Found my old HP-34C from high school

[Dieter]

(09-22-2014 11:13 AM)Steve Simpkin Wrote:  The safest method is to take the batteries out and charge them in an external AA NiCad battery charger.

For the record: here's how I use and charge batteries for my 34C, my first HP bought back in 1980.

First of all I replaced the original NiCd-pack of the day with current NiMH batteries. Two Sanyo/Panasonic AA Eneloops, to be specific, which IMHO may be the "best" general purpose NiMH batteries currently available. Besides other advantages, they have virtually no self-discharge. The Eneloops' capacity is 2000 mAh, i.e. you can expect about four times (!) the original operating time. I connected both batteries inside the battery chamber with a thin metal clip. I also tried a piece of aluminum foil (folded once or twice) and it worked fine for me either.

The batteries are charged in an external processor-controlled charger. This is both the most convenient way (insert batteries and let the charger do its job) as well as fast (typically 2...4 hours, depending on the charging current) and, most important, it is safe for the batteries themselves that are handled correctly without the danger of overcharge. A good charger does not have to be expensive. Over here in Europe, the good old BC700, for instance, can be had for less than 30 EUR.

Yes, I have to remove the batteries from the calculator every time I charge them. But due to the much higher NiMH capacity, charging is required four times less frequently than before. I got a pack of four Eneloops, so I can always have two spares in a drawer. Since there is virtually no self-discharge it does not matter if they received their last charge a week or several month ago. AFAIR the manufacturer claims something like 85% of the original capacity after a year (!).

Dieter