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Hello:

I would like to know the exact specifications of the small 4 batteries that comprise the HP82120A HP41 battery pack. I know that are NiCD, 1.2V? , but what is the capacity? Can be safely replaced by NiMh?

thanks

I can't speak to the specifics of that battery pack, but I can tell you a little about NiCd and NiMH batteries. Both are 1.2 volt nominal cell voltage. Fully charged is 1.45 volts/cell, for a new battery (they may not charge that high after they get old), it will drop off from that after you take it off the charger pretty fast to about 1.35 or so, even if it isn't under a load, 1.1 volts/cell under load is pretty close to dead. NiMH generally have a higher capacity than NiCd for the same size. Any device that works with NiCd's should work fine with NiMH. The only exception is when it is a very high current drain device like an electric model airplane or large camera flash, in that case NiCd's preform better than some NiMH's but that is less true as of late. The problem is in charging. A fast charger for a NiCD won't charge a NiMH right, However a slow charger will charge either battery fine. On slow charging the question becomes how slow is it going to be. The typical slow charge rate is 1/10 of the battery capacity. In other words if it is a 2000mAh battery you should slow charge it at 200mA, and at that rate it should take 14 hours. If you do the math you will see that it takes about 40% more energy to charge a battery than is stored in it. This same ratio works if you charge it at a lower or higher rate but you don't want to go very much above or you enter fast charging range and you need a charger specifically for that. If you go too much slower I don't think it will charge at all. I would guess that a 1/20 rate would still do the job but I don't think if you went a lot lower it would ever get fully charged.

Chris W

Thanks for your clear explanation, but I need to replace some dead small batteries from an HP82120A and I need the exact dimensions and capacity of the batteries.

I have found in http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=335

thank you very much for your explanation

The cells were 1/2N 70-80 maH NiCad cells. They are no longer being made. There are some 1/2N NiMH cells out there, but you won't be able to find them. Most people use a small NiMH cell and something to shim the space with. The NiMH cells that fit have around a 140-150maH capacity. The 1/3 AA cells are just a smidge too big to fit unless you want to carve up your case.