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Tuesday, May 12, 2009

Batteries part 3 Nickel-Metal Hydride

These neat cells are taking the NiCad cells off the market, with their higher capacity. NiMH haven't been around that long, they are one of many products refined from the original 'space race' research. NiMH cells also produce 1.25v when charged and 1.0v when discharged, but are non-toxic. They do not handle extremely high discharges as well as NiCad batteries do, but are getting close.
NiMH cells can hold up to 3x the capacity that an alkaline cell of the same size does (up to 8AH in a AA cell). However, these cells have an extremely high self-discharge rate, losing 5-10% of their charge one day after charging, and then 1% per day after that. For this reason, they are unsuitable for emergency intermittent use, but for regular use, they rule.
There is a new type of NiMH cell out now that has a greatly reduced self-discharge rate, down to less than 5% per month or even better than the NiCad. This was achieved at the loss of capacity, these cells hold less than an alkaline cell does (although better than NiCad).
NiMH cells are more finicky than NiCad cells when charging, they can suffer damage very quickly if charged incorrectly. A smart battery charger is essential for these cells. Even a trickle charger can harm them if left on it more than a day. Their charging efficiency is at 66%, so you get a maximum of 2/3 of the power put into them. They also can be damaged by discharging them completely. If treated correctly, these batteries will deliver more than 500 cycles before failure. NiCads can deliver more cycles, but their lower capacity means that more total power was stored and delivered with NiMH cells.
NiMH cells are still more expensive than NiCads, but their other characteristics make them a more suitable general use rechargeable battery. I used to be a NiCad fan, but have switched to NiMH for most rechargeable uses (they don't go in the digital camera, though).