EV charging stations are becoming more commonplace. Most charging stations you see are AC chargers. Until recently, AC chargers have been the only option.
But in recent years, companies like EvGo or Ionity have developed a new technology known as DC Fast Chargers. Companies can now apply a different type of technology to charge vehicles faster than the standard AC chargers.
The proportion of DC fast chargers to AC chargers is growing. But for many people, the question remains AC vs DC EV charging, which charging station should be used for fleet vehicles? Should companies with depots install AC chargers or DC chargers? That’s the question we’ll seek to answer in this article.
Learn more about energy management and smart charging by downloading our report, "Energy Management 101: How to Efficiently Charge Electric Fleets".
AC vs DC Current: What Is the Difference?
What is AC Charging for Electric Vehicles
Alternating current (AC) power can be described simply as the standard electricity that comes out of power stations and travels along power lines to homes and other buildings, sometimes referred to as mains or utility power.
What is DC Charging for Electric Vehicles
Direct current (DC) power, as the name suggests, is the electric current that moves in a straight line.
Direct current can come from multiple sources, including batteries, solar cells, fuel cells, petrol generators, and some modified alternators. DC power can also be "made" from AC power by using a rectifier that converts AC to DC.
Are Car Batteries AC or DC?
Batteries need DC power. And there is a simple reason. Just like any other battery - a car battery uses and provides DC power. This means the load flows in one single direction and is stored using a chemical process.
As mentioned, AC current changes direction periodically. To store AC, a battery’s terminal needs to change polarity (+ and -) at the same speed, which is not possible. Connecting an AC supply to a battery to store AC power means the battery charges only during the positive half-cycle and then discharges during the negative half cycle.
In other words, there is no way a car can store AC power in a battery.
Differences between AC vs DC EV Chargers
Your outlet delivers AC power, and your vehicle battery needs DC power.
The only way to achieve this is to convert AC power into a DC power output. This is what we call an AC to DC converter (AC-DC converter).
AC EV Chargers require you to build this converter into your vehicle. This means that the vehicle has its own small AC-DC converter. So, the vehicle receives AC power from the charge point, then the vehicle converts it to DC power.
DC EV Chargers require you to build this converter into your charge point. This means the chargers have their own built-in AC-DC converter, and the vehicle receives DC power directly. No conversion is required inside the vehicle.
EV Charging Curves
The key difference between AC and DC charging curves lies in how much power the battery can accept over time.
AC Charging Curve
AC chargers rely on your car's built-in converter, which typically handles a constant power level. This translates to a flat charging curve on a graph. The car steadily charges the battery at a set rate until full.
DC Charging Curve
DC fast chargers have a more powerful converter and deliver DC power directly to the battery. However, for battery health reasons, the ideal charging rate isn't constant. DC charging curves typically start high, allowing for rapid initial charging. As the battery fills up, the curve slopes downward, reducing the power input to prevent overheating. This creates a decreasing, curved line on a graph.
Why is DC Charging Faster than AC?
The faster you want to charge a battery - the more power you need to provide. Fast charging is usually above 50 kW, and slow charging typically between 1-22 kW to provide more power when charging a battery, you need a much larger AC-DC converter.
The problem is - converting high power from AC and DC is expensive. A large converter easily costs USD 10,000.
This high-power charging is best carried out with the converters built into the charging station rather than the vehicles so you’re not dragging around heavy and expensive converters dragged around with you in your car.
That’s the main reason why DC chargers appear to be faster than AC chargers. They aren’t really any faster; it’s just much easier and cheaper to generate high-power DC output within the charger rather than convert the output from an AC charger in the vehicle itself.
When do I Choose AC EV Charging?
AC charging is best for regular charging needs such as:
- Home
- Work
- Errands
When do I Choose DC EV Charging?
DC chargers cost a lot more than AC chargers. DC chargers require more space and require much more complex spare parts to facilitate processes such as active cooling.
In addition, high power charging requires a high power connection to the grid. With a typical residential grid connection, you won’t be able to install a DC charger.
DC fast charging is best for situations where time is tight such as:
- Road Trips
- Low Battery Emergencies
- Fleet Vehicles
The rule of thumb is: Install DC chargers when you need fast charging and install AC chargers when you don’t.
Conclusion
DC fast chargers are ideal for charging EVs that need to be back on the road quickly. But they are more expensive to install and maintain than AC chargers.
If time is of the essence for your fleet, then DC chargers are going to be the best option.
If you have a mixed fleet of vehicles - i.e., some that need fast charging and some that don’t, then a mixture of AC and DC chargers is best.
Either way, smart charging software for electric vehicles can help fleet managers to offset some or all of the extra initial outlay by reducing ongoing energy costs.
Read more about AC and DC charging here: How to mix AC and DC charging stations?
