HP 97 VERY hot charger

[Dave Shaffer]

I acquired my Dad's HP97 when I cleaned out my parents' duplex a few months ago.

I have since managed to get it to work with the battery (which may even be the almost 30-year old original!) installed after a lengthy charging time (a few days). It even worked, at least briefly, with the charger disconnected.

However, a couple of days ago, I removed the battery to take a look at the battery compartment. After putting it all back together, I plugged in the charger. A day or two later, I tried to turn it on, and nothing happened (no display). I then felt the charger and it was almost too hot to touch. I immediately unplugged it from the wall and the '97 to let it cool off, fearing that it might be fried.

However, if I check the output voltage, I get about 10 V AC, which seems reasonable when the charger (an 82059B) is not loaded. I also checked the battery pack, and get only about 1.35 V DC - clearly not right for a 4-cell pack.

Any ideas what happened? I supposed it is possible the battery pack is now more-or-less short-circuited and putting a heavy current load on the charger, but why that should have happened just when I took it out of the calculator briefly seems a bit odd. I haven't yet tried measuring the charging current. If the battery pack is shorted, I wouldn't expect the battery pack to show ANY voltage. If the battery had just become loose in its holder when I pulled it out, I wouldn't expect any charging current to flow, and the supply should at best be only a bit warm.

Any other thoughts or tests to try, other than checking the current from the charger? Given the age of the battery pack, I suppose in any case it's time to replace the NiCADs!