AA batteries reversing polarity

[jdunmyer]

I have a caller I.D. unit that uses 4, AA batteries. They only seem to last a few months, and when "dead" one cell is usually reversed polarity. The others are low in voltage, but still OK in polarity.

Is this common? Is there anything I can do about it? Batteries are "Enercell" from Radio Shack, alkaline chemistry.

[JoeBean]

The batteries are probably wired in series. One goes dead first, and the other essentially charges it in reverse. The direction of current for charging a battery is the opposite of the direction during operation, so when a battery fully drains any more current that flows would essentially be reverse-charging the battery.

There's no easy solution that I know of. The circuit in the caller ID unit should shut the unit off once voltage drops below a set amount (thus preventing this). But I don't know of any simple solution when that's not the case.

[J Tiers]

What Joe said.

Since presumably the batteries are exhausted when they are replaced, there isn't any real reason to worry about it, unless the reversed one leaks, which sometimes happens.

[A.K. Boomer]

Well - that was worth reading and makes perfect sense - nice explanation Joe.

[J Tiers]

By the way.... This exact thing can happen to NiCd cells, due to over-discharge.

Don't drain the batteries down trying to drill lust one more hole, etc. Always stop when the unit seems to be losing power and change batteries. One cell out of an 18V pack does not make much change in power, and you can ruin the pack in a matter of weeks or even less.

[TGTool]

That reminds me of something one of my acquaintances (who shall remain nameless) forwarded to me recently. It was a bit on testing batteries without a voltage tester and had a YouTube video showing (purportedly) that a good battery when dropped from two or three inches onto a hard surface will just stop, but a discharged battery will bounce. Then they go on to demonstrate. They claim to not know why it works, maybe gas change inside, but it just does. I haven't tried it, but it sounds like internet bunk to me. I could find the link, but of course it wouldn't prove anything. Any other bull**** meters going off?

[boslab]

I had a long running job xraying the battery with the gold top, if dropped the membrane sack inside can rupture resulting in battery failure, very common, the good bit was I got to keep the ones that haven't ruptured, kept me in batteries for a few years, they are or at least we're more fragile than was openly admitted which resulted in a court case revolving around coded container locks that would die in service, open house for the contents of the container!, the shipping company lost a fortune in stock.
Mark

[gvasale]

I have found once...maybe twice, but not really guaranteeing that, a battery, NEW, with a reversed polarity. AA size. Pedrsonally, I don't understand how a battery in use could possibly reverse polarity. Even if you had more than one, say 5 in a set, that one could be forced to reverse polarity. But what do I know. The brand was Ray o Vac, IRC but please don't quote me on that. I usually use at least 100 a year. I would expect 6v going backwards through a 1.5 cell would burst or cause the cell to leak at least.

I don't believe the copper top brand has much of any advantage over the other national name brands.

[jdunmyer]

Joebean,
Thanks for the explanation! I knew about the potential for reversing polarity in NiCads if they were over-discharged, so avoid that if possible.

I have a battery tester from Radio Shack that looks very much like this one: http://www.amazon.com/Gardner-Bender...battery+tester and am in the habit of checking "dead" batteries. I sometimes find that they're not really all that "dead"; my camera (2 AA cells) is a good example. Those "not so dead" cells get re-used in things like the caller I.D. unit. That's a non-critical application, as failure just means that I have to answer a few telemarketing calls until I get the batteries replaced.

[gvasale]

Supposedly load testing batteries is the "best" way to test...sometimes it doesn't make any difference. I use lots of AAs for photo flash, and typically when recycle time becomes noticably "long" they get replaced. But they still test at almost 1.4 + volts. Unused, 1.59 and occasionally 1.6 volts. They are recycled to the transistor radios & flashlights, with the radio's getting the most out of them.

[CCWKen]

That's interesting. I didn't know the polarity would reverse. I never checked old batteries. Can this happen with lead acid batteries too? I have (but not mine) a 12v car battery that is reverse polarity according to the molded-in markings. I didn't know it until I tried to use it to start an engine powered compressor. It sparked when I tried to connect it and started tracing wires. I thought my alternator was shored and replaced it. Turned out that the battery polarity is reversed.

[Don Young]

Yes, lead acid storage batteries can be charged in reverse. They will not provide much output this way. Of course it is possible for a battery to be assembled or marked in reverse also.

[CCWKen]

Well, after flipping the cables it started the engine. I wonder if it was run dead before someone charged it in reverse? It's a fairly new battery. Can I do that again and flip the polarity back where it belongs? I can't see it leaving the factory miss-marked.

[J Tiers]

The question is whether the innards are reversed..... possibly. Then it will be marked wrong but still have full capacity etc.

Otherwise it will have much lower capacity, but still might "work" until a high draw in terms of ampere hours is put on it. If it started the car, that's not a bad sign... But it is surprising how few ampere hours it takes to start a car, especially in summer.

[darryl]

Any type of battery can and will begin to charge a dead cell in reverse if you keep trying to draw current out of it. Remember- a battery is a series connection of cells, so in any pack regardless of chemistry, a dead cell once depleted is going to be subjected to voltage reversal.

I don't know if this is true about lead acid, but I have read that part of rejuvenating one is pulsing it in the reverse direction- followed of course by a charge in the right direction. Whether this would work for nicads- maybe. I wouldn't try it with the lithium chemistry.