"Rob Studdert" <[EMAIL PROTECTED]> writes: >> Do NiMH require different charger from NiCad? > > Just an extension of charge time proportional to the increase in > capacity over the original cells.
Of course, as has been pointed out here, fast charging NiMH with a peak detecting NiCd charger can cause damage through overcharging -- the extraneous heat damages the cell, and can cause venting (and, in the worst case scenario, an explosion). Overnight chargers are safe. > The beauty of the Ni-MH cells is that they are less prone to > "memory" effect than Ni-Cd and they are generally higher capacity. Ahem. The "memory" effect rightly belongs in quotation marks, since it does not exist in the real world, except if provoked by carefully creating the very special circumstances needed. It is caused by the fact that long-time trickle charging (without discharging) of the cell causes a higher internal resistance during the first discharge afterwards. This means, of course, a lower voltage during that first discharge. Now, if you take out exactly the same amount of stored power a large number of times, trickle charging the cell back up to full each time, you're eventually going to see "memory". This is because you're using the same physical part of the cell each time (or, specifically, the same outer layer of each sheet of metal), while the rest is never discharged, but being trickle charged a lot. If you then, after this has been going on for a long time, discharge the cell fully, you'll see a voltage drop when you pass the point where you've previously stopped discharging. This can cause whatever is using the cell to issue a "low battery" warning, fooling you into believing that the cell has lost the ability to store more energy than the amount normally utilized. In fact, the rest of the energy is there; it will just discharge at a lower voltage. All it takes to "erase the memory" is a single, full discharge and recharge. > However they do self discharge far more rapidly. Yup. 3-4% per day, against about 1% per day for NiCd. They also can't deliver as much current, so a camera motor drive might run just a tad more slowly with a NiMH battery than with NiCd. -tih -- Tom Ivar Helbekkmo, Senior System Administrator, EUnet Norway www.eunet.no T: +47-22092958 M: +47-93013940 F: +47-22092901
