AC vs DC Charging: What's the Difference and When to Use Each

Batteries store direct current (DC). But the electricity grid, and almost every socket in your home, delivers alternating current (AC). Somewhere between the plug and the battery, AC has to be converted to DC. Where that conversion happens is the whole difference between AC and DC charging.

AC charging: the car does the converting

With AC charging, the charger sends AC into the car and the car's own on-board charger converts it to DC. That on-board charger is small and light, so it limits the speed — typically 3.3 kW to 22 kW. That's slow, but it's perfectly matched to the times you're parked anyway: overnight at home, all day at the office, or for a few hours at a hotel, mall or restaurant. AC equipment is cheap, so AC points are everywhere.

DC charging: the charger does the converting

With DC fast charging, the conversion happens inside the charger — which can be the size of a fridge — so it sends DC straight to the battery, bypassing the car's small on-board charger. That removes the bottleneck, so DC runs at 25 kW to 150 kW and beyond. DC is what you want on a road trip or a quick top-up when you're actively waiting for the car. It costs more per unit because the hardware and power draw are expensive.

The simple rule

Charge on AC while you're parked for other reasons, and use DC only when you're waiting on the car to charge. Following that one rule keeps your costs down and your battery healthy — frequent very-high-power DC charging generates more heat and, over years, a little more wear than gentle AC charging.