Battery Pack Configuration: What S×P Does to a Cell

How a pack is built from one cell

A battery pack is just many copies of a single cell wired in a series–parallel (S×P)arrangement. Pick a cell, choose how many go in series (S) and how many of those strings in parallel (P), and the whole pack's voltage, capacity, energy, and internal resistance follow from the one cell's spec by simple circuit rules. "10S4P" means 10 cells in series, 4 such strings in parallel — 40 cells total.

Series adds voltage; parallel adds capacity

• Series (S) stacks cells to add voltage: pack voltage = S × cell voltage. Their internal resistances also add in series.
• Parallel (P) adds capacity (amp-hours) and current capability: pack capacity = P × cell capacity. Parallel resistances combine as R/P — more parallel cells means lower resistance.

Energy is the product: total energy = S × P × (cell voltage × cell capacity). So 10S4P and 20S2P and 40S1P all store the same energy from the same 40 cells — but at very different voltages and currents. The S×P split is how you hit a target voltage (which the motor/inverter needs) and a target current/energy (which the range and power need) from the same cell.

Internal resistance and voltage sag

The pack's internal resistance is R_pack = S × R_cell / P. It matters because under load the terminal voltage sags below the open-circuit value by I × R_pack. Draw a big current (hard acceleration) and a high-resistance pack droops — losing usable power and generating heat (I²·R) inside the cells. More parallel cells lower the resistance and the sag; that's often why a pack has more parallel cells than the energy alone would require.