About the Battery Pack Calculator
This battery pack calculator works out what you get when you wire individual cells into a pack. Enter how many cells are in series (S) and in parallel (P), plus the specifications of one cell, and it returns the pack's nominal, fully charged and cut-off voltages, its capacity in amp-hours, its energy in watt-hours, the maximum continuous current and power, the total cell count and weight, and how long it would run a given load.
It is built for people designing e-bike, scooter, power-wall, robot, drone and flashlight packs from 18650, 21700 or LiFePO4 cells — and for anyone checking a pack label such as "10S4P 36 V 12 Ah". Change S to hit your target voltage and P to reach the capacity and current you need.
Series cells add voltage while capacity stays the same; parallel groups add capacity and current while voltage stays the same. The figures assume identical, balanced cells and ignore wiring and BMS losses, and the runtime uses the full rated energy, so real runtime is usually 10–20% lower — especially at high current or low temperature.
With the default inputs, the pack energy is 129.6 Wh. Change any value above to recalculate instantly.
How to use the battery pack calculator
- 1Enter the number of cells in series (S) and in parallel (P).
- 2Enter the cell nominal, full-charge and cut-off voltages from the datasheet.
- 3Enter the cell capacity in mAh and its maximum continuous current.
- 4Optionally enter cell weight and a load in watts for runtime.
- 5Read the pack voltage, capacity, energy and current limits.
Formula and method
Connecting cells in series stacks their voltages while the same current flows through every cell, so the pack voltage is S times the cell voltage and the capacity in amp-hours equals that of one parallel group. Connecting cells in parallel keeps the voltage but adds their capacities and current ratings, so a group of P cells provides P times the amp-hours and P times the current.
Energy in watt-hours is nominal voltage × amp-hours, which is the same as total cells × cell voltage × cell amp-hours. Maximum continuous power is the nominal voltage multiplied by the combined current rating. Runtime divides energy by the load power and assumes the full rated capacity is usable, with no conversion or wiring losses.
- S
- Number of cells (or parallel groups) in series
- P
- Number of cells in each parallel group
- V_cell
- Cell nominal voltage
- Ah_cell
- Cell capacity in amp-hours (mAh ÷ 1,000)
- I_cell
- Cell maximum continuous discharge current
Worked examples
4S3P pack of 3,000 mAh 18650 cells
Four cells in series give 4 × 3.6 = 14.4 V nominal (16.8 V full, 10 V empty). Three in parallel give 3 × 3 = 9 Ah and 3 × 10 = 30 A. Energy is 14.4 × 9 = 129.6 Wh, which runs a 100 W load for about 1.3 hours.
36 V e-bike pack, 10S4P
Ten series cells make 36 V (42 V charged) and four in parallel make 12 Ah, so the pack holds 36 × 12 = 432 Wh from 40 cells weighing 1.88 kg. A 500 W motor at full power would drain it in 0.86 h, about 52 minutes.
12 V LiFePO4 pack from 4S1P 32700 cells
Four 3.2 V LiFePO4 cells give the standard 12.8 V pack (14.6 V charged). One 6 Ah string stores 12.8 × 6 = 76.8 Wh, enough for a 30 W load for 2.56 hours, with up to 18 A × 12.8 V = 230 W available.
Frequently asked questions
What does 10S4P mean on a battery pack?+
It means 10 cells (or cell groups) connected in series, with each group made of 4 cells in parallel — 40 cells in total. The series count sets the voltage and the parallel count sets the capacity and current capability.
How many 18650 cells do I need for a 48 V battery?+
A "48 V" lithium-ion pack is 13 cells in series: 13 × 3.6 V = 46.8 V nominal and 54.6 V fully charged. Add parallel cells until you reach the amp-hours and current your motor or inverter needs.
How do I calculate battery pack capacity in Wh?+
Multiply the nominal pack voltage by its amp-hour capacity. A 14.4 V, 9 Ah pack stores 129.6 Wh. Equivalently, multiply the number of cells by each cell's voltage and amp-hours.
Do series cells increase capacity?+
No. In series the same current flows through every cell, so amp-hour capacity stays equal to one cell (or one parallel group) while voltage rises. Energy in Wh does increase, because voltage is higher.
Why does a lithium pack need a BMS?+
Cells drift apart in charge over time. A battery management system monitors every series group, balances them and disconnects the pack on over-voltage, under-voltage, over-current or over-temperature to prevent damage and fire risk.