The present invention relates to a method for ascertaining the charge state of a battery and a control unit.
The charge state of the battery SOC (state of charge) is an important parameter for the electrical system management of vehicles. Many vehicles have so-called battery state recognition for determining the charge state. This battery state recognition normally includes an algorithm which calculates charge state SOC, e.g., from a theoretical open circuit voltage U0. Theoretical open circuit voltage U0 is in turn calculated from an initial value U00 and the integral of battery current IBatt, taking various loss factors into account, if necessary. The following equation applies, for example, to theoretical open circuit voltage U0:
U
0
=U00+1/C0·∫IBattdt (1)
where C0 is the equivalent capacitance of the battery and U00 is the initial or starting value of the open circuit voltage. Starting value U00 is normally estimated from the measured values of battery voltage UBatt, battery current IBatt and battery temperature TBatt.
This calculation of open circuit voltage U0 still delivers relatively precise results at the beginning of the integration. However, one disadvantage of this calculation is the deviation of the calculated charge state from actual charge state SOC, which increases constantly over time, due to the integral component of equation (1). This error propagates itself in the calculation of charge state SOC of the battery in which: SOC=f(U0).
Example embodiments of the present invention provide a method and a device which may be used to determine charge state SOC of a battery more precisely.
An aspect of example embodiments of the present invention is that various methods for calculating the charge state of a battery are analyzed and an error which would result from applying the individual calculation methods is calculated. According to example embodiments of the present invention, charge state SOC of the battery is finally determined based on the calculation or estimation method whose error is smaller than the error which would result from applying a different method. This has the significant advantage that it is possible to determine the charge state relatively precisely.
According to example embodiments of the present invention, at least one method is applied which calculates charge state SOC based on the open circuit voltage of the battery. Should it turn out in this case that the error of the open circuit voltage according to a first calculation method is greater than the error of the open circuit voltage when calculated according to another method, charge state SOC of the battery is ascertained based on the second open circuit voltage. Conversely, if the error of the first method (e.g., corresponding to equation 1) is smaller than the expected error which would result from applying another method, the open circuit voltage is calculated based on the first method. The ascertained value of the open circuit voltage may, for example, be used as the initial value in equation (1).
Charge state SOC of the battery is preferably determined using the method having the smallest error. If the open circuit voltage is calculated according to equation (1), various loss factors may be considered.
A second conventional method for determining the open circuit voltage is to estimate the open circuit voltage from the battery terminal variables, namely battery voltage UBatt, battery current IBatt, and battery temperature TBatt. This preferably takes place in rest phases of the battery in which the battery is not under load or is only under a slight load and thus there is only a minimum battery current flow. According to example embodiments of the present invention, the error which would result from applying this method is preferably also calculated. This error is then compared to the error which would result from calculating the open circuit voltage according to the integration method (corresponding to equation (1)) or another method.
A third conventional method for determining the open circuit voltage is to determine the open circuit voltage when the battery is fully charged. In this case, the charge current of the battery is evaluated and if it corresponds roughly to the gassing current of the battery, the battery is assumed to be fully charged. The open circuit voltage of the battery is then set to a value close to a maximum possible open circuit voltage of the battery. An associated error may also be ascertained for this method. This error is then compared to the error of one or a plurality of determination methods and depending on the size of the error the open circuit voltage determined according to one or another method is used for calculating charge state SOC of the battery.
Example embodiments of the present invention will be explained in greater detail below with reference to the appended drawings.
In the method of
The error of theoretical open circuit voltage U0, which would result from calculating the open circuit voltage according to the integral method (equation (1)), may be estimated as follows:
where err_U00 is the estimated error for U00 in V, err_Ioffset is the offset error of the current sensor in A, err_Igain is the linearity error of the current sensor, Iloss is the estimated loss current which can no longer be recovered by discharging in A and err_C0 is the determination error of the equivalent capacitance in F.
Error err_U0 estimated in this manner increases continuously during the operating time as a result of the integration.
In order to calibrate open circuit voltage U0 from time to time, the method of steps 11 through 17 is performed in the following. In this connection, it is first checked in step 11 if it was possible to determine an open circuit voltage U0 from the battery terminal variables, namely battery voltage UBatt, battery current IBatt and battery temperature TBatt. This is normally possible only in operating phases in which battery 1 is not under a load or is only under a slight load, i.e., the battery current is roughly equal to zero.
Moreover, it is checked in step 11 if the calculation error from the integration method err_U0 (corresponding to equations (1)) is greater than error err_U00
where err_UBatt is the measurement error of the voltage sensor in V, err_IBatt is the measurement error of the current sensor in A, err_TBatt is the measurement error of the temperature sensor in K, err_extrapol is the estimated determination error based on dynamic transients in V, err_strat is the estimated determination error due to built up electrolyte stratification, and Upol is the calculated compensation voltage based on the closed circuit current and the temperature.
If error err_U0 is greater than possible error err_U00
If one of the conditions of step 11 is not true (no case), it is checked in step 13 if the calculation error from the integration method err_U0, (corresponding to equations (1)) is greater than error err_U00
The error of this third method (Index fullcharge) may be determined, for example based on measurement series across a characteristic map:
err
—
U
00
fullcharge
=f(Ubatt, Tbatt, tcharge)
where tcharge is the duration of the charging phase in s.
If error err_U0 in step 13 is greater than error err_U00
If the conditions predefined in step 13 are not met (no case), it may be checked in step 15 if a result of additional methods for open circuit voltage calculation are present. After that, the associated error value is compared to error value err_U0 and values U00 and err_U00 are adjusted accordingly in step 16, if necessary.
If no additional methods for determining the open circuit voltage are provided, steps 15 and 16 are skipped.
After that, the method branches back to step 11.
Number | Date | Country | Kind |
---|---|---|---|
10 2008 041546.4 | Aug 2008 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP09/58508 | 7/6/2009 | WO | 00 | 5/4/2011 |