Abstract: A method for determining the state of charge of lead-acid rechargeable batteries comprising a) substantially simultaneously measuring measured-value pairs (Ui, Ii) of the rechargeable battery voltage and current flowing at time ti over time interval dt, b) selecting a group of said measured-value pairs (Ui, Ii) for which only a discharge current flowed in the last time interval dt, c) varying parameters Uo, R and C such that a residual sum of squares between values Ui given by formula (1)

Abstract: In a process for determining the state of charge and the peak current loadability of batteries in the currentless pauses before and after a loading phase the no-load voltages U.sub.01 and U.sub.02 are measured. From them, with allowance for battery-specific parameters, especially the time curve of the no-load voltage, the true battery rest voltages U.sub.001 and U.sub.002 are computed. During the loading phase the converted current quantity q is measured and from the relationship U.sub.002 -U.sub.001 =C.sub.1 .multidot.q/Q.sub.0 the acid capacity Q.sub.0 of the battery is found. The relative state of charge SOC.sub.1 is determined from a curve of the rest voltage U.sub.00 linearized by the formula SOC.sub.1 =U.sub.002 /C.sub.1 -C.sub.2 as a function of the state of charge of the battery from which the absolute state of charge is calculated as SOC.sub.1 .multidot.Q.sub.0.From the internal resistance R.sub.