The 4s battery pack

Four LiFePO4 cells make a nominal 12.8 V, which is the drop-in replacement shape for a lead-acid 12 V. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

Four LiFePO4 cells make a nominal 12.8 V, which is the drop-in replacement shape for a lead-acid 12 V.

The thing people get wrong

A 4s LiFePO4 bank tops out at 14.6 V, comfortably inside what most 12 V equipment tolerates — which is exactly why this arrangement replaced lead-acid rather than competing with it.

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.