Converting HP-25 to NiMH batteries, possible?

[cubytus]

Hi there,

I found a working HP-25 calculator, and quickly learnt that it requires a battery pack installed to be working properly. Only, the calculator was designed well before NiMH batteries were standard. Fast-forward 45 years, NiCad batteries are getting harder to find, mainly due to lower capacity and higher toxicity.

Using NiMH batteries kind of works as long as they're charged on a different charger, but can't be charged using the stock HP power adapter.

Given the extremely simple charging circuit, is it possible to convert this calculator to NiMH batteries? Or am I better off trying to find NiCad batteries?