Parallel acumulator wiring

Question

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?

Answer 1

is 0.3 V dangerous or negligible when wiring them in parallel?

It depends on the internal resistance of the 2 cells you are connecting in parallel. For example, if the 2 cells have a 25mOhm (quite common value for new cells), with a 0.3V difference you get a 6A surge current. This current will quickly drop to zero, though, as the two cells voltages equalize. "Aged" cells have higher internal resistance, and this mitigates the problem. Always try to do your best in equalizing the voltages before connecting the cells in parallel.

How can I check how many mAh they actually have after I wire them in parallel?

The best way is to discharge the parallel at a constant, known current down to an End of Discharge voltage (usually 2.75-3V for Li-ion), and measuring the discharge time. Then you calculate the capacity as the product of discharge current and discharge time.