HP 12c battery 1.5volt vs. 1.55volt


Would I harm my 1984 HP 12c by using 3 x 1.55 volt silver oxide batteries instead of the recommended 3 x 1.5 volt?

Thanks in advance for your response.



Silver-oxide batteries are also recommended in the HP12C OH&PSG (Owner´s Handbook & Problem Solving Guide) and in all other 3 x 1.5V Voyagers (HP10C, 11C, 15C and 16C). You won´t harm it at all.


Luiz (Brazil)

Edited: 30 Sept 2004, 8:31 p.m.


"robdi" --

I can't recall having seen a dc cell with a nominal rating of 1.55 V, but I can attest that the "1.5 V" dc cells I've bought generally deliver about 1.58-1.62 V fresh out of the package.

Three "1.5 V" alkaline or silver-oxide dc cells are recommended in the manual. Types 44, 76, or 357 are all suitable. It might be best to use the same chemistry for all 3 cells, but rechargable cells in this size are not available, to my knowledge.

-- KS


0.05 volts is a tiny difference, as far as batteries go. "Batteries" are NOT constant voltage devices, despite what the markings on the package might say. A "1.5" volt cell can be anywhere from about 0 to 1.75 volts in its lifetime. A well-engineered device will accept a pretty large range of voltages, above some minimum necessary for operation. 1.5V vs. 1.55V is nothing.


As Karl said, don't mix batteries of different chemical compositions; there is a chance that potential differentials in the wrong direction might induce leakage (though theoretically, blowing up could happen, too, but highly unlikely).


I can't see ANY possible harm in mixing battery types as long as everything is in series. In that case, one battery has no idea what other voltage sources are in the system. In any case, as the batteries age, they will age somewhat differently, and their internal voltage and resistance will all be different. Next time you need to change the AAA's in your '48, grab a voltmeter and check out the old ones. I did this a few days ago, and the no-load (except for the very high resistance voltmeter) voltages ranged from 1.1 to 1.3 volts.

Batteries in parallel, of course, is another matter!


The problem with mixing different chemistries is that when they don't all have the same life span, one with a shorter lifespan may not only get completely discharged while another battery still has useful life left in it, but worse, will get charged in reverse as current continues to flow through it after its voltage has reached 0. Then it will be likely to leak. Actually, you don't even have to mix battery chemistries to do this. It will also happen if you have all alkaline batteries but one has significantly less life left in it than another because of prior use. That's partly why they tell you to replace all the batteries at once. I've had the negative-voltage occurrence a few times (and resulting leakage a couple of times) from trying to squeeze the last bit of life out of used alkalines in projectcs cobbled together on breadboards on the workbench, where I didn't need the full voltage anyway and where leakage would easily be detected and there was no threat to expensive equipment. You can bet I don't gamble with my irreplaceable HPs though. They receive only fully new sets of batteries, and the batteries are replaced as soon as the low-battery annunciator comes on.


While actual battery voltage is generally different from the rated nominal voltage, I saw that on the Enegizer website the EPX76 is rated at 1.55V nominal.


I took a look in the technical data section of the Energizer web site:


This page indicates that the initial open circuit voltage of a silver oxide cell is 1.6 volts. Since the HP Voyager calculators impose such an extremely low current drain, the voltage when newly installed in the calculator must be nearly the same as the open circuit voltage. This would mean that the calculator would receive 4.8 volts from a new set of silver oxide cells. On another post on this forum I mentioned my experience with a friend's HP15C which refuses to operate on silver oxide cells. It runs perfectly on alkaline button cells, but just does not turn on when silver cells are inserted (this is after checking for the usual things such as poor electrical contact, cleaning off finger oil on batteries, trying another set of silver cells, etc.). Is it possible that 4.8 volts versus 4.5 volts could cause this behavior?

For the record, my own HP15C doesn't seem to care, and runs just fine on both alkaline and silver oxide cells.

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