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Abstract

A method for determining the state of charge of rechargeable batteries by measuring the rechargeable battery voltage and comparing it with predetermined families of characteristic curves, the rechargeable battery voltage U.sub.Bi, the charging time t.sub.Li and, from comparison with characteristic curves .DELTA.Q.sub.i=f(SOC,t.sub.L,U.sub.B) the charge increase are determined in each charging phase. In a subsequent discharge phase, the rechargeable battery voltage U.sub.Bi is measured in time intervals, a rechargeable battery voltage value U.sub.Bij+1 at the end of a time interval is determined from the rechargeable battery voltage U.sub.Bij at the beginning of the time interval by means of an assumed current value I.sub.Bij by comparison with families of characteristic curves U.sub.B=f(I.sub.B,R.sub.i,SOC), and a corrected current value I.sub.Bij+1 is determined by iteration from the deviation of the value thus determined of U.sub.Bij+1 and the rechargeable battery voltage actually measured at the end of the time interval. The loss of charge is calculated from I.sub.Bij+1. During the charging phase and the discharge phase, the rechargeable battery temperature T is measured and the comparison is effected with families of characteristic curves which contain the rechargeable battery temperature T as an additional variable.

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