EV charging: the difference between AC and DC

The EV charging ecosystem

Electric mobility in the UK is more popular than ever, and growing alongside it is the EV charging ecosystem. On paper, the concept is simple: instead of filling up a tank with fuel at the petrol stations, EV's plug into a charging station to top up their batteries.

In practice, however, not all chargers are created equal. EV charging stations come in many different shapes, sizes, and use cases. One of the main factors that differentiate them, however, is the type of current they use.

Difference between AC and DC

Before getting technical, there are two things you should keep in mind:

• The power that comes from the grid, i.e., your domestic socket, is always AC (alternating current).

  
  

• The energy stored in batteries is always DC (direct current).

  
  

AC and DC are two entirely different types of electrical current. Both travel in different directions, flow at different speeds, and have different applications. The hard rock band AC/DC, despite having an album titled “High Voltage,” have nothing to do with electrical currents or EV charging.

AC charging

AC is an electrical current, or flow of charge, that periodically changes direction, i.e., it alternates. AC power can be generated from renewable sources that use rotating generators, such as wind or hydropower turbines. AC can also be efficiently transported over long distances—which is why virtually all of the world’s electricity grids use AC power, and why you can find AC power flowing from the sockets in your home and office.

DC charging

DC always moves in a straight line and can be generated by renewable power technologies such as solar panels. Among other things, DC can be used for energy storage, powering electronics, and LED lighting. Batteries store DC power, and though you may have never realised it, every time you charge your laptop, the charger converts the AC power from the grid into DC power for your laptop’s battery.

In short, we get AC power from the grid and this is converted into DC power so it can be stored in batteries, such as the one used to power an EV.

What is the difference between AC and DC charging?

When we talk about charging an EV, the main difference between AC and DC charging is where the conversion from AC to DC happens. No matter whether an EV uses an AC or DC charging station, the car’s battery will still always store DC energy.

When you use a DC charging station, the conversion from AC (from the grid) to DC happens within the charging station—allowing DC power to flow directly from the station and into the battery. Because the conversion process happens inside the more spacious charging station and not the EV, more powerful converters can be used to convert AC from the grid very quickly. As a result, some DC stations can provide up to 400 kW of power and can fully charge an EV in minutes.

You might be thinking that DC charging might be bad for your battery, rest assured, it's not. We debunked this common myth here.

EV charging curves

Another key difference between AC and DC charging is how their power output fluctuates over the course of a charging session, in other words, the EV charging curve.

AC charging curve

With AC charging, the power flowing to an EV represents a flat line (so, not much of a curve at all). This is due to the relatively small onboard converter that can only handle a limited power spread over longer periods.

DC charging curve

By bypassing the car’s slower onboard converter, DC charging, on the other hand, can deliver much higher power, but its output forms a decreasing charging curve. This is due to the EV's battery initially accepting a high flow of power but gradually taking in less as it reaches full capacity.

As an example, imagine a glass as the EV’s battery, a water bottle as a DC charging station, and the water inside that bottle as the power. At first, you can quickly fill the glass with water, but you’ll need to slow down as you get to the top, so the glass doesn’t overflow.

The same logic can be applied for DC rapid and ultra-rapid charging. This is why EV's require less power once the battery is around 80 per cent full, hence the decreasing power output you see below.

Of course, charging speed and power can vary based on a number of other factors If you’re looking to know more about the factors that influence EV charging times, have a look at our in-depth article on the topic.

AC vs DC charging

By now, you probably have a sense that AC and DC charging work quite differently and serve different roles. You might be thinking that since DC charging is faster, it should be your preferred option every time.

While DC charging is fast, it also relies on bulky and expensive equipment, and requires a high-voltage connection to the power grid, which makes it impossible to install at home.

Because of this high upfront cost, you'll likely find that DC charging station operators will pass this cost down to you by setting higher prices than an AC charger would. Not to mention that even the slowest dedicated AC charging station will fill up a medium-sized EV overnight, so you probably won’t need to use DC charging for your day-to-day travels.

Where DC charging shines is for quick top-ups on long trips. Depending on your car’s charging capacity and the station’s power output, a DC charger can charge up your EV in under an hour, allowing you to quickly resume your journey.