HP-21 & HP-25 Battery Pack


How can I make or how can I get a battery pack for an HP-21 ?. Thanks


Hi Jordi.

Please have a look at


if you haven't yet; if you go through the entire section, yuo'll find some references to build or buy battery packs.

Hope this helps.

Best regards.



In my opinion, the battery packs for the HP-21 and HP-25 are the easier to rebuild, you just have to buy 2 AA NiCd batteries and put them there, if you don't want you don't even have to solder them.


Hi Jordi,
If you don't have the shell for the original battery pack assembly, you will need to acquire one, as the battery compartment door is integral with this shell. Watch on E-bay, etc, and they are sometimes available.
I just cut the center divider out of my shell, so I can snap rechargable batteries in and out of it (I marked the shell for + and -). It works OK, and I don't have the cells soldered together, as in the original pack. Occasionally I will lose power to the calc, and I have to reposition the battery pack to get it back...I think soldering the cells together would cure this problem, but it's something I can live with.
One piece of advise if I may, and my appologies if it is redundant to you, but please don't charge the batteries with the AC adapter while in the calculator...charge them externally. With woodstocks (HP 20 series) you run the risk of damaging the calculator with overvoltage if the battery pack loses contact while the calculator is connected to the AC adapter.
Best regards, Hal


Assuming you have the battery holder and simply want to rebuild it, my advice is to get rid of the charger and use ordinary alkaline cells. You can remove the plastic piece between the two batteries to allow easy replacement and carefully mark the holder with the correct polarity. E.g.



The required polarity is shown inside the battery holder I have. It is on the angled section on the long side of the bottom of the holder.


Oh wow! I never noticed this, probably because I was looking
at the bat. compartment from the wrong side.

Thanks for the very interesting remark


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