Rebuilding an HP 41 battery pack


I am in the process of rebuilding a 82120A battery pack for my 41CX with new NiCd cells. The problem I'm having is that the old cells leaked and damaged the metal contacts that mate with the calculator. They are both broken and I don't think they can be used again.

Does anyone have an ideas for a substitute for those metal contacts? My guess is they have to have some spring or flex in them so the battery pack can be removed, but maintain contact otherwise.



what about trying foam with a plate, like in all the 48 series?


That would work. I also thought of just using a small spring. I've been busy and haven't had a chance to try anything yet.




The battery pack is kept in place by the force of the 4 spiral springs inside the battery pack. For my better understanding, which contacts are coroded? The thin copper layer in the battery compartment? Or the springs in the battery holder?

In case you want to rebuild a battery case from scratch, this is very simple, based on 4 commercially available N size cell holders.



Hi Andreas,

The rechargeable battery pack that I'm trying to rebuild isn't constructed the same as the N cell pack. Part of the rechargeable pack is taken up by the charging circuit. The rest of the pack contains 4 small NiCd cells tab-welded together, then each side of the pack has a piece of spring steel that makes contact between the batteries and the copper in the calculator. It is this flex that replicates the springs in the normal N cell holder.

Of course, it is probably easier just to use N cells in the normal holder, but that's not the point!



Hi David, I am in the same situation, and I thought about springs, but ruled them out, as I doubt they would stay in place. I am going to try some thin "spring steel" shaped pretty much to the same shape as the originals. I'll let you know how I go, if it works, I can send you some of the same steel - I have enough to build 100s of 82120 :-)



Thanks for the offer John. My thinking with the springs would be to solder them (if possible) right onto the ends of the batteries. I think that, you're right, they would move and cause problems otherwise.

Fabricating a duplicate part would be ideal. Let me know how it goes.



Hi David,

Prototyping the replacement is quite sucessful, using 1/100" thick stainless steel, a little stiffer, but I think they will work fine. I take it Etienne sent some originals to you anyway, but if you want me to make you a couple contact me off forum.



Hi David,

You have mail.




Thank you Etienne for donating and sending the parts needed to complete this project. Everyone here is more than generous with their time and willingness to help others.



ciao a tutti,
I haven't tried out personally, but while I was at the shop the other day
buying spare cells, I met a guy and almost for fun we started to talk about berillium copper
which has the right flexibility for rebuilding batteries contacts
Guess what ? he told me the easiest way to get berillium copper pieces is to take apart the following ...

The inside contacts are made out of berillium copper strips

I think it's worth a try, what do you think ?
Hope is fo help to someone, take care Alberto


Beryllium is highly toxic - especially the dust, which might well be generated if you try to cut and file this to shape. Be careful!


Thank you very much for the advice !

There is always the springs option

This worked out for me, the springs are from one of my child broken toys (I swear I didn't break it just to recover the springs....)

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