The 48 V ebike pack

The 13s arrangement every 48 V ebike uses, charging to 54.6 V. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

The 13s arrangement every 48 V ebike uses, charging to 54.6 V.

The thing people get wrong

A 48 V ebike pack is 13s and peaks at 54.6 V, so a charger sold as "48 V" that outputs 58.8 V is a 14s charger and will overcharge every cell in the pack.

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.