The on-board charger is a core component of new energy vehicles. It can convert 220V/380V AC power into DC power suitable for power batteries and also supports V2G inverter charging. This article introduces its classification, composition and working principle.The power selection method is expounded. For example, it is more reasonable to select 6.6kW OBC for a 60kW·h battery. Introduce the test requirements, including six tests such as AC input voltage. Analyze failure modes, such as thermistor damage, etc., and provide rectification measures, such as replacing thermistors and modifying software, etc. Finally, it is pointed out that on-board chargers are developing towards higher integration, more efficient energy conversion, and greater safety and stability, providing a reference for subsequent development and design.