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Battery Pack Builder

Pack voltage, capacity and energy for cells wired in series and parallel, from the cell and the S and P count.

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Wire cells one way and you raise the voltage; wire them another and you raise the capacity. A battery pack is both at once, a grid of cells in series and parallel, and this works out what that grid gives you: the pack voltage, the capacity in amp-hours, the energy in watt-hours, and the total cell count, from one cell and your series and parallel numbers.

Series stacks voltage, parallel stacks capacity

Put cells in series, positive to negative in a chain, and their voltages add while the capacity stays that of one cell: four 3.7 volt cells in series make 14.8 volts. Put cells in parallel, all the pluses together and all the minuses together, and the capacities add while the voltage stays the same: two 3000 milliamp-hour cells in parallel make 6000. A real pack does both, written as S and P, so a 4S2P is four in series by two in parallel, eight cells, higher voltage and more capacity together.

Watt-hours are the true size

Amp-hours alone do not tell you how much energy a pack holds, because a volt matters as much as an amp-hour. Watt-hours do: it is the pack voltage times the capacity in amp-hours, and it lets you compare a low-voltage, high-capacity pack against a high-voltage one fairly. It is also the number that decides how far a given load will run, and the figure airlines and couriers use to limit lithium batteries, so it is worth knowing for your pack.

Nominal, full and empty

The voltage here is nominal, the average working voltage, but a cell is not fixed: a lithium cell leaves the charger around 4.2 volts and sags toward 3 as it empties, so the pack swings with it. The full-charge figure is what your charger must reach and your wiring must handle. Size connectors, switches and the charger for the full-charge voltage, not the nominal, and never discharge a lithium pack below its safe floor, which is what the protection circuit is there to prevent.

Match the cells, and protect them

Every cell in a pack should be the same chemistry, capacity, age and state of charge, because the weakest cell limits the whole pack and mismatches cause uneven wear and, with lithium, real danger. A lithium pack needs a battery management system to balance the series cells and cut off at the limits, and the cells should be joined with spot-welded nickel strip rather than soldered, which cooks them. Treat pack building as a safety job, not just a wiring one.

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