What Is an Onboard Charger, and Why Does It Limit Your Charging Speed?
Here is something that catches a lot of people out: you can buy the fastest home charger on the market and still charge at the same speed as a cheaper one. The reason sits inside the car, not on the wall. It is called the onboard charger, and understanding it saves money and disappointment.
Two Chargers in Every EV Setup
Home charging involves two devices, not one. The wall unit supplies alternating current (AC) from your home. The car's onboard charger then converts that AC into the direct current (DC) the battery actually stores. The speed of home charging is capped by whichever of the two is slower, and very often that is the car.
Why the Car Sets the Ceiling
Every EV has a maximum AC rate its onboard charger will accept. Some accept around 7kW on single phase; others accept 11kW or even 22kW on three phase. Plug a car with a 7kW onboard limit into a 22kW charger and it still charges at 7kW, the car simply will not take more. The wall unit cannot push current the car refuses to accept.
What This Means When Choosing a Charger
The practical lesson is to check the car's accepted AC charging rate before buying a high-output unit. There is no point paying for a 22kW three-phase charger if the vehicle tops out at 7kW, or if the home only has single-phase supply anyway. Matching the charger to the car's real limit is how you avoid spending on speed that never arrives.
Public Fast Charging Is Different
One source of confusion: public rapid chargers can fill a battery far faster than any home unit. That is because they supply DC directly, bypassing the car's onboard AC charger entirely. Home charging is AC and therefore bound by the onboard limit; the two are not comparable, and a slower home rate is perfectly normal and fine for overnight charging.
How to Find Your Car's AC Rate
The number you want is the vehicle's maximum AC charging rate, listed in the specifications as a figure in kilowatts, sometimes split into single-phase and three-phase values. It is separate from the much larger DC fast-charging figure, which only applies to public rapid chargers. If the specs aren't to hand, an installer can confirm it from the model. Knowing this one number stops the most common and expensive home-charging mistake: buying more charger than the car can ever use.
Why Carmakers Set the Limit
The onboard charger is sized by the manufacturer as a balance of cost, weight and how the car is expected to be used. Many mainstream EVs settle around 7kW or 11kW on AC because that comfortably covers overnight home charging, while reserving the really high speeds for DC fast charging on the road. It isn't a flaw, it's a deliberate design choice, and it's exactly why matching the home charger to that figure, rather than overshooting it, is the sensible move.
The practical takeaway is simple: find the car's AC rate first, then choose a charger and a supply that meet it, not exceed it. A 7kW charger on a single-phase home suits the many EVs that accept around that figure; three-phase only pays off when the car can use it. Getting this right is the difference between a charger that's perfectly matched and one that's quietly overspecified.
It is worth noting the onboard charger only governs AC charging, the kind you do at home. At a public DC fast charger, power bypasses it entirely, which is why those stations can charge so much faster than anything at home ever will. So a modest AC rate is no reflection on how quickly the car can fast-charge on a road trip; the two systems are separate. For home charging, all that matters is matching the wall unit to the car's AC figure.
Frequently Asked Questions
What is an onboard charger?
It is the device inside the EV that converts AC power from a home charger into the DC the battery stores. Its maximum rate sets how fast the car can charge from any AC home unit.
Why won't my car charge faster on a bigger charger?
Because the car's onboard charger caps the AC rate it will accept. If the car tops out at 7kW, it charges at 7kW even on a 22kW unit. The wall charger cannot exceed what the car allows.
How do I find my car's charging limit?
It is in the vehicle's specifications, usually listed as the maximum AC charging rate, and an installer can confirm it. Checking it before buying a charger avoids paying for unusable output.
Why is public fast charging so much quicker?
Public rapid chargers deliver DC straight to the battery, bypassing the onboard AC charger, so they are not limited the same way. Home AC charging is slower by nature and that is normal for overnight use.
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