Got a great deal (1.75 EUR) on 10 used 18650 Li-ion accumulators salvaged from old laptop batteries which should have around 9000 mAh together. I measured them with a voltmeter and some have 3.7V, some have 3.8V and some have 4V. If my understanding is correct, when I wire them in parallel they will all reach the same voltage in a short time, which will cause them to heat up. My question might sound stupid, but is 0.3 V dangerous or negligible when wiring them in parallel?
TLDR: Will 3.7V and 4V accumulators wired in parallel reach dangerous temperatures?
Bonus question: How can I check how many mAh they actually have after I wire them in parallel?