# What Is an Onboard Charger in an EV? By EV Charge Cables ยท Published 18 October 2026 **The short version** The onboard charger is the unit inside your EV that converts AC to DC, and its rating sets the maximum AC charging speed your car can accept. - Common ratings are 7.4kW, 11kW or 22kW - A cable cannot charge faster than this limit - Rapid DC chargers bypass it entirely An onboard charger is the unit inside your EV that converts alternating current from a home wallbox or public AC point into direct current the battery can store. Because it does this conversion, it sets the maximum AC charging speed your car can accept, commonly 7.4kW, 11kW, or in a minority of cars 22kW. This is why a higher-rated cable cannot make AC charging faster than the onboard charger allows: the car itself is the bottleneck. Rapid DC chargers bypass it entirely, feeding DC straight to the battery. If you are choosing a cable, match it to your car limit using the [cable finder by car](https://www.evchargecable.co.uk/ev-charging-cable). ### At a glance: onboard charger and speed | Onboard charger rating | Max AC speed | What it means | | --- | --- | --- | | 7.4kW single phase | 7.4kW | Most common, fine for home charging | | 11kW three phase | 11kW | Faster only on a three-phase supply | | 22kW three phase | 22kW | Minority of cars, needs three phase | | Rapid DC | Bypasses onboard charger | DC goes straight to the battery | ### What does an onboard charger do? It converts AC to DC so the battery can be charged. The grid and home sockets supply AC, but the battery stores DC, so a component inside the car handles the conversion during AC charging. This is the onboard charger. Its power rating decides how quickly it can do that conversion, and therefore how fast the car charges on AC. It works automatically whenever you plug into a home wallbox or public AC post using your own cable. ### Why does it cap my AC charging speed? Because conversion capacity is limited by the hardware. An onboard charger rated at 7.4kW can only process that much AC power, so even a 22kW supply and a 22kW cable will still charge the car at 7.4kW. The car cannot accept more than its onboard charger can convert. This is the single most important reason a pricier, higher-rated cable does not automatically mean faster charging: the weakest link, often the onboard charger, sets the pace. ### Why can a cable never beat the onboard charger? A cable only carries power, it does not convert it. On AC charging, the current still has to pass through the onboard charger to become DC for the battery. If that unit is rated at 7.4kW, the cable cannot push more through it. A 22kW cable on a 7.4kW car is just heavier and pricier with no speed gain. Buy a cable to match your car and supply, not in the hope of overriding the car limit. ### How is rapid DC charging different? Rapid DC skips the onboard charger. Rapid chargers do the AC to DC conversion inside the charger unit itself and feed DC straight to the battery, so they are not limited by your car small onboard charger. That is how they reach 50kW to 350kW. Rapid chargers are always tethered, so you never use your own cable. The car still has a separate DC maximum, but it is far higher than its AC onboard charger limit. ### How do I find my car onboard charger rating? Check the specification, then buy to match. - Look in the car handbook or manufacturer specification for the AC charging rate. - Common figures are 7.4kW single phase or 11kW three phase, with 22kW less common. - If your car is 7.4kW, a 7.4kW cable is all you need at home. - If your car accepts 11 or 22kW and you have three phase, a higher-rated cable helps. - When unsure, the registration checker will match a suitable cable to your car. Matching the cable to the onboard charger avoids paying for capacity your car cannot use. ### Frequently asked questions #### Does the onboard charger affect rapid charging? No. Rapid DC chargers do the AC to DC conversion themselves and feed DC straight to the battery, bypassing the onboard charger. That is why rapid charging can be so much faster than AC. Your car still has a separate maximum DC rate, but it is unrelated to the smaller onboard charger that limits AC speed. #### Can I upgrade my car onboard charger? For most cars, no. The onboard charger is a fixed component chosen by the manufacturer, so its rating is part of the car specification. If AC charging speed matters a lot to you, it is a factor to consider when buying the car. For everyday home charging, a 7.4kW onboard charger is usually more than enough overnight. #### Why does my 22kW cable only charge at 7.4kW? Because either your supply is single phase or your car onboard charger is rated at 7.4kW, or both. The cable carries power but cannot exceed the lowest limit in the chain. On a typical single-phase home with a 7.4kW car, a 22kW cable delivers 7.4kW, the same as a 7.4kW cable would, just with more weight and cost. #### Is a higher onboard charger rating always better? Only if you can use it. An 11 or 22kW onboard charger helps only where you have a three-phase supply or use three-phase public posts. On a standard single-phase home you are capped at 7.4kW regardless, so the extra capacity sits unused. It is a useful feature for some drivers, but not essential for most home charging. #### Does the onboard charger convert power both ways? The standard onboard charger converts AC to DC for charging. Some cars add extra hardware for features like vehicle-to-load, which draws DC from the battery and provides an AC socket for appliances. That is a separate capability from ordinary charging, and not all cars have it. Check your car specification if you want to power devices from the battery. #### How do I know my car AC charging limit? Check the manufacturer specification or handbook, which lists the AC charging rate, often 7.4kW single phase or 11kW three phase. That figure is your onboard charger limit and the fastest your car will charge on AC. Once you know it, choose a cable to match, since a higher-rated cable brings no benefit above that limit. To sum up, the onboard charger converts AC to DC and caps your AC charging speed, which is why a cable can never beat it. Buy a cable to match your car, not above it. To weigh up ratings, read our [7kW vs 22kW](https://www.evchargecable.co.uk/blogs/guides/7kw-vs-22kw) comparison and [AC vs DC EV charging explained](https://www.evchargecable.co.uk/blogs/guides/ac-vs-dc-ev-charging-explained). When you know your limit, use the [cable finder by car](https://www.evchargecable.co.uk/ev-charging-cable) to pick the right cable. --- Canonical version: https://www.evchargecable.co.uk/blogs/guides/what-is-onboard-charger-in-an-ev EV Charge Cables, EV charging cables matched to your car: https://www.evchargecable.co.uk. Some links in this article are Amazon affiliate links; as an Amazon Associate we earn from qualifying purchases at no extra cost to you.