Re: Lithium packs for display and storage Message #7 Posted by Ellis Easley on 24 July 2002, 1:48 p.m., in response to message #6 by Juergen Rodenkirchen (Germany)
Most NiCads seem to leak eventually - although my HP97 that I got this Spring has its original 1977 battery in it and shows no sign of corrosion or leakage. I got a set of 4 "N" NiCads to use in my HP41C about two years ago, I've only charged them three times (very gently) and one of them has the white crystals forming around the positive terminal. Here is a very good article on the subject of battery leakage and corrosion in calculators from Viktor Toth's website:
http://www.rskey.org/calccorr.htm
The usual explanation for leakage is pressure buildup caused by the temperature increase that occurs if an aggressive (non-trickle) charging current continues to be applied after the cell is fully charged. One type of "smart" charging circuit has a thermistor mounted in the battery pack to sense the temperature and stop the full charging current once the temperature starts rising quickly.
But I have the N cells and also two Classic batteries I bought brand new from EduCalc (for the used HP75/printer/disk drive set they sold) which I only charged a few times, which I stored away from the calculator (no external leakage current), which leaked over the years anyway. So I have two theories about NiCad leakage: (1) they might tend to leak less if they are used regularly - sitting idle for extended periods after complete self-discharge, they seem to leak even though they are not subjected to overcharging. (2) Smaller cells (N, AA) seem to leak more than sub-C's - in the case of normal use and occasional overcharging, this would make sense since the smaller volume would reach a higher pressure after a shorter period of abuse.
I wish Nicads came with something to absorb and/or neutralize the electrolyte that escapes, like the felt rings for car battery posts - those work very well.
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