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Among others, we have

  • HP35
  • HP97

as well as

HP 25 battery pack reconstruction

We have reconstructed the battery pack of our HP 25:

You need a knife and a couple of rechargeable batteries. The 1.2 Volt NiMH batteries that we used worked fine. There are lots of important and additional details on The Museum of HP Calculators.
There are several options, we choose to cut through the plastic strip that separate the batteries and slide out the original batteries. (These old batteries worked for a while, actually, but eventually gave up.)
Just to remind us: It is important to use the correct polarity when inserting the new batteries into the battery pack.
The old battery pack with new batteries inserted.
And we are flying again.
The same battery pack works for our HP 21.