Battery Pack Calculator - Series and Parallel Configurations

Series vs. Parallel Connections Batteries can be connected in two fundamental ways. Cells in series add their voltages together while keeping the capacity (Ah) the same, the way stacking AA batteries in a remote control adds 1.5V per cell. Cells in parallel do the reverse: they add capacity (Ah) and leave the voltage alone, like two identical batteries side by side sharing the load. A combination of both (series-parallel, or “xSyP”) achieves any desired voltage and capacity.

The S and P Notation Battery pack configurations are described as “xSyP”. For example, 4S3P means 4 cells in series and 3 such strings connected in parallel. The result: voltage = 4 × cell voltage, capacity = 3 × cell capacity, total cells = 4 × 3 = 12. A 4S3P pack of 3.7V / 3.5Ah cells gives 14.8V / 10.5Ah = 155.4Wh.

Cell Chemistry Voltages Lithium-Ion (Li-ion): 3.6–3.7V nominal, 4.2V fully charged, 3.0V minimum discharge. Used in phones, laptops, and consumer electronics. Lithium Iron Phosphate (LiFePO4): 3.2–3.3V nominal, 3.65V full, 2.5V minimum. Safer, longer cycle life (2,000+ cycles vs 500 for Li-ion), slightly lower energy density. The preferred choice for solar storage and EVs where safety and longevity matter. NiMH: 1.2V nominal, still used in power tools and consumer AAs.

A Famous Example The 85 kWh Tesla Model S pack used 7,104 Panasonic 18650 cells wired 96S74P. Sixteen modules sit in series; inside each module are six groups in series; and each of those groups is 74 cells in parallel. Sixteen times six is the 96S, and the pack sits around 350V nominal, just over 400V fully charged. The Model 3 Long Range moved to the larger 2170 cell and needs only 4,416 of them for a similar capacity. Note what the S notation buys you: the 74 cells in each parallel group behave as one enormous cell, so the pack is electrically a 96-cell series string that happens to be very good at delivering current.

The BMS Is Not Optional A Battery Management System (BMS) is absolutely required for any lithium chemistry pack. The BMS monitors each cell’s voltage and temperature, prevents overcharging (which causes fire), prevents over-discharge (which permanently damages cells), balances cells to equalize voltages, and provides short-circuit protection. Never build or charge a lithium pack without a proper BMS.

Wh Is the Real Measure of Energy Amp-hours (Ah) measure charge, not energy. Two packs with the same Ah but different voltages hold very different amounts of energy. Watt-hours (Wh = V × Ah) is the correct unit for comparing energy storage across different systems, and it is what determines how long your device runs.