Should you completely discharge a rechargeable battery every time you use it, or just the first few times?
by 
Roger Kovaciny
ANSWERS: 4
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This depends on the battery technology. A NiCd (nickel-cadmium) battery should always be fully discharged before you recharge it. This greatly extends their life, as they will develop a 'memory'. If you routinely recharge them from a partially discharged state, they will lose their ability to hold a full charge. More NiCd batteries are replaced because of this problem than for any other reason. NiMH (nickel-metal-hydride) batteries can also have the memory problem, but not as severely as NiCd batteries. Li-ion (lithium-ion) batteries are not prone to this problem. When you obtain a new NiCd or NiMH battery, you should 'season' it before use: properly charge it and then fully discharge it several times before use. This helps ensures that the new battery will be able to hold a full charge.
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Yes you always should unless it is a Li-ion battery even then it is best to do it to Li-ion because it will give you better accuracy on the battery low feature on portable DVD players and other electronics.
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Beware of completely discharging A SET OF BATTERIES, because you may encounter reverse voltage in the weakest batteries, which will do real harm to that battery cell. Most NiCad and NiMh manufactures warn about mixing old and new batteries for that reason. That is especially true in many of the higher voltage power tools where a set of batteries may contain up to 15 individual batteries or cells. Tool manufactures like DeWalt suggest people recharge batteries just after you notice they are starting to get weak to avoid this problem. They also say that even though they and most other tool manufactures make quick chargers, the best battery life can be achieved by topping off battery sets with at least a 4 hour slow charge, so that all the cells are as closely charged as possible. As a note, most NiCad and NiMh battery curves are pretty sharp. Once you notice power is running out, you have little to gain by attempting to squeeze the last 5% out of you battery set and a lot to loose if it hurts a cell in the set.
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If you really want to know a lot more about NiCad and NiMh battery charging than I could ever fit in this forum consult real experts like the one at: http://www.camlight.com/techinfo/techtips.html Read the entire article; it is written for laymen, (no real chemistry), but lots of truth about those batteries and how you should treat them if you want to get the most out of them. By the way, where I work we have “battery labs” that do nothing but try and figure out ways to make the next satellite’s batteries last as long as possible, (and weight as little as possible). When you are finished you will know hype when you see it because you will know some real facts.
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