Battery Pack Series-Parallel

Formula

Description

Battery packs combine cells in series to increase voltage and in parallel to increase capacity. A pack configured as NsSnpP has Ns cells in series and Np strings in parallel. For example, a 4S3P pack of 3.7 V, 2500 mAh 18650 cells produces 14.8 V nominal with 7500 mAh capacity. The total energy is Ns × Np × Vcell × Qcell. The forward result shows the pack voltage (Ns × Vcell). Pack capacity is Np × Qcell. Every cell in a series string must be balanced to prevent overcharge or over-discharge of individual cells.

Variables

• Ns — Number of cells in series (dimensionless)
• Np — Number of parallel strings (dimensionless)
• V_cell — Single cell nominal voltage (V)
• Q_cell — Single cell capacity (Ah or mAh)

Practical Notes

Common Li-ion configurations: 1S (3.7 V, USB power banks), 3S (11.1 V, laptop), 4S (14.8 V, RC batteries), 7S (25.9 V, e-bikes), 14S (51.8 V, e-bikes), 96S (355 V, EVs). A BMS is essential for series configurations to balance cells and prevent dangerous over-charge or over-discharge conditions. Cells in parallel should be matched for capacity and internal resistance.