This rapid charger circuit is made using MAX712, MAX713 integrated circuits manufactured by Maxim IC and can be used for rapid charging NiCd, NiMH batteries.
Integrated circuit output current is always in one of two states: high (quick charge) or low (pulse charge). Once detected fully charged, current is reduced and pulse charging is started. Circuit monitors three parameters to determine when the battery has reached full charge: voltage slope, battery temperature and charging time.
The charger circuit is powered by a voltage adapter with output voltage of 9 Vdc and nominal output current of at least 1A.
Maximum load voltage is limited to 1.65 V / battery. Fast charging is interrupted by the passage of scheduled charging time or lower battery voltage. R5's value is calculated with:
R5 = 0.25 / Irapid where irapid is desired charging current.
To charge a 1.2 Ah NiMH battery at a rate of 0.5 C (two hours), requires a charging current of 1,2 x0, 5 = 0.6 A. This means that R5 is 0.25 / 0.6 = 0.42 ohms. When the time scale for quick charge time has elapsed, D2 turns off and CI automatically switch to trickle charge pulse. Charge current pulse depends on the time scheduled for charging.
Number of elements and charge time is set using jumpers. Power transistor, T1, is mounted on a radiator. Power resistor R5 is mounted a few millimeters above the plate to help its own cooling.