This circuit came about as the result of an urgent need for a NiMH battery charger. No suitable dedicated IC being immediate to hand, the author pressed an L200 regulator and a 4.7 kΩ NTC thermistor into service. Those components were enough to form the basis of a charger with a cut-off condition based on cell temperature rise rather than relying on the more common negative deltaV detection.
The circuit uses the L200 with the thermistor in the feedback loop. When ‘cold’ the output voltage of the regulator is about 1.55 V per cell; when ‘warm’, at a cell temperature of about 35 °C to 40 °C, the output voltage is about 1.45 V per cell and the thermistor has a resistance of about 3.3 kΩ. This temperature sensing is enough to prevent the cells from being overcharged. P1 adjusts the charging voltage, and R2 limits the charge current to 320 mA. The IC is fitted with a small 20 K/W heatsink as it dissipates around 1.2 watts in use.
The charger circuit can be connected permanently to the battery pack. Charging starts when a ‘plugtop’ adaptor is connected to the input of the charger. The unregulated 12 V supply used by the author delivered an open-circuit voltage of 18 V, dropping to 14 V under load. Even though the charge voltage is reduced when charging is complete, the cells should not be left permanently on charge. The author uses the circuit to charge the battery in a torch. After three years and some 150 charge cycles, the cells are showing no signs of losing any capacity.