Charging Ni-MH batteries


I've rebuilt the power pack for my HP33C using two Ni-MH batteries - Ni-Cd batteries are becoming harder to get hold of easily and have a number of other disadvantages when compared to their Ni-MH counterparts. These appear to work fine in the calculator, however as they are strapped together in the power pack they won't fit into my conventional battery charger.

Given the warnings about charging batteries in the calculator I'd planned to charge them outside the calculator using my reserve power pack (82103A) and AC adapter (62088B) BUT before I try it, does anyone know if this will work as the power pack was obviously only designed to charge Ni-Cd batteries...

I really don't want to damage the reserve power pack (which dispite its name is actually a little battery charger into which you plug the calculator's AC adapter).


Mike T.


Hi Mike,
From my research, it looks like NiMH's need to be charged in a charger designed specifically for them. The battery voltage, as well as the battery temperature needs to be monitored during charging, and the charging rate modulated accordingly. I think it's not so much a matter of damaging your reserve power pack, but of damaging or ruining the expensive NiMH cells if you charge them in a device intended for NiCad's. If you Google Charging a NiMH battery there's lots of info on the subject that you can read.

Best regards, Hal


Quote: looks like NiMH's need to be charged in a charger designed specifically for them. The battery voltage, as well as the battery temperature needs to be monitored...

I think, that's only true if you want to charge the batteries fast. Trickle charging, as done in the old devices, shouldn't be a problem. You probably don't get the full capacity but you shouldn't be able to kill the cells.


They will charge just fine in the machine or extrnal charger. Multiply the charge time by the ratio of the increased in capacity of the new cells (usually 3-4 times).


I charged the 3000MAh sub C cells in my 97 for 2 or 3 days and they work great. Last a lot longer than original battery pack.

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