A battery electric vehicle stores energy in a high-voltage battery, converts it to three-phase AC in an inverter, and drives one or more traction motors. A DC-DC converter supplies the 12 V system, and an onboard charger converts AC supply into DC for the battery. Thermal management keeps the battery, motor and power electronics within their operating windows.
Traction current
Problem. A traction inverter delivers 150 kW from a 400 V nominal pack. Estimate the DC current drawn from the pack at that operating point.
Solution. I = 150 000 ÷ 400 = 375 A. Real current varies with pack voltage, efficiency and operating point.
Why high voltage
For a given power, raising the voltage lowers the current. Lower current means smaller conductors, lower resistive losses and less heat in the inverter and motor. That is why traction systems use hundreds of volts rather than the 12 V used for control and lighting.
- The high-voltage battery stores traction energy.
- The inverter converts DC to three-phase AC for the motor.
- A DC-DC converter maintains the 12 V system.
- The onboard charger converts AC supply to DC charge current.
- High voltage reduces current for the same power, but increases hazard.
- A BEV uses a high-voltage battery, inverter, motor and DC-DC converter.
- I = P/V gives an estimate of traction current.
- Higher voltage reduces current for the same power.
- High voltage demands qualified-person service procedures.
Worked example
At 800 V, the same 150 kW requires about 150 000 ÷ 800 = 187.5 A, roughly half the current needed at 400 V.