EV Charging: AC vs DC chargers

Posted 5th April 2024

As the UK shifts towards a greater use of electric vehicles effective charging infrastructure must be in place. When looking at the range of charging options available to you, it’s important to understand the difference between AC and DC chargers.

AC and DC Electric Vehicle charging

For electric vehicles, we don’t talk about ‘fuel’ in the conventional sense, but as there is a difference between a car that uses diesel and a car that is powered by petrol, there are also two types of “fuel” for electric cars, AC (alternating current) and DC (direct current). AC power comes from the grid, whereas batteries store power as DC, which is what the battery in an electric vehicle will store. Most electronic devices that you use daily will have a converter built into the plug, meaning that every time you charge your phone or other device, the plug is converting AC into DC. 

Home chargers use AC whilst DC chargers are used for rapid charging. DC chargers are the perfect solution to charging fleets.

The reason DC chargers are faster comes down to the vehicle’s limitation on converting AC to DC. Whereas a DC charger negates the need for the vehicle’s inbuilt charging system to convert AC to DC, and so it can take more KW per hour direct from DC – thus massively improving the charge rate.

Charging your car with AC or DC energy does not change the characteristics of how this energy is used by the vehicle powertrain (Electric Motors), this ‘fuel’ is more around the rate in which the battery charges at. The battery will use the energy stored in the same matter, regardless of how it was charged.

What is the current UK EV charging infrastructure like?

The use of electric vehicles is gaining in popularity in the UK year-on-year, but for more people to drive electric vehicles on the roads there needs to be greater provision of electric vehicle charge points for both commercial EV charging and residential charging. The electric charging ecosystem certainly needs to develop, but this is where the difference between AC and DC becomes more apparent. In principle, it is a simple process for electric vehicles to recharge. Whereas a traditional car will stop at a petrol station to fill the car up with petrol or diesel, an electric vehicle needs to plug into a charging station to recharge the battery. This is the ‘refuelling’ process for an EV. Chargers have different power capabilities so it’s important to understand exactly what chargers your EV fleet needs in order to minimise downtime.

What is AC charging for electric vehicles?

AC (alternate current) is a flow of charge that periodically changes direction. It can also be transported over long distances. This is why most of the electricity grids in the world use AC power, and why it is the most common method to charge electrical items in your home and workplace. For electric vehicles, there is an onboard charger that converts AC to DC, feeding the energy into the battery inside the car. For electric vehicles this is the most common method used for the charging process, as most chargers will use AC power, which needs to be converted for the battery in the car. 

What is DC charging for electric vehicles?

Power that comes directly from the grid will always be AC. What this means is that there must be a conversion process for the power to charge the electric vehicle it is going into. The difference between the two types of charging is the location of the conversion, either inside the car or outside the car. A DC charger has the converter inside the charger itself, which means that it can take power and directly feed it into the battery inside the car without the need of the onboard charger to convert it (as mentioned above with AC charging for electric vehicles). DC chargers are much bigger, but also much faster and will likely be seen as a big breakthrough in the evolution of electric vehicles and the potential for EV charging networks. 

Where is AC charging and DC charging found?

Most electric vehicle charging stations that you will see use AC charging. This is most commonly at a charging speed of 22 kW, but this will depend on the power available to that particular charging infrastructure, as well as the car that is being charged.

AC charging locations

AC charging stations are currently the best option for charge points in the workplace and for individual use at home, because it allows for more time to load and to fully charge the vehicle. Electricity at home is usually provided using alternating current, and this is at a low speed of 7kW. If you use a home charger for your EV, the power will travel through a socket and into a converter in the car. This converter will then convert the electricity into DC to charge the car battery. 

DC charging locations

When it comes to DC charging, this is more common in public charging stations and alongside large road networks, where there isn’t as much time to recharge the EV battery and people want to be moving on to another destination as soon as possible. The evolution of DC charging in the home and workplace will offer new possibilities. 

How many public EV charge points are there in the UK? 

According to Zap Map, at the end of February 2024 there were 57,290 charging points for electric vehicles in the UK. These were spread across 32,575 unique locations. This is a 47% increase on the total number of charging devices in the year since February 2023. It is getting easier to find an EV charge point no matter what type of electric vehicle you have and what type of long journey you are doing. As more charge points are added to the UK EV infrastructure it will become easier to charge your vehicle wherever you are. 

Is AC charging a less efficient process than DC charging?

EV batteries store DC energy, and though drivers of an electric car can still use an AC charging point to power the car up and start driving, this is down to the converter in the car that allows AC electricity to be stored and used as DC in the vehicle’s battery. If you are planning a long journey, it is best that you consider whether you’ll be using AC or DC charging, as this will have an impact as to the length of time you need to recharge along the route. This is because the conversion process from AC energy to stored DC energy in the car battery leads to a slower charging rate than if you utilise DC charging only. 

Is DC charging always faster than AC?

The efficiency question above has been answered, and although in theory DC charging will always have the potential for higher charging speeds, this will vary though depending on a few different factors. This could be down to your make and model of electric vehicle and the type of charger that is being used. 

It is recommended to confirm the charge rates of your vehicle through your manufacturer’s vehicle manual, to identify the maximum rates on AC & DC Chargers. Note – different specifications of the same vehicle can have different charge rates.

The difference between AC and DC charging

The main difference is that AC power is what we get from the grid and this needs to be converted into DC power to be stored in batteries. This is the type of power that is stored in batteries for electric vehicles. The batteries both recharge and deliver energy to the motor(s) through DC energy.

 When you plug your car into a DC charging station, the conversion from AC electricity from the grid to DC power to be stored in the car battery, takes place within the charging station. This allows for DC power to flow directly from the charging station and into the car battery. As the conversion process happens inside the charging station, there is more space and more powerful converters can be used to convert the AC from the grid at a fast pace. This means that some DC stations can provide power over 400kW, which allows for an EV to be fully charged in minutes.

The difference between the two in simple form is that AC charging relies on the electric vehicle to do the hard work of charging the car, and DC charging is where the hard work is done outside of the vehicle, at the charge point, before the vehicle has even plugged in. This allows for more power to be delivered in one go and at a fast speed. 

What is an EV charging curve?

One of the other big differences between AC and DC charging is how the power output fluctuates during the charging session. There is an EV charging curve to consider. 

The AC charging curve

The AC charging curve is one where the flow of power to an EV is represented by a flat line. There is not much curve at all, due to the relatively small converter that is required on board. It can only handle a limited spread of power over longer periods, so there is less curve. 

The DC charging curve

For DC charging, there is the ability to deliver a much higher power, as it bypasses the onboard converter (which is much slower). Despite the higher power, the output has a decreasing charging curve. This is due to the point that the EV battery initially accepts a high flow of power, but it takes in less electricity as it heads towards its full capacity. Charging speed and power will vary depending on a few different factors, including the type of electric vehicle and the charging station being used. The reason the battery slows down it’s rate of charge as it nears capacity is to maximise the life of the battery.

Think of AC charging as going for a walk, no warmup or cooldown is needed. Whereas DC is like going for a run, it needs to warm up and cooldown – and thus the charge rate slows slightly during these periods. In most use cases this is within the 0-20% & 80-100% battery capacity, whereas the 20-80% range is when the battery is charging at its fastest rate on DC.

The cost of AC vs DC charging

It might seem like DC charging should be the first choice every time, due to the fact it is a faster charging process. However, it currently relies on equipment that is bulky and more expensive than AC charging, due to the requirement for a high-voltage (3-phase) connection to the power grid. This is why you do not currently find DC charging stations in domestic residential properties. The infrastructure and technology associated with DC charging will improve in future, and this will make it easier to charge electric vehicles in a greater number of locations, but as it currently stands, DC charging is best for commercial vehicles that are designed for long travel. The key for industries like haulage and logistics is that their fleets have minimal downtime making DC charging essential. 

AC charging on the other hand is perfect for day-to-day travel needs, where you can charge your car overnight or when at home or in the workplace for a few hours at a time.

 ElectrAssure provides an intelligent EV charging solution and Charge Point Management System for EV networks. Our team work with a wide range of industries making the switch to a fleet of electric vehicles. Our goal is to make this transition seamless and stress free. From an initial survey all the way through to installation and ongoing maintenance the ElectrAssure team is your perfect EV partner. If you are planning your move to electric charging infrastructure please contact our team for a no-obligation chat, we would love to guide you through the process.