High-Voltage Contactors and Pre-Charge Circuit
High-voltage contactors and the pre-charge circuit form the controlled connection between an EV battery pack and the vehicle's external high-voltage bus.
Main components
- High-voltage contactors are electrically operated, high-current switches. The main positive and negative contactors connect or isolate the pack terminals from equipment such as the inverter, onboard charger, and DC-DC converter.
- The pre-charge circuit uses a resistor and a controlled switching path to charge capacitors in that equipment before the main connection is completed.
How they work together
Connecting an uncharged high-voltage bus directly to the battery could create a large inrush current. During a typical startup, the controller checks relevant safety conditions, closes the pre-charge path, monitors the voltage rise beyond the main contactors, and closes the main path when the difference is within its permitted limit. The exact switching order varies by design.
Pre-charge reduces stress on contactors, connectors, fuses, and power electronics; the main contactors then carry normal vehicle current. Opening them disconnects external equipment, but it does not remove voltage or stored energy from the cells and internal battery busbars.
For the surrounding protection system, see EV Battery Safety and Failure Management.