What Is an Onboard Charger?

The onboard charger (in-vehicle charging unit) is the unit inside the electric vehicle that, during AC charging, converts the incoming alternating current into the direct current the battery needs. The upper limit of AC charging speed is usually set not by the station but by this unit: for example, a vehicle with an 11 kW onboard charger charges at roughly 11 kW even when connected to a 22 kW-capable station, because the bottleneck is in the vehicle. That is why the answer to 'my station is 22 kW, so why isn't it charging faster?' is usually the vehicle's onboard charger capacity. In DC fast charging, however, the conversion is done inside the station, so the onboard charger is bypassed and the vehicle's AC limit does not apply; power is delivered directly to the battery. When choosing the right AC device, knowing the vehicle's onboard charger power is important so you don't spend money on an unnecessarily high-power device. Bemis AC wallbox devices offer options suitable for common onboard charger powers (3.7–22 kW). A practical example: for a user whose vehicle only accepts 11 kW, buying a 22 kW device for home brings no extra speed; in that case an 11 kW model suited to the vehicle is both sufficient and more economical.