The 4s2p pack

Eight cells at a nominal 14.4 V, the usual arrangement for power tools and small inverters. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

Eight cells at a nominal 14.4 V, the usual arrangement for power tools and small inverters.

The thing people get wrong

Parallel groups should be built from cells at the same state of charge. Joining a full cell to an empty one in parallel dumps the difference between them as current, limited only by their internal resistance.

Questions

What does 3s2p mean?

Three cells in series, then two of those strings in parallel — six cells in total, at three cells’ voltage and two cells’ capacity. Series multiplies voltage and keeps capacity; parallel multiplies capacity and current capability and keeps voltage. The series number always comes first.

Do I need a BMS?

For any lithium chemistry, yes, and this site will not design a pack without one. A BMS is the only thing that stops one cell being driven under-voltage on discharge or over-voltage on charge, and the divergence between cells is invisible from outside the pack. Lead-acid and NiMH can be built without one; lithium cannot.