Re: Spares for vintage calculators HP-25, HP-27?


Since the original hp charging scheme was for NiCads, has anyone made a mod. to properly charge a pair of NiMH batteries in the unit, so the battery pack does not have to be disassembled for re-charging? (Does anyone make an aftermarket HP25 battery? Or, is it always a matter of putting new batteries in the original plastic shell?)

There are instructions on the net how to cut these packs so that putting cells in and take them out is almost as easy as any other electronic consumer item.

That said if you really want to use modern NiMH then no mod is really needed. All you need is those nice low self discharge ones like Sanyo Eneloop, Panasonic Infinium etc. Yes you must charge a longer time but as they're LSD types they'll accept the charge just fine. Charging ordinary NiMH too slow and they take ages because they then waste too much of the already small current. Also you must not charge NiMH too fast without a dedicated charger. Really the "charger" isn't really a charger. It's just current limited charge method. And that works for NiMH too they're not just as tolerant as the old NiCd but since their capacity is enormous compared to the original NiCd they're going to be just fine.

I don't know the charging current of the 25 but it would surprise me if an overnight charge with LSD NiMH isn't in the right ballpark for a totally flat pack - just like the slow overnight charge was the gentle method for NiCd back in the days.

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