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Posted by D. Sherman [24.32.202.83] on Wednesday, January 20, 2010 at 15:20:17 :
In Reply to: Re: O/T Flash Lights posted by Franz© [4.156.234.111] on Wednesday, January 20, 2010 at 14:36:25 :
The rule of thumb we always used for nicads, which is probably pretty close to working for most batteries is that you could charge at up to the "C/10" rate without any fancy gimmickry or damage to the number of charges the battery would take. "C/10" means that you divide the amp/hour rating of the battery by 10 and if you don't push more than that many amps through it while charging, you're safe. Typical AA and sub-C cells (like are used in portable power tools) are rated at somewhere around 2000 mAH, which means you can draw 2000 mA (2 amps) out of them for an hour in use. That's pretty optimistic, but that's the rating. So, you could recharge it at a rate of 200 mA without hurting it. Another way of looking at it is that if the batteries are designed to power the tool or flashlight for 1 hour when fully charged, you'd best figure on 10 hours to re-charge it without damage.
Obviously the power tool chargers push it a lot harder than that. They get away with it because they have clever circuitry and proprietary battery packs with built-in temperature sensors. Still, overall the more gently you charge a battery, the longer it will last. For car batteries, the rule of thumb has always been 5 amps for a slow charge. You can put 5 amps into a car battery for a long time without damaging it, even after it's fully charged.
12 hours as a rule of thumb for a "dumb" charger is certainly safe, but a smart charger that knows all about the battery chemistry, the temperature, the condition of each individual cell in the pack, and so on, really can charge some batteries a lot faster without any decrease in their life. Even still, you're looking at hours rather than minutes. The portable tool chargers definitely cut the life of their very expensive battery packs by pushing for the quickest charge possible, because to the guy who's trying to do a job, waiting for the battery to charge is more expensive than replacing the battery more frequently. It would be nice if they had a switch on the chargers, though, so you could set it to "quick charge" only when you really needed it.