How series and parallel work
Cells in series (S) add up their voltages; cells in parallel (P) add up capacity and current. A 4S2P pack has four groups in series, each made of two cells in parallel: eight cells in all.
V = S × V_cell Ah = P × Ah_cell Wh = V × Ah I_max = P × I_max,cell
Example with Samsung 30Q cells (3000 mAh, 15 A): 4S2P gives 4 × 3.6 = 14.4 V nominal, 16.8 V full and 10 V at the 2.5 V cut-off, 2 × 3 = 6 Ah, 14.4 × 6 = 86.4 Wh, and up to 2 × 15 = 30 A.
Common configurations
| Series | Nominal (3.6–3.7 V per cell) | Full charge (4.2 V per cell) | Often called |
|---|---|---|---|
| 1S | 3.6–3.7 V | 4.2 V | |
| 2S | 7.2–7.4 V | 8.4 V | |
| 3S | 10.8–11.1 V | 12.6 V | 12 V |
| 4S | 14.4–14.8 V | 16.8 V | |
| 5S | 18–18.5 V | 21 V | |
| 6S | 21.6–22.2 V | 25.2 V | |
| 7S | 25.2–25.9 V | 29.4 V | 24 V |
| 10S | 36–37 V | 42 V | 36 V |
| 13S | 46.8–48.1 V | 54.6 V | 48 V |
| 14S | 50.4–51.8 V | 58.8 V | 52 V |
Li-ion cells: 3.6–3.7 V nominal and 4.2 V full. The cut-off is 2.5–3.0 V per cell depending on the cell; LiFePO4 cells are 3.2 V nominal and 3.6–3.65 V full.
BMS, charger and fuse
The BMS (battery management system) watches every series group and disconnects the pack on over-charge, over-discharge, over-current and short circuit. Choose one for the same number of series groups and the same chemistry, rated for more than your highest load current. The charger must be a CC/CV charger set to S × 4.2 V (S × 3.6–3.65 V for LiFePO4). Put a fuse on the main lead, sized between your load current and what the cells and wires can carry.
Flying with a battery pack
IATA rules for spare lithium-ion batteries carried by passengers: up to 100 Wh in cabin baggage; from 100 to 160 Wh only with the airline’s approval and at most two; above 160 Wh only as dangerous-goods cargo. Spare batteries never go in checked baggage, and airlines can be stricter. Only batteries of a UN 38.3-tested type may fly, which a home-built pack normally isn’t; tape or cap the terminals.
Frequently asked questions
How many 18650 cells for 12 V?
Three in series (3S): 10.8–11.1 V nominal and 12.6 V full. If the voltage must stay above 12 V for most of the discharge, use four (4S, 14.4 V nominal, 16.8 V full) with a regulator. Add cells in parallel for capacity.
And for 24 V or 36 V?
24 V devices usually take 7S (25.2 V nominal, 29.4 V full); 36 V e-bike packs are 10S (36 V nominal, 42 V full). With LiFePO4, 4S gives 12.8 V and 8S 25.6 V.
How do I work out the Wh?
Multiply the nominal voltage by the capacity in Ah. A 4S2P pack of 3 Ah cells is 14.4 V × 6 Ah = 86.4 Wh, which is also eight cells of 10.8 Wh each.
What BMS do I need?
One for the same number of series groups and the same chemistry (a 4S Li-ion BMS for a 4S Li-ion pack), with a continuous rating above your highest load current. A balancing BMS keeps the groups at the same voltage.
Can I mix cells of different capacity?
No. In series the smallest cell empties first and sets the capacity of the whole pack; in parallel, cells of different type or age share the current unevenly. Use cells of the same model, ideally from the same batch.
18650 or 21700?
A 21700 cell is 21 × 70 mm instead of 18 × 65 mm: 4000–5000 mAh instead of 3000–3500 for about 45 % more mass. The same pack needs fewer cells and fewer welds.
Why does the voltage drop under load?
Every cell has a small internal resistance: at high current its voltage sags, and it recovers at rest. The cut-off then comes sooner, so the capacity you get at high current, and in the cold, is lower than the rated one.