The Sofirn 4000mAh 21700 (got it bundled with Sofirn C8F-21700) appears to have 4000mAh capacity (MC3000: 4.20v drain at 0.75A discharge current, cut-off 2.50v)
The iJoy 3750mAh 21700 only registered 3600mAh.
The IR tests are the lowest results after reseating the battery a few times… Looks like they’re similar IR.
(it seems the contacts of my MC3000 is not so good nowadays, I generally have to reseat batteries several times to get the lowest readings, unlike before when I only did to reseat maybe twice to get a good reading…)
You might want to wipe the rails (where the springed connections are mounted) thorougly with some dry (or slightly alcoholic) cloth periodically (once per 3-6 month probably will be enough).
I had the exact same problem with Miboxer C2-4000, and after I did as above (I did not remove the cover, just wiped, where I could access them), my readings were much better.
I also have SKYRC MC3000 - and probably I will wipe its rails, too.
I bought 20 new LG 2850mah cells, and they all came between 2830 and 2849. Same for dozens of other cells and capacity. The ONLY two cells to ever come up over capacity is my sofirn 4000 mah 21700 and my wurkkos 4800mah. Thats it. I think that two high out of dozens…if not hundreds … of cells is probably unlikely. That includes liion and nimh… aa,aaa,26650, 18650, 21700, 14500.
You must have gotten lucky and gotten the 4800 mAh cell in the 4000mAh wrapper. Nice. Funny enough, Sofirn’s store page lists the IF25a as coming with a 4800 mAh cell but the pic shows the 4000 mAh one so there could indeed be a bit of cell lottery going on. I vaguely recall reading something similar with regards to their 5500 mAh 26650’s.
Yesterday, I got in a wurkkos hd20… thats the 4800mAh 21700 I mentioned above. It came in at exactly 5000maH. I suspect that they are rewrapping the same third party cells. But the 4000 came in at 4330 and the 4800mAh came in at 5000mAh. None of my sofirn 18650s or 26650s has ever come in above stated capacity. Only these 21700…