The 3s2p pack

Three in series for voltage, two in parallel for capacity and current. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

Three in series for voltage, two in parallel for capacity and current.

The thing people get wrong

The notation is always series first: 3s2p is six cells at three cells’ voltage and two cells’ capacity. Reading it backwards designs a pack at half the voltage and three times the current.

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.