battery testing

Notes

These are excerpts from a Discussion on Discord.

Reddawg99:

You actually don't need a fancy tester. A bit of math and the battery's base voltage with a simple multimeter is all you need to determine if a lead acid battery is good or not. Unlike lithium batteries, which maintain a constant voltage despite total capacity, the less total capacity a lead acid battery has left affects the output voltage.

Lithium is trickier, but a typical "battery capacity tester" actually damages lithium batteries. Lithium reacts very badly to being fully drained, and that is what those testers are designed to do.

Woozle:

I think what I want is a dedicated device that doesn't cost as much as a full-blown voltmeter -- but it sounds like it'd be simple enough to find a meter-component somewhere and rig it with the appropriate resistor to get a meaningful readout.

Longer-term, I'd like some kind of networkable multiplexed sampling circuit that will let me automate the monitoring functions on a bank of batteries and automatically notify when one needs replacing... but one step at a time.

But anyway: so, what's the process for determining charge capacity? Charge for X amount of time, disconnect from charge, measure voltage?

Reddawg99:

Harbor freight sells 7 function digital multimeters for $8 btw.

What you want to do is fully charge it, even trickle charge it over night. Next day, put a mild load on it to discharge any over voltage, then measure the output volts. Then compare that to the standard output voltage of the battery. If it is off by more than 1.5v, it is a dead battery.

For lithium, you just have to run a load test with a stopwatch, and try to catch it before it totally drains.

I'll give you a example of a lithium test. Lets say I wanted to test the capacity of one of my 18650 lithium vape batteries. I know from the factory it puts out 3.7 volts steady, with a maximum capacity of 3500mAh. mAh is milliamp hours, so that translates to 3.5 amps over one hour. If I can drain the battery with a constant 12.95 watt device, it should last one hour. (3.7v times 3.5a = 12.95 watts). So I hook up 24 2w LEDs, I should get just over 15min of power. Then a stopwatch, and I watch for the LEDs to get dim.

I'd have to adjust that a little, since LEDs run at typically 5v minimum, so the amp draw is higher per LED than I just listed, but you get the general idea.

If the UPS batteries used modular 18650 cells, getting a solid lithium battery tester for those is simple, and not too expensive. Since you are dealing with batteries that are sealed with a unknown cell configuration however, you'd be talking about a lithium multi-function tester that costs at least a thousand, and that tester is a cheap knockoff.

[...]

...apparently someone made a cheap lithium tester that they sell on Aliexpress. https://www.youtube.com/watch?v=3dmWqHR7b9w