18650 lithium ion parallel

[johnnym]

I have a bunch of used 18650 lithium ion batteries (laptop batteries) and want to put them to good use. My buddy is an IT nerd so instead of recycling he is giving them to me.

What is the issue with wiring batteries in parallel? What I keep reading is that it can be dangerous. Voltage is supposed to be the same for all cells if you want to wire them in parallel right...? Otherwise the other cells drag down to the lowest voltage cell or other weird things.

What if you have this:

Five batteries at 3.91v
Three batteries at 3.9v
Two batteries 3.89v

Would that difference really matter if they wired them in parallel? It is so tiny.

[joestue]

Issue is one cell shorting out takes the rest with it.

Used batteries have rather high internal resistance so as long as the batteries are mostly the same voltage you can connect them together.

[OperaHouse]

It is much like fusing solar panels. If one shorts, the others provide all the current to it. If you go above two or three in parallel, fuse each one.  Ihis can be as simple as having a very small short strand of wire to each one. That will prevent many tens of amps flowing into one cell and only adding negligible resistance. For example a #26 strand of copper wire fuses at about 20A.