EV Battery Packs and BMS

Cells, series-parallel configuration, capacity and the BMS that keeps them safe.

Intermediate

Packs are built from cells in series (voltage) and parallel (capacity), written e.g. 96S2P. Pack voltage ≈ $N_s \times$ cell voltage (96 × 3.6 V ≈ 346 V); energy (kWh) = voltage × capacity (Ah) / 1000.

Common chemistries: NMC (high energy density) and LFP (safer, longer life, cheaper; widely used in India's two- and three-wheelers and many cars).

The BMS:

monitors every cell's voltage and temperature;
estimates state of charge (SoC) and state of health (SoH);
balances cells (passive bleed resistors or active transfer);
protects against over-voltage, under-voltage, over-current and over-temperature to prevent thermal runaway.

India's AIS-156 standard sets safety requirements for EV battery packs.

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