Chemical engineers with a specialty in Alkaline batteries?

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crfriend
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Chemical engineers with a specialty in Alkaline batteries?

Post by crfriend »

This is a long-shot to say the least, but I'm wondering if there are any chemical engineers with a speciality in battery design here.

I tried firing up my digi-cam this afternoon to take a couple of shots and was greeted by a brick. This is usually down to flat batteries, so I'm entirely familiar with that idiom. Replacing the batteries (piles [0]) fixed the immediate problem.

What remained perplexes me, though. I put the four cells into a tester, and three of the four were marginal (which wasn't terribly surprising) but the fourth one tested distinctively negative! A chemical device that's supposed to be pitting out +1.5 VDC is, instead, putting out -0.785 VDC. No wonder my camera was confused.

What gives?


[0] There's a dialect pun in there.
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Re: Chemical engineers with a specialty in Alkaline batterie

Post by Caultron »

crfriend wrote:...I put the four cells into a tester, and three of the four were marginal (which wasn't terribly surprising) but the fourth one tested distinctively negative! A chemical device that's supposed to be pitting out +1.5 VDC is, instead, putting out -0.785 VDC. No wonder my camera was confused.

What gives?...
The fourth one must be antimatter. Congratulations! By sheer luck you're created Earth's first warm drive engine! Stop posting anything about this online and get thee to a patent office!
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Re: Chemical engineers with a specialty in Alkaline batterie

Post by pelmut »

crfriend wrote: I put the four cells into a tester, and three of the four were marginal (which wasn't terribly surprising) but the fourth one tested distinctively negative! A chemical device that's supposed to be pitting out +1.5 VDC is, instead, putting out -0.785 VDC. No wonder my camera was confused.

What gives?
I'm no expert on the chemistry, but it seems that rechargeable alkaline cells, which work fine as individuals, have a degredation mechanism when several of them are combined in a battery. Obviously each of the cells has sightly different capacity, so one of them is bound to run out slightly before the others. Unless the battery is protected in some way, the remaining cells will continue to drive current through the discharged cell and will, in effect, try to charge it backwards. As the reverse voltage and internal impedance of the discharged cell rises, the overall fall in battery voltage will be noticed by the user, who will then stop using the battery and will recharge it.

After recharging, the battery will appear to be all right, but the cell which was reverse charged will have gassed and lost a small amount of its internal water through a safety pressure vent. This means that this cell will now have slightly less capacity than it did before - and will run out even sooner. Each time the battery is discharged beyond the point of slight voltage drop that indicates one cell is exhausted, more damage will be done until eventually the weakest cell will be reverse-charged almost as soon as the battery is put into use.

That, I think, explains what you have discovered.
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Re: Chemical engineers with a specialty in Alkaline batterie

Post by crfriend »

pelmut wrote:That, I think, explains what you have discovered.
It makes eminent sense for rechargeables, but in this case it was a non-rechargeable cell in play. The overall theory, however,may well hold.

The camera has a voracious appetite for power, and so eats batteries at a prodigious pace. My usual tactic is to select the strongest ones from the last batch and use them until most of them are pretty close to flat. That alone would tend to induce the sort of battery behaviour you describe. Since the camera is always drawing a little bit of power (for internal clock/calendar) reversing a very weak cell would certainly be possible whereas if I simply removed all four cells it wouldn't. Perhaps I'll start doing the latter.
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Re: Chemical engineers with a specialty in Alkaline batterie

Post by Kilted_John »

For photo gear, it's a good idea to splurge a bit and buy the lithium AA (and button cells for analog gear) batteries. Typically, you have full power until they finally go dead, and they last longer. Also, there's no leakage, causing the dreaded corrosion. I've even stopped using silver-oxide batteries in my Nikon F2, F3, and FM2 bodies. Since all three cameras take two, I can instead use a Lithium CR 1/3N battery and not worry about battery corrosion if I happen to leave a battery in there with the cameras stored away.

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