Abstract: An automatic electric battery charger which detects when the battery is fully charged by monitoring the rate of rise of battery voltage; when the rate of rise falls below a predetermined value, the charge is terminated. The rate of rise of voltage is detected by comparing an unprocessed battery voltage signal with a signal which follows the battery voltage signal with an exponential lag having a time constant of about 45 minutes. With a low rate of rise of voltage, such as occurs when the battery approaches the fully charged state, the lagging signal will tend to catch up with the unprocessed signal, and when the difference falls below 11 mV per cell, the charged is terminated.In the preferred embodiment, the signal processing is digitally implemented, using a microprocessor.