In this article I discuss the two methods of NiCd and NiMH charging -- standard and trickle. The "overnight" charger that comes with most rechargeable powered products charges at a rate of C/10 (the C rate is the hour capacity of the battery, i.e. a typical AA NiCd battery of 600mAh capacity has a C rate of 600mA, and a C/10 rate of 60mA). There is a very good reason why the manufacturer chose this rate. If the charger uses a higher rate, it would have to detect when the batteries are fully charged and shut off, or risk damaging them. This would make the charger more complex, and hence more expensive. Lower charge rates than C/10 unnecessarily extend the charging time, and in fact at very low rates (below C/50) the batteries never fully charge no matter how long you wait.
Thus the C/10 charging rate is a compromise between keeping the charger simple, yet charging the batteries in an acceptable amount of time. At the C/10 rate the battery will reach a full charge after approximately 14 to 16 hours. If the actual battery capacity was the same as its rated value, and its charging efficiency was 100% then only 10 hours would be necessary to fully charge a battery. But, actual capacity is usually greater than rated and charging efficiency is always less than 100%, thus 14 to 16 hours of C/10 charging ensures a fully charged battery. At this point any further charging only results in an increase in temperature and internal cell pressure. This does not damage the battery, although it accelerates their deterioration thus reduces their reliability.
Now that we know what the standard charger is, how do we use it? Most important, follow the manufacturer's instructions. A typical manufacturer recommends a 15 hour charge to fully charge a discharged battery. If your battery is only partially discharged, your can prorate the charge time. For instance, a battery that is one third discharged will fully charge in only 5 hours. If you don't know the condition of your battery, then you should charge it for the full 15 hours. What happens if you forget to disconnect the charger and end up charging for more than the required amount? If you overcharge by only a few hours, don't worry. If you leave your charger connected for a couple of days, then you're unduly stressing your batteries. If you're the forgetful or worrying type, then you might want to use a timer. I like to use the standard 24 hour security timer, used to switch lights on and off when you're away. It is sold almost anywhere, usually less than $10 on sale. To use a timer with removable ON/OFF pins for standard charging, first rotate the time dial until 12 midnight. Now insert an OFF pin at the 3pm position (15 hours) and remove any remaining pins (figure 1). Now plug your charger into the timer and the timer into a wall socket. Rotate the on/off switch on the timer until your charger turns on, and leave it alone. After 15 hours your charger will shut off and remain so. If you have a timer without removable pins set it up as above, but instead of inserting an OFF pin at 3pm, set an ON/OFF pin (or if separate pins, the ON and OFF pins together) at 2:30pm such that the timer will turn on at 2:30pm and off at 3pm (figure 2). It will operate similar to a single cycle timer, except on subsequent days the charger will turn on for one half hour each day (actually an advantage as we'll see later).
