Method for monitoring an energy store in a motor vehicle
A method for monitoring an energy store in a motor vehicle, wherein the energy store supplies at least one, in particular safety-relevant, consumer, preferably for an automated driving function. In the method, at least one characteristic variable of the energy store is predicted. At least one measured variable of the energy store is recorded and at least one state variable of the energy store is ascertained as a function of at least the measured variable. The characteristic variable is predicted using a variable that has been stored in advance as a function of at least one state variable and that represents an influence of a loading history of the energy store on the characteristic variable. The variable is selected as a function of the state variable.
1 . A method for monitoring an energy store in a motor vehicle, wherein the energy store supplies at least one safety-relevant consumer, the method comprising the following steps:
predicting at least one characteristic variable of the energy store;
recording at least one measured variable of the energy store;
ascertaining at least one state variable of the energy store as a function of at least the measured variable;
wherein the characteristic variable is predicted using a variable that has been stored in advance as a function of at least one of the state variables that represents an influence of a loading history of the energy store on the characteristic variable, wherein the variable is selected as a function of the one of the state variables.
2 . The method according to claim 1 , wherein the energy store is loaded with a load profile including at least one base load and one peak load, and, after a rest phase, until an open-circuit voltage of the energy store has been reached, the energy store is loaded with a further load profile including a peak load, and wherein an arising voltage curve is evaluated to ascertain the variable.
3 . The method according to claim 2 , wherein the arising voltage curve is evaluated to ascertain a time constant with which voltage of the energy store increases again in the rest phase, and/or to ascertain a voltage level to which the voltage increases again at a start of the loading with the further load profile, and/or to ascertain a dynamic voltage including a voltage difference of a voltage drop during the loading with the load profile including the peak load and previous base load, and a voltage drop during a subsequent loading with only with the peak load.
4 . The method according to claim 2 , wherein the voltage curve and/or the variable which includes a dynamic voltage, is ascertained again as a function of at least one of the state variables in the case of a changed state variable.
5 . The method according to claim 1 , wherein the variable, which includes a dynamic voltage (Udyn), is specified by an equation as a function of at least one of the state variables of the energy store by the following equation: Udyn (Q. U0, T)=a*exp(b(U0)/T)*Q+c, wherein a, c, d are constant parameters to be determined depending on measured values, and T represents a temperature, Q represents a charge, and b represents a variable dependent on a state of charge or the open-circuit voltage (UO).
6 . The method according to claim 1 , wherein a time constant including a relaxation time, which represents a relaxation behavior of the energy store is used as the variable which represents an influence of a loading history of the energy store on the characteristic variable.
7 . The method according to claim 6 , wherein the time constant is ascertained as a function of a temperature of the energy store and/or is stored in an assignment table.
8 . The method according to claim 6 , wherein a state detection of the energy store ascertains, as a function of at least the measured variable, at least the state variable of the energy store, the state variable including an internal resistance of the energy store and/or a state of charge of the energy store and/or an open-circuit voltage of the energy store and/or a temperature of the energy store and/or a charge dependent on the time constant of the energy store.
9 . The method according to claim 8 , wherein: a sensor is provided which: (i) senses the measured variables and/or (ii) includes the state detection and/or (iii) is configured to store the variable.
10 . The method according to claim 6 , wherein the relaxation behavior of the energy store is represented by a state variable in the form of a charge, wherein the time constant is used to ascertain the state variable.
11 . The method according to claim 6 , wherein the relaxation behavior of the energy store is represented, via an expected voltage curve, how much the voltage curve increases in the rest phase.
12 . The method according to claim 1 , wherein the predicted characteristic variable depends on an open-circuit voltage of the energy store and/or on a dynamic voltage of the energy store and/or on a voltage drop across an internal resistance.
13 . The method according to claim 1 , wherein the characteristic variable is predicted as a function of a ratio of an internal resistance and a polarization resistance of the energy store, wherein, as a function of at least one current state variable of the energy store and/or of at least one current measured variable of the energy store, an associated ratio of the polarization resistance to the internal resistance is selected and is used for the prediction of the characteristic variable.
14 . The method according to claim 1 , wherein the predicted characteristic value is ascertained as a function of a polarization voltage and/or of a ratio, stored in advance, of a polarization resistance to an internal resistance, multiplied by a currently ascertained internal resistance and multiplied by a characteristic variable describing a peak load of the load profile including a peak current.
15 . The method according to claim 1 , wherein, when the predicted characteristic variable reaches a limit value, a measure is initiated including a blocking of a function that is safety-relevant and/or outputting a warning.