HP-41 rechargeable pack


Hi. Is there an article somewhere showing how to recell a HP-41 rechargeable pack that have dead Ni-Cd batteries.

I intend to install Ni-MH batteries in there, is there a problem with the wall charging system for the 41 and these Ni-MH batteries insted of Ni-Cd batteries.

I have new batteries WITHOUT tab, is it ok (i didn't open the pack yet to see inside)?

Thanks for tips!


I was going to post the same exact question this morning, but you beat me to it. I am looking for a step-by-step procedure; How to get into an HP-41 battery pack (Part #82120A) and replace the batteries. Please check the web page for some info on replacement batteries. http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=254

Does anyone have any recommendations for the type, make, model of replacement batteries??


To get into the pack, I used an x-acto saw. Not a knife, it's a very fine serrated blade. Gets you in very neatly without breaking the plastic. Saw around the perimeter where the plastic halves are joined. Works on other HP battery packs too. Be careful not to cut into the cells. To reassemble, just use some plastic cement.


The charger is okay, but you'll need batteries with tabs so you can solder to them.

I rebuilt a pack using 1/3AAA ni-mh from Gold Peak. I bought them from the Battery Station listed in the Museum repairs section:


IIRC cells were 2.50 ea. You must ask for TABS on the batteries, they are free. The capacity is about double that of the original ni-cads. My 41C with a card reader has been working for a month or more on the original charge, the pack voltage hasn't dropped more than 50 mv.


If you are buying your cells from a Batteries Plusm Batteries Etc, etc just have them weld the string up for you... no need to risk damaging these very delicate cells with soldering iron heat. The smaller the cell, to more likely it will be damaged by heat.

I cut my packs open with a razor blade (or two or three depending upon how many I break). The 82120A packs are very well sealed, but do give in to a little razor work (and sometimes a tap or two with a hammer... wear eye protection).

Then I bring the half of the case with the little cut out windows and the charger circuit (with the old cells still connected) to the battery store so they can get the cells spaced properly.

Normally I just have the battery store cut open and rebuild packs, but not for the 82120A. I want to test the charger circuit before I seal it up, and I don't trust them to get the contact springs installed properly.

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