A traction battery is built from cells grouped into modules and then into a pack. Cells in series add voltage; cells in parallel add capacity. The battery management system monitors cell voltages and temperatures, estimates state of charge and state of health, balances cells and controls the contactors that connect the pack to the vehicle.
Pack voltage and energy
Problem. A pack uses 96 cells in series, each with a nominal voltage of 3.7 V, and the pack capacity is 100 Ah. Find the nominal pack voltage and energy.
Solution. V_pack = 96 × 3.7 = 355.2 V. E_pack = 355.2 × 100 = 35 520 Wh = 35.52 kWh.
C-rate
C-rate expresses current relative to capacity. A 1C discharge of a 100 Ah pack is 100 A. The acceptable continuous and peak C-rates depend on the cell chemistry, temperature and design, and are specified by the cell manufacturer.
Current at a given C-rate
Problem. A 100 Ah pack is discharged at 0.5C. What current does that represent?
Solution. I = 0.5 × 100 = 50 A.
- Series cells add voltage; parallel cells add capacity.
- The BMS monitors cells and controls the pack contactors.
- State of charge is an estimate, not a direct measurement.
- C-rate relates current to capacity.
- Cell limits are chemistry- and temperature-dependent.
- V_pack = n_series × V_cell.
- E_pack = V_pack × Ah.
- Series adds voltage, parallel adds capacity.
- The BMS monitors, balances and protects the pack.
Worked example
A 400 V pack built from 3.7 V cells needs about 400 ÷ 3.7 = 108 cells in series. With 100 Ah capacity, its nominal energy is roughly 400 × 100 = 40 kWh.