Method and device for determining the charge status of a battery

[0005] In a subsequent step 103, the integral of the battery current over time is determined. Step 103 can be carried out immediately after initiation of the starting operation, or it may also be carried out at any desired time during battery operation. Suitable periods of time over which the integral is determined are obtained in a conventional manner.

[0006] The operability of the starter battery of the internal combustion engine in a motor vehicle (or any other system powered primarily by an internal combustion engine) is one of the most important prerequisites for ensuring operating reliability. An internal combustion engine is generally started by an electric starter which draws power for the starting operation from the starter battery. The starter must briefly apply a torque capable of turning over the internal combustion engine at a certain minimum rpm. To do so, a sufficiently high current must flow through the starter or a sufficiently high voltage must be applied to the starter. If the starter battery is very cold, partially discharged or severely aged, its internal resistance may be so high that insufficient current and voltage are available to ensure this starting operation. Such a problem is described in German Patent No. 19705634 C2, for example.

[0007] An attempt has been made to determine the charge status of a battery, in particular an automotive lead-acid battery, based on a known initial charge status, e.g., a full charge, by integration of the battery current over time. However, in this case the charge status must be recalibrated at certain intervals because of errors that occur in measuring the current and in current integration and because of battery gassing currents. For example, it is conceivable here to use the open-circuit voltage, which can be determined with a battery with no load connected to it after a lengthy resting phase, in particular lasting for more than four hours, and is proportional to the charge status. However, it has been found that the accuracy of such methods is inversely proportional to the length of the resting phase, i.e., such methods lead to unsatisfactory results in the case of operating cycles with short resting phases such as taxi operation.

[0008] An object of the present invention is to improve on the accuracy of methods of determining the charge status of a battery, in particular for operating cycles with short resting phases.

[0009] In contrast with known methods of determining the charge status, where current integration and recalibration must be performed, in the method according to the present invention the current integral is not replaced by a calibration value but instead it is corrected by an instantaneously measurable no-load voltage U₀to form a weighted mean. It is also possible with this measure to use short resting phases, in particular resting phases lasting less than four hours, with a battery under no load for correction of the current integral, so that errors occurring in current integration can be minimized relative to the errors occurring in traditional methods, in particular with corresponding operating cycles with short resting phases (especially in taxi operation). Another advantage of the method according to the present invention is that errors in determination of the open-circuit voltage from longer resting pauses have less effect on the charge status determined. Thus on the whole, the accuracy in charge status determination when using current integration methods is improved by the method according to the present invention.

[0010] According to a preferred embodiment of the method according to the present invention, the weighted mean is used to determine charge status soc on the basis of an equation in the form:

[0011] soc=(1−gew)*soc_i+gew*soc_U0  (1)

[0012] where gew is a weighting factor for which the inequality 0≦gew≦1 holds. The introduction of such a weighting factor can be handled relatively easily by computing, and the software complexity required to carry out the method according to the present invention can be kept low.

[0013] It is especially preferable that the weighting factor is selected on the basis of an equation of the form:

[0014] gew=1−exp (−t_R/T_R)  (2)

[0015] where t_Ris the duration of a resting phase prior to the measurement, and T_Ris the time constant of the transient response of no-load voltage U₀until it reaches open-circuit voltage U_Rof the battery. In conventional terminology, open-circuit voltage U_Ris understood to be a no-load voltage U₀which is ultimately established after a prolonged period of no load condition and which depends on the battery temperature and its history (charging or discharging before the resting phase).

[0016] FIG. 1 shows a flow chart to illustrate a preferred embodiment of the method according to the present invention.

[0017] FIG. 2 shows a schematic block diagram of a preferred embodiment of the device according to the present invention.

[0018] According to a preferred embodiment of the method according to the present invention, the no-load voltage of a battery whose charge status is to be determined is itself determined in a step 101. This step is preferably carried out during or immediately after a resting phase or immediately before a starting operation of an internal combustion engine, which is in operative connection with the battery. In a step 102, charge status soc_U0of the battery is calculated in a known manner on the basis of no-load voltage U₀thus determined.

[0019] In a subsequent step 103, the integral of the battery current over time is determined. Step 103 can be carried out immediately after initiation of the starting operation, or it may also be carried out at any desired time during battery operation. Suitable periods of time over which the integral is determined are obtained from the concrete givens in a known manner.

[0020] Then in a step 104, another charge status soc_iis calculated on the basis of the battery current integral thus determined.

[0021] Finally, in a step 105 an average is formed from the determined and calculated charge status values soc_U0and soc_ito obtain an average charge status soc.

[0022] Weighting for the averaging of step 105 is preferably performed on the basis of an equation of the form:

[0023] soc=(1−gew)*soc_i+gew*soc_U0where 0≦gew≦1.

[0024] Weighting factor gew is selected as a function of an accuracy, in particular a known or estimated accuracy, of open-circuit voltage U_Rthus determined, which in turn depends on duration t_Rof the resting phase. It has proven to be expedient to select a weighting factor gew according to an equation of the form:

[0025] gew=1−exp(−t_R/T_R).

[0026] Value T_Rcorresponds to the time constant of the transient response of no-load voltage U₀until it reaches open-circuit voltage U_Rwhich depends on the battery temperature and its history. For example, if values of soc_i=0.8 and soc_U0=0.6 are determined, and if a resting phase t_R=0.5 T_Ris assumed, this yields a weighting factor of 0.39, from which it is possible to calculate the battery charge status as follows:

[0027] soc=0.61*0.8+0.39*0.6=0.722.

[0028] A deviation in charge status soc thus determined from a simple average can thus be seen.

[0029] For the case when t_R=T_R, this yields a weighting factor of 0.63, which in the above example leads to a battery charge status soc of 0.674.

[0030] The method according to the present invention has the effect that the influence of no-load voltage U₀, which is always known with greater accuracy as the resting phase becomes longer, has a greater influence on the results of averaging.

[0031] FIG. 2 shows a device 20 for measuring the no-load voltage of a battery. Furthermore, it also shows a device 21 for measuring the battery current. These devices 20, 21 are connected, optionally with an A/D converter (not shown) connected in between them, to a computer or controller device 22 which calculates battery charge states soc_U0and soc_ion the basis of the measured values thus obtained and performs the averaging described above to obtain an average battery charge status soc.