method according to the application, the SOH is determined at least SOH_C, preferably not only SOH_C but also SOH_R. Further criteria, such as the aging of the battery cells, the number of charging cycles or the overall converted energy, can also be taken into account in the determination of the SOH.

[0062] In the method according to the application, a charging profile with a lower charging current is selected at a poor SOH. Furthermore, temperature limit values T max or T min (at which the charging profile is changed or the charging is interrupted in order to temper the battery cells) can be determined in dependence on the SOH, so that in battery cells with a poor SOH, narrower limit values are applicable in order to prevent a further acceleration of the aging and to prevent possible damage.

[0063] The selection of the charging profile P 1…N is therefore made at least in dependence on the SOC of the battery system and the T and SOH of the battery cell units. However, the selection can also be made taking into account other external conditions, for example taking into account a specification for the available charging time. If sufficient time is available, a more conservative charging profile with a lower charging current can be selected if necessary in order to prevent a hasty aging of the battery system.

[0064] Furthermore, the charging can also be interrupted before the target SOC is reached, for example by a user input or by the battery management system, for example in order to prevent damage when an abnormal operating state, for example a strong temperature increase, is detected in one of the battery cells during charging.

Claims

1. Used for rapidly charging a battery system from its initial State of Charge (SOC) 0 to a predetermined target State of Charge (SOC). ziel The method wherein the battery system comprises multiple lithium-ion battery cells, wherein, The battery cell units, consisting of individual lithium-ion battery cells or stacks of lithium-ion battery cells connected in parallel, are connected in series, and further include a mechanism for measuring at least one component of the voltage and impedance of these battery cell units. The method includes: - Continuously or intermittently determine the cell voltage and impedance values ​​of a single battery cell, wherein the impedance values ​​include one or more components of impedance at one or more frequencies; - The state of charge (SOC) of the battery system is determined by the voltage of each individual battery cell. - The temperature T of each individual battery cell is determined by the determined impedance value. 1…N ; - Determine the aging status (SOH) of each individual battery cell. 1…N The aging state includes at least the capacity-related aging state SOH_C. 1…N ; -The first charging curve P1—which utilizes the initial state of charge SOC0 and the temperature T 1…N and aging state SOH 1…N The selection is based on the detected value—charging the battery system until the first state-of-charge threshold value SOC1 is reached, or until a predetermined maximum temperature T is exceeded in one of the battery cells. max，1 or below the lowest temperature T min，1 The initial state of charge (SOC0) is 10-30% of the capacity, and the first charging curve P1 is a charging curve with a constant charging current in the range of 2.0 to 10.0 C. -With one or more other charging curves P 2...N —It utilizes the initial state of charge (SOC0) and the temperature (T) 1…N and aging state SOH 1…N The selection is based on the values ​​detected separately—charging the battery system until the corresponding second state-of-charge (SOC) threshold is reached for the corresponding charging curve. 2...N Or until the predetermined maximum temperature T is exceeded in one of the battery cells. max，2...N or below the lowest temperature T min，2... N , Until the target state of charge (SOC) is reached ziel Or interrupt the charging process, wherein the target state of charge (SOC) is reached. ziel The previous last charging curve P2 or P N It is a charging curve with a constant voltage.

2. The method according to claim 1, wherein, Choose charging curve P from charging curves with constant current, charging curves with constant voltage, charging curves with constant power, and combinations thereof. 1…N .

3. The method according to claim 1 or 2, wherein, Charging is performed in pulses.

4. The method according to claim 1 or 2, wherein, The state of charge (SOC) of the battery system is determined by a given impedance value.

5. The method according to claim 1 or 2, wherein, The aging state SOH 1…N This also includes the aging state SOH_R, which is related to internal resistance and is determined by a defined impedance value. 1…N .

6. The method according to claim 1 or 2, wherein, Target State of Charge (SOC) ziel It is 60-80% of the capacity.

7. The method according to claim 1 or 2, wherein, The impedance value includes the real and imaginary parts at at least two different frequencies.

8. The method according to claim 1 or 2, wherein, The method is applicable above the maximum temperature T. max，1 or T max，2... N or below the lowest temperature T min，1 or T min，2... N This includes interrupting the charging process and adjusting the battery system temperature to the rated temperature before the charging process continues.

9. A battery system configured to implement the method according to any one of claims 1 to 8, the battery system comprising: Multiple battery cell units, which are composed of individual lithium-ion battery cells or stacks of lithium-ion battery cells connected in parallel, are connected in series with each other. A signal generator designed to apply an alternating current signal as an excitation signal to all lithium-ion battery cells or stacks, or one or more signal generators designed to apply an excitation signal individually to lithium-ion battery cells or stacks. At least one voltage measuring device for each lithium-ion battery cell or stack, designed to measure the entire cell voltage U and the AC voltage component. One or more controllers are designed to determine the impedance value from the AC voltage component of the excitation signal and the cell voltage. as well as A battery management controller for controlling the charging process, designed to implement the method according to any one of claims 1 to 8.