Converting HP-25 to NiMH batteries, possible?

[BobVA]

(07-21-2022 01:40 AM)cubytus Wrote:  About using Ni-MH batteries: even if it's not ideal, if I let them charge using the original charger, at some point display segments flash randomly. Question: as I don't have a scope on hand, would it be reasonable to assume the Ni-MH chemistry doesn't provide the regulation function like the Ni-Cd did?

+1 to Maximilian's comment. The flickering display is not a good sign, but I don't think it's caused by the difference between NiMH and NiCad cells.

Either you've got an out-of-spec component in the charger or the calculator, or a possibly a poor connection to/between the batteries. (In particular the connection to rivets on the circuit board, which form the battery contacts, can degrade over time so there may be some resistance there.)

The safest path is to just not to connect the charger to the calculator and to charge the cells with an external charger appropriate to the battery chemistry.
It only takes a second to swap the cells and modern, high capacity NiMH batteries give very long run times. Just watch the polarity.

This advice may sound overly cautious since HP clearly sold a lot of Woodstock calculators and never changed the design. But there's some speculation that this is more of an issue now, due to degradation of passive components and perhaps increasing fragility in IC's as they approach 40-50 years of age.