Method for Controlling a Battery System, a Battery System, and Motor Vehicle

A battery system comprises at least one battery cell and a high-voltage network connected thereto which includes a pre-charge circuit having at least one pre-charge resistor. The battery system further comprises a component including a link capacitor with a specific capacitance. A method for controlling the battery system includes measuring a first voltage at the link capacitor before charging, charging the link capacitor, and measuring a second voltage at the link capacitor after charging. The method further includes forming a voltage difference from the first and the second voltage, and determining an energy received by the pre-charge resistor based on the voltage difference at the link capacitor and based on the capacitance of the link capacitor.

DETAILED DESCRIPTION

Within the scope of this patent application, the terms “received energy” and “output energy” are used. Received energy is electrical energy as a result of a flow of current. An example of the received energy will be discussed within the scope ofFIG. 2. The output energy is thermal or heat energy.

FIG. 1shows a battery system100in accordance with an exemplary embodiment of the disclosure that illustrates a series connection of a plurality of lithium-ion battery cells102and a high-voltage network104, wherein the high-voltage network104is connected to the series connection of the plurality of lithium-ion battery cells102. The high-voltage network104can be connected to a further component106, for example a link capacitor108, a discharge circuit, pulse-width-modulation inverter, or other consumers, such as electric motors, etc.

The high-voltage network104comprises a pre-charge circuit that in turn comprises an operational contactor110and a series circuit formed from a pre-charge contactor112and a pre-charge resistor114, wherein this series circuit is connected in parallel to the operational contactor110. The pre-charge circuit and the link capacitor108form a series circuit with the lithium-ion battery cells102. The lithium-ion battery cells102form a battery or an accumulator.

The discharge circuit comprises an end relay116and a discharge resistor118connected in series to the discharge relay116. The discharge circuit is connected in parallel to the component106or to the link capacitor108.

The battery system100further comprises a battery management unit120, which is designed to carry out one of the methods described hereinafter with reference inter alia toFIGS. 4 and 5.

The operation of vehicle components, such as electric motors, booster generators, and charging units as consumers at the battery system100causes peak loads, in particular when said vehicle components are switched on or off, said peak loads normally being buffered by an electronic storage device in the battery system100. The link capacitor108forms such an electronic storage device. If a vehicle component is connected to the battery system100, the link capacitor108is thus initially charged, then the vehicle component itself can be put into operation. The link capacitor108is charged from the lithium-ion battery cells102via the pre-charge resistor114, wherein the battery management unit120controls the pre-charge contactor112in such a way that it connects the pre-charge resistor114to the lithium-ion battery cells102. Once charging of the link capacitor108is complete, the battery management unit120closes the operational contactor110in order to put the vehicle component into operation.

Quick charging of the link capacitor108generates high currents in feed lines and in structural elements of the high-voltage network104. The high currents reduce the service life of the feed lines and structural elements concerned. Slow charging would indeed be gentle on the structural elements, but requires correspondingly more time before the vehicle component can be put into operation.

The current through the link capacitor108decreases to the extent that it is charged. In this regard,FIG. 2shows the curve of the power received by the pre-charge resistor114during charging over the period tl. The area below the power curve shown inFIG. 2corresponds to the energy received by the pre-charge resistor. Here, with the onset of charging of the capacitor, a high power Ppis received in the resistor and is converted into heat.FIG. 2shows three successive pre-charging operations, for example.

Here, the loading of a plurality of charging operations carried out corresponds to the individual loads added over the period of time tv, as is shown inFIG. 3by the power curve Pc.

A method400in accordance with an exemplary embodiment of the disclosure is shown inFIG. 4. In a first step402, the battery management unit120measures a first voltage at the link capacitor108. In a subsequent step404, the battery management unit120controls the pre-charge circuit in such a way that the link capacitor108is charged. In a subsequent step406, the battery management unit120measures a second voltage at the link capacitor108after the charging operation and then forms, in step408, a voltage difference from the first and the second measured voltage. The battery management unit120, in a subsequent step410, determines an energy received by the pre-charge resistor.

A method500in accordance with a further exemplary embodiment of the disclosure is shown inFIG. 5. In a first method step502, a start temperature of the pre-charge resistor is extracted from a non-volatile memory, or for example is determined at an ambient temperature of approximately 60° C. In a subsequent step504, an energy output from the pre-charge resistor in the form of heat is determined, wherein the heat output causes a cooling of the pre-charge resistor.

The heat output and therefore a temperature change of the pre-charge resistor can be determined in step504as follows:

In this case, T is the determined current temperature of the pre-charge resistor; W is the energy output by the pre-charge resistor; Cpis the heat capacity of the pre-charge resistor in the unit of joules per kelvin; Gthis the thermal conductance in the unit of watts per kelvin at ambient temperature; Tambient,maxis the maximum ambient temperature and telapsedis the time elapsed during the heat output.

In a subsequent step506, an energy received by the pre-charge resistor in the worst-case scenario is determined as follows:

In this case, Ww.c.is the energy received by the pre-charge resistor114in the worst-case scenario in the unit of joules, given from the total voltage of the battery cells102, that is to say the battery voltage Ubattery, the ohmic resistance Rvof the pre-charge resistor, and the time t during which energy is received.

In a subsequent step508, the temperature Tw.c.of the pre-charge resistor114in the worst-case scenario is determined as follows:

In this case, T is the current temperature of the pre-charge resistor114, that is to say for example the ambient temperature in a first method run-through.

In a subsequent step510, the condition as to whether the temperature in the worst-case scenario Tw.c.is less than a fixed maximum temperature for the pre-charge resistor is checked. If the condition is met, the method branches to a subsequent step512, in which the link capacitor108is charged or pre-charged. If the condition in step510is not met, the method500branches back to step504, in which the pre-charge resistor114is cooled or the cooling of the pre-charge resistor114is determined.

In step514, the energy actually received by the pre-charge resistor is determined as follows:

In this case, W is the determined energy that is received by the pre-charge resistor; Ubatteryis the total voltage of the battery; wherein the voltage of connection elements Ulinkbetween the battery cells and the pre-charge resistor114is subtracted from said total voltage; Rvis the ohmic resistance of the pre-charge resistor; and tchargeis the time elapsed during the charging of the link capacitor108.

In a subsequent step516, the condition as to whether the pre-charging operation is complete is checked. If the condition is met, the method500branches to the next step518. If the condition in step516is not met, the method500branches back to step514and the link capacitor108is charged further, during which time the energy received by the pre-charge resistor continues to be determined.

In step518, the heating or the temperature T of the pre-charge resistor114present once the pre-charging of the link capacitor108has ended is determined as follows:

The methods described with reference toFIGS. 4 and 5can be used for thermal protection of the pre-charge resistor114in the battery system100, wherein the battery management unit120is designed to carry out such a method. The battery system120can in turn be used in a motor vehicle and can provide a greater reliability of the motor vehicle.

In accordance with Ohm's law, instead of determining the voltage before and after the pre-charging operation, the flow of current during the pre-charging operation can also be measured. The formulas for the received energy of the pre-charge resistor apply accordingly in this regard.

In a further exemplary embodiment, a further pre-charge circuit can be used instead of the pre-charge resistor in order to charge the link.