Method for charging an energy storage device

Abstract

Methods for charging an energy storage device, where a charging current (IL) for charging the energy storage device is regulated depending on a voltage (Ubatt) of the energy storage device, characterized by the fact that a level of the charging current (IL) is regulated such that the level of the charging current is reduced when a lower threshold value (Uu) of the voltage (Ubatt) of the energy storage is exceeded, wherein the lower threshold value (Uu) of the voltage (Ubatt) of the energy storage is below a maximum permissible value (Umax) of the voltage (Ubatt) of the energy storage and above a setpoint value (UmSOC) of the voltage (Ubatt) of the energy storage and wherein the setpoint value (UmSOC) is the value of the voltage (Ubatt) of the energy storage that is established in the currentless state of the energy storage when the energy storage has a state of charge that is below a maximum possible state of charge.

Description

[0001] The invention relates to a method for charging an energy storage device, for example a battery.

[0002] Electrically powered vehicles generally contain energy storage devices to provide energy to propel the vehicle. Such energy storage devices can be, for example, batteries, particularly lithium-ion batteries, which provide a voltage (terminal voltage) to operate an electric motor. The voltage of such an energy storage device is influenced in particular by the charging or discharging current. Technical causes for this can include, for example, the internal resistance of the energy storage devices (ohmic), transient polarization voltages (capacitive), or similar phenomena. In the following, only the voltage-influencing effect of the charging or discharging currents is considered, not their physical cause.

[0003] Various methods for charging energy storage devices are known from the state of the art: US 5 973 478 A discloses a method for charging batteries in which the battery voltage is monitored and the charging current is adjusted accordingly. US 2015 / 0 022 160 A1 describes a system and method for controlling the charge of energy storage devices, taking into account voltage thresholds. US 2011 / 0210695 A1 shows a method for charging batteries that takes the battery characteristics into account and controls the charging current accordingly. US 2007 / 0 188 134 A1 discloses a charging method in which the battery voltage is monitored during the charging process and the charging current is adjusted accordingly. JP 2007-151 261 A describes a method for charging batteries taking into account voltage thresholds and charge states. US 2011 / 0037 438 A1 shows a system for controlling the charge of energy storage devices, which monitors the battery voltage and controls the charging current accordingly.

[0004] The following boundary conditions should be observed when charging or discharging an energy storage device: a) The energy storage device should be charged in the shortest possible time, taking into account the permissible charging currents. b) The permissible maximum and minimum voltages of the energy storage device should also be taken into account. c) Charging of the energy storage device should, particularly for reasons of service life, take place in a range above the minimum possible state of charge of the energy storage device, i.e., above a value of 0% SOC (state of charge), and below the maximum possible state of charge, i.e., below a value of 100% SOC. For example, the state of charge of the energy storage device should be between at least 10% and 90% of the maximum possible state of charge. The state of charge should be determined based on the voltage of the energy storage device in a de-energized state, or it should be determined by integrating the battery current over time. d) When the desired charge level of the energy storage device is reached, a charger or discharger should be switched off. Typically, controllers are used for this purpose, which, due to internal losses, generate unwanted AC losses and noise even at a DC current of 0 A, and are subject to undesirable thermal stress, and therefore should be switched off. e) The energy storage device shall indicate its current state of charge and the permissible charging current via a suitable data interface. The state of charge and the permissible charging current shall be calculated from various internal battery conditions and parameters. The energy state and the charging current shall be read and observed in the charger (controller), for example, the on-board charger. f) It is assumed that no detailed internal information of the energy storage device, for example on transient polarisation effects or resistances, which could be used to take (complicated) compensation mechanisms into account during charging, is available.

[0005] For charging energy storage devices, such as lithium-ion batteries, the so-called CCCV (constant current-constant voltage) charging method, also known as the IU charging method, is used. The energy storage device or battery is charged with a constant charging current until a predefined maximum voltage Umax is reached. After that, the charging process continues with the maximum voltage. Common shutdown criteria for conventional battery chargers include detecting a drop in the charging voltage (dU < 0), detecting a voltage maximum (dU / dT = 0), or detecting a flattening of the charging voltage curve (d 2 U / dT 2= 0). All of these detection mechanisms are useless for lithium-ion batteries, especially when operated below the maximum possible state of charge. In this case, with an operating maximum state of charge lower than the maximum possible state of charge, the maximum charging voltage may never be reached.

[0006] Instead, for example, shutdown can occur when a defined voltage value UmSOC is reached during charging, which corresponds to the desired maximum state of charge. However, this generally leads to an undesirable effect: When the charging current is switched off, the storage voltage drops below the desired maximum operating value, i.e. the energy storage device is not fully charged. If the charging process then starts again, the voltage of the energy storage device rises again to a value above the selected shutdown value, so that shutdown occurs again. This behavior continues indefinitely, resulting in undesirable "two-point behavior", i.e. the charger constantly switches on and off, with long charging times.

[0007] Another alternative for charging a charge storage device, especially a battery, is to specify a voltage UmSOC in the CCCV charging characteristic instead of the maximum voltage Umax, which corresponds to the desired maximum operational state of charge in the de-energized state. However, the device only shuts down when the charging current is zero. This means that as the battery approaches the desired state of charge, the available voltage difference between the voltage value UmSOC and the battery's countervoltage becomes very small, allowing only very small charging currents to flow, resulting in an undesirably long charging time.

[0008] Another option for charging an energy storage device could be to terminate charging when the desired state of charge is reached, rather than depending on the measured voltage of the energy storage device. However, since calculating the current state of charge of an energy storage device can generally be error-prone, such a method is risky, as it could lead to the unintentional exceeding of the maximum permissible voltage Umax of the energy storage device or, at least, provoke undesirable overvoltage protection reactions of a battery due to measurement tolerances.

[0009] An aim of the present invention is to provide a method for charging an energy storage device with which the charging of the energy storage device can be carried out efficiently, wherein in particular a two-point behavior during charging and at the same time a charging of the energy storage device beyond the desired SOC is prevented.

[0010] The object is achieved by a method for charging an energy storage device as specified in patent claim 1.

[0011] According to the proposed method for charging an energy storage device, in particular a battery, a charging current IL for charging the energy storage device is regulated as a function of a voltage of the energy storage device. The level of the charging current is regulated such that the level of the charging current is reduced when a lower threshold value of the energy storage device voltage Uu is exceeded. The lower threshold value Uu of the energy storage device voltage Ubatt lies below a maximum permissible value Umax of the energy storage device voltage Ubatt and above a setpoint value UmSOC of the energy storage device voltage Ubatt. The setpoint value UmSOC is the value of the energy storage device voltage Ubatt that occurs when the energy storage device is de-energized and has a state of charge that is below a maximum possible state of charge.

[0012] The setpoint is, for example, the value of the voltage of the energy storage device that occurs when the energy storage device is de-energized and has a charge level of approximately 90% of its maximum possible charge level.

[0013] According to one possible embodiment of the method, the charging current level is regulated such that the charging current level decreases linearly or is switched off after the lower threshold value of the energy storage device's voltage is exceeded. The charging current level is reduced according to a predetermined switching-off characteristic. Depending on the energy storage device's reaching a predefined state of charge, the charging current is switched off. According to the specified method, the energy storage device is thus charged by defining a voltage-dependent switching-off characteristic for the charging current. Instead of applying a voltage criterion to switch off the charging current, the charging current is switched off using a state of charge criterion (SOC criterion).

[0014] According to a further embodiment of the method, the charging current level is regulated such that the charging current level has a constant value before the lower threshold of the energy storage device's voltage is exceeded. This means that the energy storage device is charged below the lower threshold of the energy storage device's voltage at a constant charging current level.

[0015] According to a further development of the method, the charging current level is regulated such that it drops to zero when the energy storage voltage exceeds an upper threshold Uo. The upper threshold of the energy storage voltage is below the maximum permissible value of the energy storage voltage Umax and above the lower threshold of the energy storage voltage Uu.

[0016] The method for charging the energy storage device can be used, in particular, for charging a battery. The battery can be configured as a traction battery for powering a vehicle. The method can preferably be used to control the vehicle's on-board charger.

[0017] The process enables a short charging time for the energy storage device and avoids long controller operation. During charging, two-step behavior of the charger, i.e., frequent switching on and off, is avoided. No detailed knowledge of internal battery parameters is required to apply the process.

[0018] By avoiding two-point charging of the energy storage device, both the charger and the energy storage device or battery are protected. This results in an extended battery life with a short charging time. The short charging time results in short turnaround times and, consequently, no additional vehicle demand in operation, especially for vehicles subject to a timetable, because the charging time for a vehicle battery would be too long. The method can therefore be advantageously used for vehicles subject to a timetable, for example, rail vehicles.

[0019] The specified procedure for charging the energy storage device can also be applied analogously for discharging the energy storage device.

[0020] The invention is explained in more detail below with reference to figures that illustrate embodiments of the present invention. They show: Fig. 1 a qualitative example of a discharge characteristic for executing a method for charging an energy storage device; Fig. 2 shows a qualitative example of a temperature characteristic curve for determining a lower threshold value of a voltage of an energy storage device as a function of the temperature of the energy storage device for carrying out a method for charging an energy storage device; and Fig. 3 an embodiment of a control structure for controlling a charging current for charging an energy storage device.

[0021] Fig. Figure 1 shows a characteristic curve (discharge characteristic curve) for regulating a charging current IL for charging an energy storage device, in particular a battery. The charging current IL for charging the energy storage device is regulated as a function of a voltage Ubatt of the energy storage device, for example, a battery voltage.

[0022] The level of the charging current IL is determined according to the Fig. 1, the characteristic curve is controlled in particular such that the level of the charging current is reduced when a lower threshold value Uu of the voltage Ubatt of the energy storage device is exceeded. Before the lower threshold value Uu of the voltage Ubatt of the energy storage device is exceeded, the level of the charging current IL can be controlled to a constant value Imax. The energy storage device is thus charged with the constant charging current level Imax. When the lower threshold value Uu of the voltage Ubatt of the energy storage device is exceeded, the level of the charging current IL can be reduced continuously, for example, linearly.

[0023] The charging current IL is switched off depending on the energy storage device reaching a certain charge level. As can be seen from the discharge characteristic of the Fig. 1, the level of the charging current IL is regulated in such a way that it drops to a zero level when the value of the voltage Ubatt of the energy storage device exceeds an upper threshold value Uo.

[0024] The upper threshold value Uo of the energy storage device's voltage Ubatt is below a maximum permissible value Umax of the energy storage device's voltage Ubatt. The maximum permissible battery voltage Umax can be found, for example, in the energy storage device's data sheet. Furthermore, the lower threshold value Uu of the energy storage device's voltage Ubatt is above a target value UmSOC of the energy storage device's voltage in the de-energized state.

[0025] The target value UmSOC is the voltage value of the energy storage device that occurs when the energy storage device is de-energized and has a desired state of charge that is below the maximum possible state of charge. The target value UmSOC of the voltage of the energy storage device is, for example, the value of the voltage Ubatt that the energy storage device has at the maximum operational state of charge. The maximum operational state of charge can, for example, be 90% of the actual maximum possible state of charge when the energy storage device is de-energized. The target value UmSOC of the voltage of the energy storage device can, for example, be determined from a data sheet or by measurement. For physical reasons, it must be below the maximum permissible battery voltage Umax.

[0026] To determine the steepness of the discharge characteristic, the upper threshold value Uo of the voltage Ubatt and the lower threshold value Uu of the voltage Ubatt of the energy storage device are defined. The upper threshold value Uo of the voltage Ubatt of the energy storage device lies between the voltage value UmSOC and the voltage value Umax. The lower threshold value Uu of the voltage of the energy storage device is lower than the upper threshold value Uo and can also lie between the voltage value UmSOC and the maximum permissible battery voltage Umax. The lower threshold value Uu of the voltage of the energy storage device can also be below the voltage value UmSOC.

[0027] According to the Fig. 1, the charging current IL is linearly controlled between the lower threshold value Uu and the upper threshold value Uo of the voltage of the energy storage device from a maximum permissible level Imax of the charging current IL to the zero level.

[0028] The following describes the system's behavior during charging. When the energy storage device voltage exceeds the lower threshold Uu, the charging current IL decreases slightly. As a result, the energy storage device voltage also begins to drop slightly, because this voltage is dependent on the charging current, particularly due to the internal resistance of the energy storage device. Therefore, only a slight decrease in the charging current IL occurs. Only as the battery's state of charge increases does the battery voltage Ubatt increase and the charging current IL decrease.

[0029] When the desired state of charge of the energy storage device, for example 90% of the maximum possible state of charge, is reached, charging is stopped and the charging current is switched off. Up to this moment, the charging current IL was still relatively high because it was charged with a charging voltage greater than the voltage value UmSOC. After the energy storage device is disconnected from the charger, the battery voltage usually drops to the voltage value UmSOC. The energy storage device is now "fully" charged in the sense of the desired operational value, e.g. 90% of the maximum possible value. If this is not the case, the state of charge of the energy storage device may have been calculated too inaccurately using the temporal integration of the battery currents. The charging process can then be started again after the calculated state of charge has been corrected.

[0030] Unintentional safety shutdowns of the battery or impermissibly high charges are automatically avoided because the upper voltage value Uo, at which no charging current can flow, is sufficiently far below the maximum voltage Umax of the battery.

[0031] The following describes how to adjust the slope of the discharge characteristic between the lower threshold Uu and the upper threshold Uo of the energy storage device's voltage. In principle, it is possible to derive an optimal discharge characteristic with precise knowledge of the battery parameters that influence the voltage and the timing of the charging controller. However, often not all of these parameters are known or they are highly variable. Therefore, the following presents an efficient and practical approach for adjusting the slope of the discharge characteristic.

[0032] The upper threshold value Uo of the voltage Ubatt results as the distance to the maximum possible voltage Umax of the energy storage device from the measurement and control tolerances of the charging system and, if applicable, depending on other desired safety distances to the maximum permissible battery voltage Umax.

[0033] The steepness of the characteristic curve is essentially determined by the choice of the lower threshold value Uu of the battery voltage. If internal battery parameters are unknown, the lower threshold value Uu of the battery voltage can be adjusted depending on the behavior of the energy storage device. In the simplest case, the lower threshold value Uu of the energy storage device voltage can be set equal to the voltage value UmSOC. The behavior of the charging current IL is then monitored.

[0034] If the level of the charging current IL fluctuates greatly when the lower threshold value Uu of the battery voltage Ubatt is exceeded, or even drops to zero, the lower threshold value Uu of the battery voltage must be corrected. In this case, the lower threshold value Uu of the battery voltage is reduced, e.g., set below the voltage value UmSOC. On the other hand, if it is determined that the level of the charging current IL continuously decreases with increasing battery state of charge, the lower threshold value Uu of the battery voltage can also be increased, e.g., set higher than the voltage value UmSOC, in order to shorten the battery charging time. To optimize the lower threshold value Uu of the battery voltage, known optimization algorithms, for example, an interval halving method, can be used, taking into account the two aforementioned criteria.

[0035] Since internal operating parameters of the energy storage device or the battery, in particular the internal resistance, can be highly temperature-dependent, the lower threshold value Uu of the battery voltage is preferably set at the lowest expected operating temperature Tmin. This results in a control characteristic with a low gradient, i.e. a relatively "flat" characteristic with a large difference between the lower threshold value Uu and the upper threshold value Uo of the battery voltage, so that the charging current IL does not oscillate even at higher temperatures, but the charging currents are comparatively small. Alternatively, a certain oscillation of the charging current IL can be accepted at low temperatures. However, this approach is generally not preferred because the batteries usually have a defined "high" operating temperature during operation.

[0036] According to a further development of the method, the lower threshold value Uu of the voltage Ubatt of the energy storage device can be adjusted depending on the temperature Tbatt of the energy storage device. For example, if the charging time is to be minimized, the lower threshold value Uu of the voltage of the energy storage device can be increased depending on the temperature.

[0037] Fig. Figure 2 shows an example of a possible temperature characteristic curve for the lower threshold value Uu of the battery voltage. In the solution shown, the lower threshold value Uu(Tmin) of the battery voltage is increased linearly with the temperature Tbatt of the energy storage device at the lowest expected temperature Tmin of the energy storage device up to a value Uu(Tmax) determined at the normal (maximum) operating temperature Tmax of the energy storage device.

[0038] Alternatively to the Fig. 2 for setting the lower threshold value Uu of the battery voltage, the temperature characteristic can be further optimized by knowing the internal battery parameters.

[0039] Fig. 3 shows an embodiment of a control structure for controlling the charging current IL, in which the charging current IL is determined as a function of the current battery voltage Ubatt according to the Fig. 1 shown characteristic curve.

[0040] As the output signal of a control system RS, the battery voltage Ubatt(t) is fed to a circuit P. The circuit P represents a P-controller with a corresponding gain. The slope of the ramp in the discharge characteristic represents the internal resistance of the battery and can be considered a gain within the control system. A limiter B is located behind a summing element S1, which feeds values ​​between 0 and 1 to a multiplier M depending on the battery voltage Ubatt(t).

[0041] After multiplying the desired charging current Ibattsoll, for example the maximum possible charging current Imax, by the supplied value between 0 and 1, the desired charging current IL_soll is produced as the reference variable, which is fed to a further summing element S2. The actual charging current IL from the controlled system R is fed to the summing element S2 as a feedback variable, so that a control deviation results at the output of the summing element S2. This control deviation is fed to an element PI. The element PI is designed as a PI controller, the output of which is connected to the controlled system RS. The controlled system comprises an actuator SG, which sets the charging current IL. A manipulated variable d is fed to the actuator from the PI controller PI. Due to the given physics of the energy storage device, e.g. by multiplying it by the internal resistance of the battery Rbatt(T), the battery voltage Ubatt(t) is ultimately produced at the output of the controlled system.

[0042] The steepness of the ramp in the discharge characteristic represents the internal resistance Rbatt(T) of the battery and can be considered a gain within the control loop. Due to the influence of temperature, the internal resistance of the battery changes, and thus also the loop gain. The controller gain of the P-controller P must be temperature-dependent in order to quickly reach the desired state of charge of the energy storage device and to keep the control loop stable.

Claims

[1] Method for charging an energy storage device, wherein a charging current (IL) for charging the energy storage device is regulated as a function of a voltage (Ubatt) of the energy storage device, characterized by , that a level of the charging current (IL) is regulated such that the level of the charging current is reduced when a lower threshold value (Uu) of the voltage (Ubatt) of the energy storage device is exceeded, wherein the lower threshold value (Uu) of the voltage (Ubatt) of the energy storage device is below a maximum permissible value (Umax) of the voltage (Ubatt) of the energy storage device and above a setpoint value (UmSOC) of the voltage (Ubatt) of the energy storage device, and wherein the setpoint value (UmSOC) is the value of the voltage (Ubatt) of the energy storage device that occurs when the energy storage device is in the currentless state when the energy storage device has a state of charge that is below a maximum possible state of charge. [2] Method according to claim 1, wherein the level of the charging current (IL) is controlled such that the level of the charging current (IL) decreases linearly after the lower threshold value (Uu) of the voltage (Ubatt) of the energy storage device is exceeded. [3] Method according to one of claims 1 or 2, wherein the level of the charging current (IL) is controlled such that the level of the charging current (IL) has a constant value before the lower threshold value (Uu) of the voltage (Ubatt) of the energy storage device is exceeded. [4] Method according to one of claims 1 to 3, wherein the charging current (IL) is switched off as a function of reaching a charge state of the energy storage device. [5] Method according to one of claims 1 to 4, wherein the level of the charging current (IL) is controlled such that the level of the charging current (IL) drops to a zero level when the value of the voltage (Ubatt) of the energy storage device exceeds an upper threshold value (Uo). [6] Method according to claim 1, wherein the setpoint value (UmSOC) is the value of the voltage of the energy storage device which is established in the currentless state of the energy storage device when the energy storage device has a state of charge of approximately 90% of the maximum possible state of charge. [7] Method according to claim 1 or 6, wherein the upper threshold value (Uo) of the voltage (Ubatt) of the energy storage device is below the maximum permissible value (Umax) of the voltage (Ubatt) of the energy storage device and above the lower threshold value (Uu) of the voltage of the energy storage device. [8] Method according to one of claims 1 to 7, wherein the lower threshold value (Uu) of the voltage (Ubatt) of the energy storage device depends on a temperature of the energy storage device. [9] Method according to claim 8, wherein the lower threshold value (Uu) of the voltage of the energy storage device increases linearly with the temperature of the energy storage device. [10] Method according to one of claims 1 to 9, wherein the energy storage device is designed as a battery for driving a vehicle.

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