Give it a voltage, get a pack

Say what the pack has to do and this works out the arrangement, the cell count, what it will really deliver, and the protection it must have. Every assumption behind the answer is shown and editable.

Protection is part of the answer, not an add-on

Every lithium result here carries its BMS requirement, a fuse size, a charge profile and the temperature limits, whether or not you asked for them. A pack design without those is incomplete, and presenting them as optional extras is how they end up omitted.

Some designs are refused rather than computed

Lithium with no BMS, lithium cells in series without balancing, and cells asked to carry more than they are rated for all return a refusal with the reason instead of numbers. A refusal is an answer. Printing an arrangement alongside a warning would just invite somebody to build it and treat the warning as decoration.

Every figure says how confident it is

A number computed from what you typed and a number resting on a datasheet-typical value are different kinds of claim, so each is labelled as exact, derived or typical along with what it was computed from. That is the difference between an answer you can check and one you have to trust.

Questions

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.

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.

Why is the usable capacity lower than the cells add up to?

Because running a lithium cell flat every cycle destroys it quickly, and because cells lose capacity as they age. The default here is 80% depth of discharge with a 20% ageing margin, so a 12 Ah pack delivers about 7.7 Ah of real work. Both figures are inputs you can change, and the result always says which were used.

What is C-rate?

Current expressed as a multiple of capacity. A 3 Ah cell at 3 A is 1C; the same cell at 15 A is 5C. It is the number that decides whether a cell is being used gently or punished, and the rating that matters is the one on that cell’s own datasheet rather than a rule of thumb.