Calculate S/P battery pack layout, voltage, capacity, energy, cell count, and simple load checks from your cell data.
Table of contents
How to use our Battery Pack Series and Parallel Calculator
- Choose What do you want to do?: Build from S and P if you already know the layout, or Meet a target if you want the calculator to round up to a minimum voltage and capacity.
- Enter Single cell voltage and Single cell capacity using the same type of cell for the whole pack; if your cell rating is in mAh, divide by 1000 before entering Ah.
- Enter Cells in series and Cells in parallel for a known layout, or enter Target pack voltage and Target pack capacity for target mode.
- Open Advanced options only if you have Cell max continuous current, Expected load current, or Cell internal resistance and want runtime, current headroom, or voltage-under-load checks.
- After you click Calculate, sanity-check the Pack layout, Total cells needed, and Pack energy; if the layout seems too large or the voltage jumps above your target, recheck the cell voltage and target units.
Definitions
S/P: Short layout notation for series and parallel, such as 3S4P, meaning 3 cells in series and 4 cells in parallel in each series group.
Cells in series: Cells connected end-to-end. Series raises pack voltage [1].
Cells in parallel: Cells connected side-by-side in each group. Parallel raises pack capacity and helps share current [1].
Single cell voltage: The voltage of one cell. Nominal voltage is usually best for normal pack sizing.
Single cell capacity: The charge capacity of one cell in amp-hours, written Ah. A 2500 mAh cell is 2.5 Ah.
Pack energy: The estimated energy stored by the pack, in watt-hours, written Wh. Wh is useful for comparing packs with different voltages.
Current headroom: Estimated pack current limit minus Expected load current. Positive is under the entered limit; negative is over it.
Cell internal resistance: A small resistance inside each cell that causes voltage to drop when current flows.
Common mistakes and quick fixes
Mistake: Entering Single cell capacity in mAh, such as 2500, when the field needs Ah.
Fix: Convert mAh to Ah first, so 2500 mAh becomes 2.5 in Single cell capacity.
Mistake: Using full-charge voltage in Single cell voltage when you want the normal Pack voltage.
Fix: Use nominal cell voltage for normal sizing, and use full-charge voltage only when checking charger or BMS limits.
Mistake: Swapping Cells in series and Cells in parallel.
Fix: Put the voltage-building count in Cells in series and the capacity-building count in Cells in parallel.
Mistake: Treating Estimated pack current limit as a guaranteed safe current.
Fix: Use Cell max continuous current from the datasheet, then also check the BMS, wiring, heat, and connector ratings.
Mistake: Entering Expected load current as peak current when you want Estimated runtime.
Fix: Use average Expected load current for runtime, and check peak current separately against Current headroom.
Mistake: Ignoring Estimated voltage under load when Cell internal resistance and Expected load current are entered.
Fix: If Estimated voltage under load is very low or negative, lower the load, increase parallel cells, or recheck the resistance value.
Limitations & Key Assumptions / Boundary Conditions
- The calculator assumes all cells are the same chemistry, model, capacity, age, state of charge, and condition.
- Pack voltage is based on the Single cell voltage you enter. Nominal, full-charge, and empty voltages give different results.
- Target mode rounds up Cells in series and Cells in parallel, so the final Pack voltage or Pack capacity may be above the target.
- Estimated runtime assumes a steady Expected load current. Real runtime changes with cutoff voltage, temperature, cell age, discharge rate, and device behavior.
- Estimated pack current limit assumes current shares evenly between parallel cells. A real pack can be limited by the BMS, nickel strips, wires, fuses, connectors, and heat.
- Estimated voltage under load uses a simple resistance model. It does not model changing cell voltage during discharge or heating during long high-current use.
- This math does not verify safe pack construction, charger settings, BMS series count, balancing, insulation, fusing, or enclosure design.
Methodology
Core pack math
In Build from S and P mode, the calculator uses your entered Cells in series and Cells in parallel. In Meet a target mode, it rounds up each count so the calculated pack is not below the target voltage or capacity.
pack_voltage_v = series_count * cell_voltage_v
pack_capacity_ah = parallel_count * cell_capacity_ah
pack_energy_wh = pack_voltage_v * pack_capacity_ah
total_cells = series_count * parallel_count
Target mode
Target mode uses ceiling rounding, which means it rounds up to the next whole cell count when the target does not divide evenly.
series_count = ceil(target_voltage_v / cell_voltage_v)
parallel_count = ceil(target_capacity_ah / cell_capacity_ah)
target_voltage_margin_v = pack_voltage_v - target_voltage_v
target_capacity_margin_ah = pack_capacity_ah - target_capacity_ah
Optional load checks
If optional current and resistance values are entered, the calculator adds simple design checks. Blank optional fields are ignored.
max_pack_current_a = parallel_count * cell_max_current_a
current_headroom_a = max_pack_current_a - load_current_a
estimated_runtime_hours = pack_capacity_ah / load_current_a
load_power_w = pack_voltage_v * load_current_a
pack_resistance_ohm = series_count * (cell_internal_resistance_mohm / 1000) / parallel_count
voltage_drop_v = load_current_a * pack_resistance_ohm
voltage_under_load_v = pack_voltage_v - voltage_drop_v
Mini-example
For 3.7 V, 2.5 Ah cells in a 3S4P layout, Pack voltage is 3 * 3.7 = 11.1 V, Pack capacity is 4 * 2.5 = 10 Ah, Pack energy is 11.1 * 10 = 111 Wh, and Total cells needed is 3 * 4 = 12 cells. If Cell max continuous current is 10 A and Expected load current is 5 A, Estimated pack current limit is 4 * 10 = 40 A and Current headroom is 40 - 5 = 35 A.