Short answer: The component inside your car that converts AC electricity from the grid into DC electricity that the battery can store.
Explanation
The onboard charger is a power electronics module built into your EV that converts alternating current (AC) from the charging cable into direct current (DC) that the battery can store. Every EV has one, and its capacity determines the maximum speed you can charge on AC.
Onboard chargers are rated in kW. Common ratings are 7.4 kW (single-phase), 11 kW (three-phase), and 22 kW (three-phase). If your car has an 11 kW onboard charger and you plug into a 22 kW AC charger, you will only charge at 11 kW because the onboard charger is the bottleneck. The extra capacity of the wallbox goes unused.
During DC fast charging, the onboard charger is bypassed entirely. The DC charger converts AC to DC inside its own cabinet and sends DC power directly to the battery through the CCS2 connector's DC pins. This is why DC chargers can deliver much higher power than any onboard charger, reaching 350 kW compared to the onboard charger's typical maximum of 22 kW.
Where you'll see this
- In vehicle specifications
Common confusion
People sometimes upgrade their home wallbox to 22 kW thinking it will charge their car faster, without checking if the car's onboard charger supports more than 11 kW. Always check your car's AC charging limit before investing in a higher-powered wallbox.
Example
A Renault Megane E-Tech has a 22 kW onboard charger, meaning it can use every kW from a 22 kW wallbox. A Hyundai Ioniq 5 has an 11 kW onboard charger and cannot charge faster than 11 kW on AC regardless of the wallbox.
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