Abstract: The charging voltage U.sub.L of the battery being charged is sensed and amplified by an amplification factor. The amplification factor is determined by the sensed charging voltage. This provides automatic scaling of the charging voltage. The amplification factor K can be repetitively or continuously adjusted during battery charging. The time behavior of the input signal is monitored. The monitoring unit determines whether the time behavior of the input signal fulfills predetermined or given criteria. When a criterion is met, a check-result signal is generated, which controls charging, for example, by stopping charging when the time behavior indicates charging is completed. Time behavior is monitored by monitoring a second order time derivative of the input signal. The monitoring unit performs optimally within a certain magnitude range. Therefore, the amplification factor is adjusted to set the input signal at a magnitude within that range.