The RC LiPo pack

Four pouch cells at a nominal 14.8 V, sized for burst current rather than runtime. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

Four pouch cells at a nominal 14.8 V, sized for burst current rather than runtime.

The thing people get wrong

LiPo packs are chosen on C-rate, not capacity: a 5000 mAh 50C pack is claiming 250 A, and that claim is a marketing figure far more often than it is a datasheet one.

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.