After disconnection from the power supply system, a discharge
characteristic of the no-load voltage is recorded automatically by means
of a control unit across a discharge resistance during a stage-by-stage
discharge cycle from a charged battery which is being operated for the
first time on the power supply system. This stage-by-stage discharge
cycle is repeated at times which can be defined, in order to record an
instantaneous discharge characteristic of the no-load voltage, and an
alarm no-load voltage is determined from the instantaneous discharge
characteristic of the no-load voltage from a previously defined reserve
time and from the final discharge voltage as determined in the first
measurement, which alarm no-load voltage represents a measure of the
energy which in each case still remains in the battery, and on reaching
which during operation on the load, an alarm is produced for the operator
of the power supply system.