Low voltage cutoff?

[Madbison]

I have some 18650 batteries. Some are about 10 years old, and some are brand new. I have a tester that will test batteries at a chosen amp draw, and will stop the test when the voltage reaches a set point. The old batteries are rated at 3100 mAh, starting at 4.2v, cut off at 2.5 volt, 0.2A draw (according to spec sheet).

My question is re the cutoff voltage. If I run a 1 Amp load test, and set the voltage cutoff at 2.6v, then it does cut off at 2.6v, but then floats back up to about 3.15v fairly quickly. So I assume that the cutoff voltage, is always under load?

I only want to get a baseline for the batteries, particularly the new ones, so that I can closely match the cells, as they will be used in packs of 4. Some are going in a torch that is a very high draw, and some are going into a power supply for an esp32 device, that only turns on every 6 hours, so is a very very low draw. I just chose a 1 Amp draw as a nice round number, and a somewhat quick test, unlike the 0.2A draw that would take over 14 hours.

If there are any good links to details on battery testing like this, I would be interested to know about them.

Thank you
David

 

[sunshine_eggo]

This 2.5 vs. 2.6 vs. unloaded vs. loaded is noise and irrelevant, but yes, the cutoff voltage is always under load.

Any common-sense CONSISTENT method can yield good results. Since 3.7V chemistry has very little capacity below 3.0V, I would limit it to 3.0V.

Given that the spec sheet indicates a very low C rate (0.64C) and you are proposing something closer to 0.33C, I would be concerned their internal resistance may be too great to give meaningful results.

If your tester can also measure internal resistance, that would be a very useful filtering and matching criterion.

Suggest you run some tests on a single cell and make a decision:

Discharge @ 0.2A to 3.0V, record value
Continue discharge @ 0.2A to 2.5V and record value (confirming very little is left between 3.0V and 2.5V).
Recharge to 4.2V
Discharge @ 1.0A to 3.0V, compare to above values.

If you get > 80% of the 0.2A to 3.0V yield @ 1.0A, then you can get meaningful results with a 4.2 to 3.0V @ 1.0A discharge for comparison between cells.

Don't be afraid to throw out outliers, i.e., cells that test notably outside what is typical for the group should likely not be used at all.