An on-board charger, or OBC, is an automotive power electronics device that achieves electrical power conversion through high-frequency switching operations. OBCs are classified into unidirectional OBCs and bidirectional OBCs. The unidirectional OBCs can only charge traction batteries, while the bidirectional OBCs can invert the direct current from high-voltage traction batteries to alternating current (220V AC 50Hz/110V AC 60Hz). An electromagnetic relay is a switch-controlled component that operates through electromagnetic effect and consists of a set of input terminals for single or multiple control signal(s) and a set of operating contact terminals. In an OBC, the AC input and the high-voltage DC output are usually configured with one relay each. Before confirming with the charging pile to initiate charging, the relays ensure the electrical isolation of the OBC from the power grid; in V2L mode, the relays in the same position control whether power is released to AC loads, to ensure the safety of the user and the external loads.

Taking a 6.6kW, 400V single-phase OBC as an example, the current is 32A_RMS at 220V rated input and the maximum output current is about 30A_DC during battery discharge. High-power operation requires the relays in the OBC to withstand approximately 400Vpk voltage and carry 60A over-current. An electromagnetic relay mainly consists of a coil, core, contacts, and springs. When the coil is energized, the core generates a magnetic field, closing the contacts; when the coil is de-energized, the core loses its magnetic field and the contacts are disconnected by the spring.