Series and parallel, in one notation
Battery packs are described as "13s4p": so many cells in series, so many strings in parallel. Series adds voltage — thirteen 3.6 V cells make a 46.8 V nominal string. Parallel adds capacity and current — four strings of a 3.0 Ah cell give 12 Ah and four times the current budget. Multiply through and a 13s4p pack of 18650 cells is 52 cells, 46.8 V, 12 Ah, and about 562 watt-hours.
Why "48 V" packs aren't 48 volts
Nominal voltage is a nameplate, not the real range. A "48 V" e-bike pack is almost always 13s lithium-ion: 13 × 3.6 = 46.8 V nominal, but 54.6 V fully charged (13 × 4.2) and about 32.5 V empty (13 × 2.5). Build the same "48 V" from LiFePO4, whose cells sit at 3.2 V nominal, and you need 16s: 16 × 3.2 = 51.2 V. The chemistry sets the cell voltage, so the same nominal number means a different cell count and a different full-to-empty range.
What the BMS cutoffs mean for usable capacity
The voltage range matters because your battery management system enforces it. The pack is "full" at the top and "empty" when the BMS cuts off at the bottom — not at zero volts. The usable energy lives between those limits, which is why the min/max range is worth knowing, not just the nominal.
Current and safety
Parallel count sets your current ceiling: each cell can only deliver so many amps, and putting cells in parallel multiplies that. A pack asked to deliver more than its cells' combined rating overheats — the C-rate is not a suggestion. And the broader point: lithium cells pack enormous energy into a small space. Any real pack needs a proper BMS, appropriate fusing, and cells that are matched in age and chemistry. This calculator does the arithmetic; it is not a substitute for a build guide or basic battery safety.
Related tools: the battery life calculator for runtime, the solar calculator for off-grid storage, the EV charging calculator, and the wire size calculator for pack leads.