Make a simple battery capacity test

I want to check the capacity of maybe (40) 26650 cylindrical lithium iron cells at once. I planned to charge them all to exactly the peak rated voltage of 3.6, then hopefully making a circuit where individual cells would be discharged at an exact rate and to the exact rated low voltage I think 2.5v. And timed.

I know little about electronics and initially was hoping to just use an led as the load and the indicator but think they don’t open circuit at an exact voltage and a transistor is maybe needed

if you want to time it, then you will need a constant current load, which an LED is not without some external circuitry, an op amp, a sense resistor and a mosfet could make an alternative constant current load without too much complexity, with a second op amp to detect when the under voltage threshold has been reached, and disable the load / maybe flag a buzzer so you know one is ready?

The currents are low so you can get away with quite cheap mosfets, use a cheap quad op amp package so they are shared across 2 channels, and power externally,

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You have used up all my free support for the next few months or so.


lithium ion. Cheaper to manufacture, one less r.

I’m thinking I can get away with just a resistor as a load. It would vary as the voltage drops but it will do that consistently from cell to cell and I’ll be able to compare in that way

then I’d just need a simple timer and way to get that timer and the discharging to stop at a very accurate voltage.

This problem is much more complex than you think. A small led many run for days or weeks. The test needs to reflect fa actual battery usage. If you want a av rapid test, the load will get hot.

As @iabarry points out, a standard LED might safely dissipate say around 100mW (20mA at 5v) and your 26650 batteries should be good for 5000mAH so your LEDs might run for as long as 250hrs. 10 days later… :slight_smile:

There are quite a few designs for battery capacity testers knocking around on the internet - some more complex but others pretty simple to build. I have not tested or validated either of these so ymmv.

There is a straightforward example here which has a schematic to follow on the website. Might make a good KiCad starter project.

There is a rather more full featured design here which is in my notebook as something to build when I get some spare time.

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There are very cheap electronic loads available from AliExpress and the like, even with logging capabilities, they willl serve you better than a resistor and a LED

Some examples:



For a DIY version with a bit of background:

Disclamer: I haven’t tested any of them and do now know how good they will work.