Strange HP41 battery pack


Happy New Year Everybody !!

Having some spare time at the begining of the year, I was sorting out
some items and I have found this :

Has anyone ever seen a battery pack with three pins ??

What is the third pin for ??

Take care and again, Happy 2011 !! Alberto


Happy New Year to you, too:-)

Yes, I have seen batt packs with 3 pins. Some time ago there also was a related thread here in the forum.

And no, I don't know exactly what the central pin is used for, but it *might* have been planned for use with a similar AC adapter as on the classics.



It is the original design 82120A, later changed to a two pin design.

Think of the 3 pin circuit being the same as the classics, when you plug the charger in, the calculator runs from a separate 5 volt source from within the battery pack while the battery charges. Remove the charger and the calculator runs from battery. The switch is that copper color tab on the center pin of the pack in your photo.

They eliminated the switch in the later design as it could glitch and crash the 41 if it was on when the charger was removed. Any 3 pin packs I rebuild, I modify the circuitry to the later design. This is usually a must as with even minor corrosion, the crazy little flex circuit strip is destroyed beyond reuse. It's easier to toss the extra parts and modify to the later design.


Thanks for the clarification,
does anyone know where I can find on TAS the necessary batteries for rebuilding this pack ?
take care Alberto


Hello Alberto,

I have bought such batteries from the seller pierreh0.
He lives in Belgium. You may contact him directly.



Hello Alberto, i'd like to rebuild a rechargeable battery pack too. If you decide to buy them from this contact, we could group our order together in order to save the shipping costs to Milan.

Please let me know,


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