There’s no doubt that greenhouse gas (GHG) emissions are a big problem in the modern world. They have been linked to a general reduction in air quality and climate change.
Transportation is one of the biggest culprits. Approximately 30 to 40 % of all greenhouse gas emissions in large cities are caused by combustion engine vehicles.
30 to 40 % of all greenhouse gas emissions in large cities are caused by combustion engine vehicles
That’s why science advisors and governments around the world are pushing for the adoption of electric vehicles (EVs). By increasing the use of electric vehicles and EV chargers, governments will reduce harmful emissions and achieve their transportation decarbonization goals.
As a result, 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 Office of Energy Efficiency and Renewable Energy states that around 16% of the 70,000 charging stations in the U.S. are DC fast chargers (DCFC). In Europe, we see that around 11% of all chargers are DCFC.
The proportion of DCFC to AC chargers is growing. There are new state initiatives and incentives to promote DC fast chargers. Utility providers are being incentivized to help with the installation of DCFC chargers.But for many people, the question remains - 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.
What is the difference between AC and DC EV chargers?
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.
Direct current (DC) power, as the name suggests, is the electric current that moves in a straight line.
As stated in our last article AC and DC current were invented by Thomas Edison and Nikola Tesla. And believe it or not, they have never been best friends...
How does a vehicle battery work?
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.
In technical terms: 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 you can store AC power in a battery.
There is no way you can store AC power in a battery.
Differences between AC and DC 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).
You have two options:
A) you build this converter into your vehicle,
B) you build this converter into your charge point.
AC chargers use option A. 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 chargers are option B. 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.
Why are DC chargers so much faster?
This is an important question.
Let’s look at things logically...
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.
Therefore, 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.
It’s pretty obvious that you don’t want heavy and expensive converters dragged around with you in your car. So, high-power charging is best carried out with the converters built into the charging station rather than the vehicle.
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.
Are DC chargers always better?
It would appear that DC chargers are a much more sensible and efficient option then, right?
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.
Also, trying to install ten 150 kW chargers is a complex and expensive project. On the other hand, installing ten 11 kW chargers is fairly simple and more affordable.
The rule of thumb is: Install DC chargers when you need fast charging and install AC chargers when you don’t.
What are the best-charging stations for fleet vehicles?
For the rest of this article, we’ll focus on depot EV charging.
For the purposes of this article, we won’t consider employee fleets that charge at more than one depot, as this will make the analysis more complicated.
For traditional single depot charging, we’ll study the first major use-cases: overnight depot for delivery vehicles
We’ll take a look at their daily operations and how AC or DC charging would work in this cases. You’ll see big differences between them.
For each use-case, we’ll apply the following figures:
Number of vehicles: 100
Stay-time/dwell time: 10-12 hours (depot, night-time)
Grid capacity: 500 kW
Here is how vehicles are arriving at the depot:
DC charging only
We used our software to simulate how the fleet would charge with DC chargers only. We considered 10 x DC chargers with 500 kW each. Due to the power limit at the location, we can't install more charge points.
The simulation shows that
- The DC chargers are fully active the entire night
- Not all vehicles are able to charge
- Some vehicle charged but couldn’t reach a full charge on-time
One main reason for that result is that batteries will not charge at 50 kW DC power the entire time. Without going into details of that problem, the charging power will drop to 10 kW and later 5 kW (depending on the vehicle). This is called constant voltage phase (CV phase)
AC charging only
For the second simulation, we considered a different setup. We equip the location with 100 AC chargers and use intelligent load management from Ampcontrol to optimize the utilization.
The simulation shows that:
- the AC chargers could charge even more vehicles
- all vehicles are able to charge
- all vehicles receive a full charge
Clearly, this scenario shows that AC charging would be sufficient for the fleet depot. But that really depends on the scenario and there is no general answer.
AC charging is capable of charging the vehicles within the longer dwell times and fully guarantees on-time departure.
DC charging would also do the job. DC chargers could charge the vehicles within 2 hours or less. However, you’ll need to keep in mind that batteries can’t charge a full-power for the entire cycle.
Also, charging too many vehicles at DC power would cause a surge in power demand.
The good news is that by applying smart charging software such as Ampcontrol, fleet managers can ensure that all-electric vehicles are ready on time. The software coordinates vehicles and charging stations at the fleet depot.
Mixing AC and DC charging
Let’s say that we have a few vehicles that need fast charging and need to leave the depot within 2 hours. That requires DC fast chargers. However, the majority of vehicles stay up to 12 hours at the depot.
Do we need to install DC fast chargers only? No, we can mix AC and DC chargers intelligently to benefit from both.
How? We allocate the vehicles to the right charging stations and manage the power that goes to each charge point. For instance, high power can be delivered to DC chargers if the vehicle needs to leave sooner. Normal power can be delivered with AC chargers if the vehicle stays more than 9 hours.
These use cases truly benefit from smart charging software for electric vehicles.
Operating these mixed installations for fleet vehicles without smart charging software is next to impossible as it requires constant monitoring of power, drivers, time, operation, and staff.
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?