Alkaline cell behaviour

In message <t9u85u$3b4qm$ snipped-for-privacy@dont-email.me, Harry Bloomfield Esq snipped-for-privacy@harrym1byt.plus.com> writes

Someone with more knowledge will be along shortly, however, consider the direction of current flow from the two sound cells.

I had a fairly new cell in a gate alarm transmitter fail. The other cell was still reading 1.65V.

Bill

I'd say not usually, but I have seen it in triple A cells on my remote temp sensor. One will be almost dead and the other not so much. MY guess is that for quite a while the resistance of the slightly worse cell is still enough to allow the circuit to operate, then it gets reverse charged through the load and dies very fast indeed until its such a high resistance no current flows from the better cell. Brian

Absolutely, that's how it works.

You can also take advantage of that, on a LED torch blending a new good cell, with two half-worn cells. The light is not as bright, but in the long run, you get more total usage (watt hours) out of the cells. As eventually, you grind all of them down. But one always dies first, since one will hit the shoulder and rapidly discharge to zero volts and head towards reverse-bias country.

LED torches give "some" light, well down into the discharge curve of the battery. Whereas incandescent, the light would go "browner" sooner. The cells are not "matched" well enough, to all die in the same second.

You can even take cells out of an incandescent torch and get further usage out of them in a LED torch. However, the incandescent torch typically uses larger cells than the LED ones do.

Paul

Zero volts implies an internal short-circuit. Otherwise, after a short recovery I would expect to see a volt or so.

I would expect a 3-cell LED torch would need more than 2 working cells in series, so might not expect a faulty/low capacity cell to be appreciably reverse biased. Plus, an alkaline has a soft discharge characteristic. Towards it's EOL it's internal resistance climbs acting as a current limit.

TBH I've not see this sort of failure, but then I would probably change all 3 cells without checking and not see a single failure.

It is always worth doing this. I have a stack of older already used once batteries that are fine for low current devices but are well past it for high current (and I have some very high current devices).

Wall clocks and similar will run for another year or so on a spent cell. (they are also handy for powering red LED astronomy torches)

Not really unless the control of the capacity of each cell at manufacture is hopeless.

One will always die first but the other two should also be on their last legs by then too. I test all batteries for utility under load before throwing any away. I have never seen the sort of single point failure fault that you describe (except with physical damage to the cell).

I have sometimes used JCBs PP9s and not noticed any problems with them.

My favourites at the moment are Eveready and Panasonic.

I have given up on Duracell - they have leaked on me far too often.

on 05/07/2022, Martin Brown supposed :

In the drawer where I keep cells and batteries, I also keep a quick tester which applies a load to the item under test. Pointless just checking the voltage without any load applied.

The very worst cells I have come across, are the Kodak ones sold in pound shops.

Yes, I have also has some expensive leaks from Duracells.

John

My tester is a specific load test (sharp current spike).

Although they are poor quality and old style zinc chloride cells they are OK for powering LED lanterns overnight on a near throw away basis. Anyone who pinches those deserves what they get!

Price is hard to beat 10 (sometimes +2 free) for £1.

You sometimes see one duff cell reverse polarised.

Particularly if users have mixed some partially spent cells and new ones in a multi battery device (doesn't matter that instructions say DON'T).

Almost all modern LED torches now use a chip that converts voltage to a constant current drive so that they can hammer their battery into the ground. A higher power version of the joule thief.

I have a 'smart' charger that shows the internal resistance of cells. If they're only about 50 milliohms it charges them at 70 mA, but higher resistance ones only get charged at 30mA. I was amazed at some cells I'd been using for ages had resistances of 500 milliohms - no wonder they didn't last long after being charged. I threw them out.

The charger I used before charged at as high a current as possible - maybe that's what killed them.

yesterday I bought a pack of 2 LR44 button cells branded Panasonic in a local 'sells everything' store for £2.99.

Youf assistant tried to sell my a pack of 'JCB' button cells for £1.99. It contained about 10 cells but only two were LR44.

He couldn't understand why I declined to buy the cheaper pack. The additional unwanted cells have already consumed non-renewable resources and would just be discarded. He also didn't know that 'JCB' make diggers so it is just a brand name slapped on an unrelated product.

That's expensive. I usually buy 10 Duracell LR44s for £5.65 through Amazon Marketplace. I've found a local hardware store that sells cards of 10 (unknown brand) LR44s for £1.99. I know how long the Duracells last in a couple of clocks that use them, so I shall be able to make the comparison.

Are they genuinely LR44? Those cards of button cells have a weird selection that probably aren't much use for anything.

The cards of coin cells sold by "pound" stores are good value as they have CR2016s and CR2032s. Only the CR2025 have I no use for.

Pad the CR2025 cell out with a fold of aluminium foil on the +ve side and use in place of a CR3032. It won't last as long of course, but it's an acceptable substitute for a while.

Good idea. But they're so cheap it's hardly worth it.