Battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs) use high-voltage battery systems to drive the cars. These battery systems require a method of charging when not in motion. The most common systems use 300 V-400 V batteries, but some manufacturers have begun specifying 800 V battery systems to improve vehicle energy efficiency. Regardless of battery voltage or battery type, they all require a method of daily charging.

One method is to use a stationary charger, which can be installed in a garage or public place. However, there may be a need to charge the vehicle in remote locations where there is no dedicated charging source. In this case, there must be a charger in the vehicle that can use typical AC supply voltages and connections. This requirement forms the basis for the need for an on-board charger.

Today's OBCs employ many different designs while always balancing their power levels with cost and weight. Vehicle manufacturers must define the appropriate on-board charging power level based on the vehicle requirements. Chargers come in many different power levels. The higher the power level, the shorter the charging time. These chargers require a large amount of AC power and are powered by either single-phase or three-phase power, depending on the design of the on-board charger.

Four general power levels have been developed, depending on the typical AC power source available worldwide. 3.3 kW and 6.6 kW chargers have become the basic building blocks used for all power levels. 11 kW and 22 kW chargers both combine three single-phase units, each running one of the three phases. The most popular power levels are shown in Table 1.