DIY battery pack question.

[mojoo999]

I just bought 50 Samsung 30Q cells from NKON. Most are 3.42V and 3.43V, but some are 3.45V and 3.46V. Are they all safe to use in the same pack? I heard they need to be within 0.01V of each other.

 

[KD5ZXG]

This might not be the best answer, but I'll give it a try.

Cells are often shipped at less than full charge, and
might make sense to weld them in that state, rather
than full.

So get a resistor and bleed them all down to 3.3V
Open the circuit occasionally to measure, not just
how it pulls down with the resistor load. Should all
reveal about the same open vs loaded if they are
really alike. But even that doesn't assure equality
at full charge.

You need BMS or charger that has balance function.
And charge very slowly as you approach full. Most
balance circuits can't shunt away more than 50mA
of overcharge.

 

[eric1565]

I build packs, and I think that is fine. If you are worried, you can measure them all and sort by voltage. Then when you are building your pack you can put the same volt cell in the same parallel group. And then use a balance charger to balance the pack out. But I have found it much faster to charge each parallel group individually to max, and once you do that to all the groups, the pack is balanced.