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) Ui=Uo−R*Ii+1/C∫Idt  (1) and measured values U(ti) is minimized, wherein Uo represents no-load voltage, R represents resistance and C represents capacitance, and d) calculating the state of charge of the rechargeable battery from the no load voltage obtained from formula (1).