Battery pack of HP 25 calculator

[Sadsilence]

Both battery types NiCD and NiMH have right nominal voltage of around 1.2V per cell, together 2.4-2.5V. Necessary calculator curcuits' voltages are created internally.

You can open battery case with a litte patience on bottom side and glue parts together again after battery exchange, so you do not have to cut out middle bar.

Personally I prefer flat top NiMH cells with low self discharge characterics. Most famous member of this battery type are eneloops (but no flat top). Flat top cells cause genuine ones were flat top, too. So less mechanical pressure on calculator contacts. No matter if flat top or not. So called LSD batteries are still with load even after a few months storage time without use.

This one.

Load current with HP charger is around 50mA, which perfectly matched genuine HP 450-500 mAh batteries. Load time then was around 12 hours.

Cells mentioned above have a capacity of 2100 mAh. Take it multiplied with 1.2 - 1.5 (due to loss during charge procedure) and divide result of around 2500 through loader current of 50 mA. Batteries are fully charged within around 2 days! Advantage is: you simply cannot overcharge and damage batteries that way. Still it makes more sense to charge cells with a modern racing car charger or a lab power unit. Loading "adapter" can easily made by using neodymium magnets. Or indeed cut out middle bar, so cells can easily removed and loaded seperately with a default nimh charger.