This circuit includes a battery charger (MAX712) and step-down switching regulator (MAX5089) for handling the power portion of the charger. By controlling the regulator the MAX712 acts as a battery-charge controller, producing outputs of 7V to 16V. It operates from any DC source capable of delivering the desired fast-charge current, with this condition: the output voltage must equal 1.7V multiplied by the sum of (two plus the number of batteries to be charged in series).

All battery chargers can be regarded as constant-current power supplies, but they differ from power supplies in two important respects: battery chargers (by design) block all discharge paths from battery to charger, under any conditions. They also include circuitry that decides when the battery has taken a full charge (signaling when the full-charge current must be reduced), and when the charging process should be terminated.

Several techniques are available for deciding when a NiCd or NiMh battery is fully charged. The most common of these relies on terminating the charge when the battery terminals reach a particular voltage level, based on a characteristic increase in the positive slope of voltage versus time.

Download the full circuit and application note from Maxim: