HP-38C + NiMH Batteries: Bad Idea?


Having a nice 38C with no battery I tried the AA alkaline with aluminum foil "trick" with the cells in the correct polarity position. Worked fine. Got too ambitious and tried the same with a pair of 1650mah NiMH batteries. Correct polarity again. They charged up nicely and ran the calc with no problems. For a while that is. After about an hour the display froze. Turned the calc off then on. Nothing. Even the double alkaline does not work. Have I fried some internal component? Simple thing like a resistor? Or have I baked something that can't be replaced.



The batteries are not the problem... I have used NiMH cells for years in all my HP calculators. You can get over 1800maH cells in the AA package. They can take three times as long to charge as the originals, but run for three times longer.


After sitting for 24 hours, the 38C with alkaline batteries seems to work OK. Don't know what happened. Still would like to know if NiMHs are ok for this calc. David Smith has indicated no problems. Are the Spice Continuous Memory calcs an exception?


If possible, it is VERY MUCH advisable to charge the NiMH outside your HP38, with an external (NiMH specific) charger.

The recharge cycle for NiMH is different than for NiCd, and you will also avoid the risk of "frying" some internal parts of your calculator if anything goes wrong with the internal charging circuit. A simple dirty contact or momentary misconnection will allow more than 8 volts inside tha calculator, and the chips will be damaged irreversibly.

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