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HP Forum Archive 08

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Replacement battery for HP-41C rechargeable pack
Message #1 Posted by Dave Mabry on 29 June 2002, 6:17 a.m.

I know this has been discussed before and is in the archives, but I'm hoping there is some new information. Can someone point me to a good replacement battery for the rechargeable pack from HP for the 41C? It seems the exact replacement is no longer made and some recommend a 1/3 AAA. The capacity is pretty low on that cell and it seems there is a reasonable chance of overcharging it.

Can anyone suggest a good replacement cell and a source to buy that cell? Thanks for any help.

Re: Replacement battery for HP-41C rechargeable pack
Message #2 Posted by Ron Meek on 11 July 2002, 1:29 p.m.,
in response to message #1 by Dave Mabry

I have been using "N" rechargeable batteries in my calculator since it was new. I recharge them with an old Motorola pager that had been run over, but still had the charger and the cradle. You could probably use almost any charger, provided the charge current didn't exceed 10% of the rated capacity of one of the batteries. Voltage is not too important, current (milliamps) is.

Message #3 Posted by Andrés C. Rodríguez (Argentina) on 11 July 2002, 4:59 p.m.,
in response to message #2 by Ron Meek

Well, voltage and current are usually related (the simpler case is Ohm's Law, where V = I * R); even in a charger there is a relationship, but depending on the design the relation may be not so simple.

Simple chargers apply a current which usually is 10% of the rated capacity of the cell, so a 500 mAh cell receive 50 mA. Usually it takes some 14 hours to charge a fully discharged cell, and about 8 hours to replenish a partially discharged one.

Intelligent chargers allow for shorter charging times, but to avoid overcharging, overheating, venting or even explosion of the cell, they monitor internal resistance, charge level, temperature, or other condition of the charging cell, and adjust the current as a function of such.

As different cell chemistries (NiCd, NiMH) have different behavior about charging times, current profiles, temperature increases during charging, etc; it is advisable to use an external charger for the specific kind of batteries you are working with.

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