> I've only used the NiMH cells in classic models and
> HP-25's. If your charge time is specified at eight
> hours or longer (i.e. trickle charge) you are probably
> safe with NiMH cells, if it is a quick charge (4 hrs
> or less) definitely stick with nicads. Dumb charge
> circuits are generally more compatible with NiMH
> retrofits than smart circuits.
Beware of what HP say. For example, the HP41 nicad pack specifies 12 to 16 hours for a full charge (suggesting C/10 or thereabouts), however the charge current is 16mA (this is from memory, I gave a more detailed answer when I sat down with the packs in front of me several months ago) for a 55 mAHr battery, which is C/3.5.
Whilst measuring the internal temperature during charge of a pack inside an hp41 tape drive (could have been a printer) I observes a rapid temperature rise after about 6 hours (again from memory -- check my posting).
Both of these suggest a charge rate in excess of C/10.
Oh, and the charger circuit for the 82120A (both of them) have a full wave rectifier, and some filtration. Whilst the re is also going to be some variation in voltage during charging, it will be far less than the older diode/resistor combination, and in the case of the newer version, far less.
I also had a concern that I believed that NiMh didn't like to be completely discharged, which is why many devides that use them (e.g. mobile phones) turn themselves off when they detect low battery.
I am also uncertain of the typical failure mode of NiMH if overcharged. I do know that they can get rather hot (even with correct charging).
I'd *love* to use NiMH but I would want to be assured that i'm not going to have my calculator explode in front of me whilst on charge :-)
I have found it quite difficult to get good information on NiMH cells, and without such information I have tended to steer away from them. About the limit of such information appears to be "NiMH require a special charger -- do not use a NiCad charge unless it is designed to charge NiMH". What I want is charts, technical information, information about the charging end point, etc, etc. Why do manufacturers seem to think this is proprietry -- or have I been looking in the wrong place.