I've got a few 12V lead acid batteries knocking about and I'm not sure of their condition. They measure over 12V but I'm not sure I could rely on them (variously, old, pulled from something while working). Also, I'm finding that a few NiCad AA and AAAs are starting to react unpredictably. And I've just measured the voltage of a couple of button cells I've replaced that measure over 1.5V. So I'm reluctant to get rid of batteries that might have some life left.
Is there a decent method of testing? I gather for car batteries one of these:
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- something that applies a load. But I'd like something more versatile, that I could use across the range. Is there such a thing?
A crude battery test that works for primary cells is to apply a load and look for voltage sag. A big voltage sag indicates the cell won't power anything that takes much load. It might have residual voltage to power a small thing like a clock, but if the clock has a chime on it the voltage might sag when it chimes and the clock might stop. Similarly the battery could be ok for running a car radio but not starting the car. This is referred to as an impedance test.
For rechargeables, a better test is a full charge-discharge cycle, measuring how many Wh you put into the cell and how much you get out (at a given current).
You can do the first crudely with a resistor and a voltmeter, but there's no chance that you use the same setup on a car battery and a button cell - one you need to take hundreds of amps, the other hundreds of mA. I suppose it could select from a different set of resistors, but if you took hundreds of amps from a button cell it could catch fire.
For single cell rechargeables (AA, AAA, 18650, etc) these are the gold standard charger-analyser used by 'DIY powerwall' types:
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note that a full charge-discharge cycle takes several hours, so they often have a bank of them running in parallel.
Its 'quick test' gives an impedance test - tells you the number of ohms of the battery, which is an indication of how much it will sag under load. On older batteries the impedance figure creeps up as they age. It might work for primary 1.5v cells (don't leave them in there for more than a few seconds in case it tries to charge them) but won't for a car battery.
I have a voltage double based small battery tester that lights an LED if the battery is able to provide sufficient voltage under real load. But it is specific to cells with nominal 1.1-1.7v terminal voltages.
You could design a load test for larger batteries but as the batteries bigger the load has to be more aggressive and possibly on a heatsink.
Terminal voltage at a discharge rate of C/10 or C/5 is a decent guide.
Plenty of UPS batteries seem to look OK whilst on float charge but give up the ghost almost immediately they are called upon to do any work. Likewise with emergency lighting SLAs.
In the interests of science I put in some alkaline cells on Quick Test: (which worked fine, no explosions). I think these are in ohms:
Part used Philips alkaline C cell: 148, 162, 218, 171, 217 New Duracell plus C cell: 206 Part used GP AA cell: 282, 326, 260
Part used GP AA Greencell (zinc chloride?): 359
Fully charged LG F1L lithium ion 18650 3350mAh 4.9A: 128, 131, 168, 120 (these are high capacity low current lithium cells intended for powerbanks rather than power tools)
so I suppose you would need an idea of what a 'good' cell should offer, because the ability to supply current also depends on the cell's chemistry and construction.
(also to note that C cells don't fit this charger very well, so it's possible to get high resistance contacts that still measure but as too high readings)
you monitor the behaviour of rechargeable cells/batteries throughout the discharge cycle. Not really helpful with your used button cell (although you could test one to full discharge to see how others might perform) but a fun toy if you're so inclined.
For any battery, you need a device that applies the normal load expected to see what the voltage is in use. Also with multi cell systems you need to be able to measure each cell under the load as well. Many button cells look good but have such a small unpolluted contact are of on the electrodes inside that the voltages vanishes under any load. I mean a couple of dies similar metal skewers in a Lemon can power an lcd device, but try to power an led and you are out of luck!
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