The 24 V pack

Nominal 25.2 V from seven li-ion cells in series — scooters, small e-mobility and mid-size inverters. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

Nominal 25.2 V from seven li-ion cells in series — scooters, small e-mobility and mid-size inverters.

The thing people get wrong

Seven in series is an awkward number for off-the-shelf BMS boards, which cluster around 4s, 8s, 10s and 13s. Check the BMS exists before committing to the cell count.

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.