The 36 V ebike pack

Ten cells in series at a nominal 36 V, charging to 42 V — the older and lighter ebike standard. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

Ten cells in series at a nominal 36 V, charging to 42 V — the older and lighter ebike standard.

The thing people get wrong

The controller’s low-voltage cut-off matters more than the pack’s: many 36 V controllers stop at 30 V, well above the 25 V the cells could actually give, so part of the capacity is unreachable by design.

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.