About the Battery Pack Calculator
Building a battery pack means arranging cells in series, which adds voltage, and in parallel, which adds capacity. This calculator does the sums both ways. Enter the number of cells in series (S) and in parallel (P) and it returns the nominal pack voltage, the capacity in amp-hours, the energy in watt-hours and kilowatt-hours, the maximum continuous current and power and the full-charge voltage. Or enter the voltage and capacity you want and it finds the layout you need, such as 14S4P. Choose a common cell like the 18650 or 21700, or type your own values. It is a planning tool, so always follow the cell maker's limits and use a proper battery management system.
Updated October 2026
How to use the Battery Pack Calculator
- 1Choose whether you want to find the pack from S and P, or the layout for a target.
- 2Pick your cell type or type a custom voltage and capacity.
- 3Enter the series and parallel counts, or your target voltage and capacity.
- 4Read the pack voltage, capacity, energy and current.
- 5Copy the layout, for example 13S4P, into your build notes.
Series and parallel in plain words
Connect cells in series, plus to minus, and the voltages add while the capacity stays that of one cell. Connect them in parallel, plus to plus, and the capacity adds while the voltage stays that of one cell. A pack combines both, so a 10S3P pack has 10 cells in each of 3 strings joined side by side.
The more cells you put in parallel, the more current the pack can supply, because each cell shares the load. That is why a high-power pack for an electric bike uses more cells in parallel than a small pack for a light.
Safety basics
- Use cells from a trusted maker with a datasheet.
- Never mix old and new cells or different brands in one pack.
- Always add a BMS rated for your series count and current.
- Use fuses and proper thick wiring for the maximum current.
- Charge only with a charger made for your chemistry and voltage.
Related tools
Estimate how long the pack will run with the Battery Runtime Calculator, and work out the charging time with the Battery Charge Time Calculator. Convert between milliamp-hours and watt-hours with the mAh to Wh Calculator, and size the cabling with the Wire Gauge Calculator. Lithium batteries can be dangerous if built or used wrongly.
Features
- Pack from S and P
- Layout from target voltage and capacity
- Li-ion 18650 and 21700, LiFePO4 and NiMH presets
- Custom cell values
- Energy in Wh and kWh
- Maximum current and power from C-rate
- Full-charge voltage
Frequently asked questions
What does 13S4P mean?
13 cells in series and 4 such strings in parallel, so 52 cells. With 3.6 V, 3 Ah cells that is 46.8 V and 12 Ah.
How do I calculate pack voltage?
Multiply the number of cells in series by the cell voltage. A 10S Li-ion pack at 3.6 V per cell is 36 V nominal, and 42 V when full.
How do I calculate pack capacity?
Multiply the cells in parallel by the capacity of one cell. Four 3 Ah cells in parallel give 12 Ah.
How much energy is in a pack?
Energy in watt-hours is voltage multiplied by amp-hours. 46.8 V times 12 Ah is about 562 Wh.
What is the C-rate?
It is the current as a multiple of the cell capacity. A 3 Ah cell at 1 C delivers 3 A and at 2 C delivers 6 A. Check your cell's datasheet for its safe limit.
Do I need a BMS?
Yes. A battery management system balances the cells and protects against overcharge, over-discharge, short circuits and overheating. Lithium packs without one are dangerous.
Are the numbers exact?
They use nominal cell values. Real voltage varies with charge level, temperature and load, and real capacity depends on age and discharge rate.