Starting at 7A, either one of the cells or the test equipment seems to have problems, the discharge curves look somehow jagged. Can you elaborate on this?
Looking closer in your comparator (the most useful tool on the internet BTW), I see that also the other cell has a somehow ragged discharge curve at higher currents.
So either there was a contact problem during measurement, or these cells do have problems at 10A or more.
The deep dip at the beginning of the discharge curve suggests that the cells can only sustain current because the internal resistance is much reduced due to self-heating. The example with higher capacity has a deeper dip, it has problems from 7A onwards, where it’s voltage starts falling below the other cell’s.
Could it be that they heat up extremely and that this causes contact problems due to thermal expansion? AFAIK you don’t use real four-terminal battery contacts, so fluctuations in contact resistance would be visible in the measurement, even it they are only in the low milliohm range.
Personally, I wouldn’t use them at more than 10A continuous until the cause of the jagged discharge curves is determined.
Have you considered repeating this measurement under closer control?
You are correct, it is either a contact problem or the batteries and it is difficult to see, but I do not really have time to investigate.
I would love to use full 4 terminal measurement, but finding something that can be used at 30A and with hot batteries is difficult. The ones from Fasttech are fine for lower current and not so hot batteries and I uses them for that.
I’ve been using the Fasttech ones for quite some time now, for up to 20A and for batteries too hot to touch. However, I don’t go beyond 70-80°C because I want to keep and use the batteries I test, so I try not to ruin them.
20A is rather high for the 18AWG wires they have for current.
Because my testing is automatic I am usual not around when doing the high current tests, this means I would need two bench multimeter's connected to my computers to check the temperature and stop if it gets to high. It would be very useful recording the temperature when discharging, but I have never gotten around to do it with LiIon.