Nickel-Metal-Hybrid (NiMH) batteries are now widely available. A typical one is the 120Ah, which has a capacity of 1.2Ah in size AA. This type does not lose more than 45% of its capacity through self-discharge over a period of 25 days at a temperature of 20oC. Recommended fast charging instructions are charging with a current of 0.3C for not more than 2.5 hours, or until the cell voltage has risen to 1.49V, or the cell temperature rises above 40oC. The cell is than charged upto 75% of its capacity, after which it should be charged with a current of 0.1C, which it can stand for a long period. The circuit shown enables these instructions to be adhered to.
NiMH battery Charger Circuit Diagram:
Output CT13 of IC2 goes high after 2.5 hours, whereupon charging stops. At the same time the output of IC1a goes high thyristor Th1. The output potential of IC4 then can’t rise above 1.2 V. Since this is lower than the battery voltage, D5 will be reversed biased. Further charging of the battery is via R15 . The value of tis resistor (in Ω) is obtained by dividing the difference between the supply voltage (in V) and battery voltage by 0.12. The quotient should be rounded off to the nearest standard value (the exact value is not terribly important).
The level of the supply voltage depends upon the number of cells contained in the battery to be charged. Its minimum value is 4V plus 1.5V times the number of cells. It is necessary to mount IC4 on a small (10 KW-1) heat sink.
Two keys must be pressed to switch the charger on. S1 resets timer IC1 and S2 switches off the thyristor Th1. These switches could be combined, but double-pole key switches are not generally available.