The Solar battery bank

A nominal 25.6 V LiFePO4 bank for off-grid storage, sized from daily consumption rather than from a load. The designer below starts from this arrangement; change anything and the numbers follow.

What this arrangement is for

A nominal 25.6 V LiFePO4 bank for off-grid storage, sized from daily consumption rather than from a load.

The thing people get wrong

Storage banks are sized in kilowatt-hours per day and days of autonomy, not in amps. Two cloudy days of reserve at 80% depth of discharge is roughly two and a half times the daily figure.

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.