The 12 V lithium pack

The lead-acid replacement: same nominal voltage, a third of the weight, and several times the cycle life. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

The lead-acid replacement: same nominal voltage, a third of the weight, and several times the cycle life.

The thing people get wrong

A "12 V" lithium pack is 4s LiFePO4 at 12.8 V, not 3s li-ion at 10.8 V. Both get called 12 V and only one will run 12 V equipment properly.

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.