Battery Pack Designer — series, parallel, voltage and C-rate

Series/parallel cell configurations, pack voltage, watt-hours, C-rate and continuous current limits for 18650 and LiPo packs.

Example: 4S2P of Li-ion (NMC/LCO) gives 14.4 V nominal, 6 Ah and 86.4 Wh, with 12 A continuous.

Inputs

Chemistry

Series (S)

Parallel (P)

Cell capacity(Ah)

3 Ah is typical for a good 18650.

Cell max discharge(C)

1C means it can supply its own capacity in one hour.

Load current (optional)(A)

Result

Nominal voltage

14.4 V

Capacity: 6 Ah
Energy: 86.4 Wh
Max continuous current: 12 A
Max continuous power: 172.8 W
Voltage range: 12 V … 16.8 V
Cells: 8

The pack swings from 16.8 V fully charged to 12 V empty — a 33% span. Everything downstream must work across all of it, not just at the 14.4 V nominal.
4 cells in series need a balancing BMS. Cells drift apart in capacity and self-discharge, and without balancing one cell hits its limit first — overcharged on charge, over-discharged on load. Lithium cells fail dangerously when that happens.
C-rate is a per-cell property. Parallel cells share current so the pack's limit scales with P; series cells all carry the same current so S changes nothing. People routinely get this backwards and undersize the parallel count.

Configurations with 8 cells

Config	Voltage	Capacity	Energy	Max current
1S8P	3.6 V	24 Ah	86.4 Wh	48 A
2S4P	7.2 V	12 Ah	86.4 Wh	24 A
4S2P	14.4 V	6 Ah	86.4 Wh	12 A
8S1P	28.8 V	3 Ah	86.4 Wh	6 A

Formula

V_pack = S × V_cell    capacity = P × cell Ah    energy = V × Ah

Worked example

4S2P of Li-ion (NMC/LCO) gives 14.4 V nominal, 6 Ah and 86.4 Wh, with 12 A continuous.

V_pack = S × V_cell
4 × 3.6 V
14.4 V
capacity = P × cell capacity
2 × 3 Ah
6 Ah
I_max = C-rate × cell capacity × P
2 × 3 × 2
12 A
Series count does not appear — every series cell carries the same current.

Frequently asked questions

What does 4S2P mean?

Four cells in series, two of those strings in parallel — eight cells total. Series multiplies voltage, parallel multiplies capacity and current capability. A 4S2P pack of 3.6 V 3 Ah cells is 14.4 V and 6 Ah.

Does adding cells in series increase current capability?

No. Every cell in a series string carries the identical current, so the string's limit is one cell's limit. Only parallel raises it. Undersizing the parallel count is one of the most common pack design mistakes.

Do I need a BMS?

For any lithium pack with more than one cell in series, yes. Cells drift apart in capacity and self-discharge rate, so without balancing one reaches its limit before the others — overcharged when charging, over-discharged under load. Lithium cells fail dangerously in both cases.

Why does my pack voltage vary so much?

Because nominal voltage is a midpoint, not a constant. A 4S Li-ion pack is 16.8 V full and 12 V empty — a 33% span. Every regulator, motor driver and sensor downstream must work across that whole range, not just at the nominal figure.

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