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New batteries in old HPs

Posted by W. Bruce Maguire II on 27 July 2001, 4:24 p.m.

Hello all:

I just wanted to share my recent battery adventures with the faithful. I made a few battery packs for the HP classics using new NiMH 1600 mAh batteries (Sanyo industrial and Kodak retail). Unfortunately, a standard classic charger will take 36-48 hours to fully charge these batteries (and they might not fully charge since the current is relatively low). So, I modified a charger to output about 180mA on the battery-charger circuit rather than the normal 50-60mA. This should yield about a 12 hour charge time. It was a very easy modification. In my wall transformer, there is a 13 Ohm resistor being fed into a transistor. This resistor determines the current available from the "current source" circuit on the battery-charging side. By putting two 10 Ohm resistors in parallel with the 13 Ohm, I was able to get the net resistance to about 3.7 Ohms. This resistance theoretically should yield 190mA, but I measured less--about 175mA.

Now, the really good news: these 1600mAh NiMH batteries last a LONG time. I decided a good test would be to start my HP-55 timer (with hundredths-of-a-second displayed---meaing eight digits and two colons), and see how long it lasted. I started to get the all-decimals low-battery indicator after 9 HOURS and 45 MINUTES. I finally gave up (and went to bed) after 10 HOURS and 20 MINUTES (it was still running fine at that point).

A test in my HP-67 yielded similar results, in that a running program lasted about 8 HOURS. Unfortunately, the 67's low battery LED comes on after only an hour or two---even though the calculator runs many more hours. I haven't quite figured this one out. I'm not sure why the low-battery threshhold on the 67 would be different than the 55.

Anyway, I'm a happy camper! 8 to 10 HOURS on a charge. Woo-Hoo!

Bruce.