How to mix AC and DC charging stations?

In the last article, we explored the difference between AC and DC charging.

We concluded that a small number of fleet operators would benefit from implementing all AC chargers or all DC chargers, but most companies need a mixture of both.

In this article, we’ll quickly refresh your memory on AC and DC chargers, and the pros and cons of each, before moving on to investigate how to implement a mix of the two effectively.

What is the difference between AC and DC charging?

Alternating Current (AC) is the standard electricity that comes out of power stations. It travels along power lines to homes and other buildings, sometimes referred to as mains or utility power. It is described as “alternating” as the current flow changes direction regularly.

Direct current (DC), as the name suggests, is an electric current that moves in a straight line.

Vehicle batteries need DC power. In other words, there is no way you can store AC power in a battery.

As DC can charge faster than DC chargers, it is often wrongly assumed that DC chargers are better. Actually, the truth is that AC chargers are usually a lot cheaper and easier to install.

High power DC chargers sometimes even require specific security measures due to their value.

For some use-cases, the best approach is to mix AC and DC charging.

In this article, we seek to answer the following questions:

• Why does mixing of AC and DC charging make sense?
• Which use-cases are ideal for a mix of AC and DC?
• What are the potential problems when mixing charging stations?
• How can intelligent software help to solve these problems?

When do we use AC chargers and when DC?

Some charging networks offer only AC charging, while others offer DC only. So, when is it preferable to mix AC and DC chargers?

Electric vehicle batteries need DC power to charge. As it stands, there is no way to charge batteries with AC power directly. The problem is that the grid only provides AC power. Consequently, there is a mismatch between the type of power the grid provides and what EV batteries need.

That is easily solved with an onboard converter situated in the vehicle that takes the incoming AC power and converts it to DC power to charge the vehicle. If you want to charge a Tesla vehicle within 12 hours overnight, this AC charger will work perfectly.

Problems arise when you are short of time. Let’s assume you are running out of energy on the highway, and you’re only halfway to your destination. You simply don’t have 12 hours to recharge your EV battery. You need to be able to charge a decent amount within roughly 30 minutes. AC can undoubtedly deliver some charge to your battery through a built-in DC converter, but it will only convert approximately 11–20 kW. That means for a 50% charge; you need to wait nearly 3 hours.

The solution to this problem is a DC charger. The DC charger pre-converts the AC grid power to DC power. This means that the vehicle’s input is already DC power that can be sent directly to the battery.

When should we mix AC and DC chargers?

If AC is good for long stay-times and DC is better for short stay-times, why do we need to mix?

The answer is simple — when you have some vehicles with long stay-time and some with short stay-time. This situation occurs at certain public locations such as airports, train stations, for example. The other places where it frequently occurs are fleet depots for buses or delivery vehicles.

You cannot simply install a large number of DC chargers, as they are very costly, and your utility provider won’t allow such a high power installation.

And you cannot rely solely on AC chargers, as they are not rapid or flexible enough.

Let’s take a look at an everyday use case for mixed chargers.

Imagine that we have a taxi fleet that runs two primary operations. The first one operates 24 hours per day as it connects essential city areas. The second one operates during the daytime only as it is mainly used by commuters to and from train stations and other transport hubs.

The commuter taxis will stand for 10–12 hours at the depot, allowing for longer charging times. The 24-hour taxis only stand during shift changes for a short period, twice per day.

In some cases of high usage, the commuter taxis also come back during the day to charge during the lunch break.

The question here is — when should the driver charge their vehicle at the AC charger, and when should they use a DC charger?

Secondly — how many DC chargers and AC chargers does this depot need?

To answer these questions, we need to examine the fleet energy requirements and how the fleet operates closely.

What problems arise when mixing AC and DC chargers?

Problem 1 — Drivers struggle to make the right charging decisions

When a fleet driver is faced with a choice between several charging stations, both AC and DC, how do they decide which is the best for them?

To make the best decision, they need to plan their arrival and departure times carefully.

Is it possible for drivers to do this accurately? Almost certainly not.

A mix of charging stations makes it very difficult, or even impossible, for drivers to make the correct charging decisions.

Similar to vehicle maintenance and cleaning. Ideally, a central fleet manager will coordinate and monitor activities in the depot, including making charging decisions.

Problem 2 — Nearly-full vehicles block charging stations and reduce throughput

Vehicles with full or near-fully charged batteries are harming the throughput of your fleet depot.

The obvious step is to unplug vehicles once they are charged at 100%. But did you know that your vehicles behave differently after hitting a battery charge level of 60–80%? The charging will slow down.

In other words, a vehicle plugged into a DC fast charger of 100 kW may charge at 10 kW only. The vehicle may block the charger for one more hour, although it is already at a 90% battery level.

Problem 3 — Power management and fleet management are not connected

Typically, you have an automated fleet management system (e.g. routing system) that coordinates maintenance, routes, and maybe drivers.

The switch to electric vehicles means you’ll likely consider getting a load management system to manage the charging stations.

The problem is — your load management system doesn’t know what your fleet management does and vice versa.

So what should you do?

A quick fix would be a fleet manager who looks at two screens and tries to talk to the drivers simultaneously. That isn’t sustainable. Sooner or later, the manager will face a situation where the vehicles will not be ready, and drivers are sitting around waiting. That’s a disaster of apocalyptic proportions for fleets!

Uncoordinated charging and missing intelligent connections between fleet operation and energy management are highly damaging to the survival of any fleet company.

How can smart charging software help to mix AC and DC charging?

At Ampcontrol, we’ve taken a closer look at these problems. The solution is an intelligent software system that makes decisions for the electric fleet in real-time.

Imagine that every time a vehicle arrives at the depot, a driver plugs in, or the charging station sends new meter values, everything is incorporated into your fleet management system.

At the same time, imagine that every time you adapt the fleet schedule and driver allocation, it is instantly taken into account for EV charging.

The number of errors will decrease massively. Even in the case of shortage due to high fleet utilization, you’ll be able to predict this hour before it actually happens.

You can react before it actually happens.

Intelligent software systems for electric fleets are still relatively rare in the industry but will become the standard in a few years. There is nearly no limit to what you can achieve and how far you can optimize your charging.

Once you have expanded your fleet and need new charging stations, the same software will help identify how many chargers you need and how to mix AC and DC chargers efficiently.

Conclusion

Mixing AC and DC chargers is an essential strategy for many fleet businesses, especially those that have EVs with different stay-time requirements.

The most effective way to mix AC and DC chargers is by applying intelligent charging software such as Ampcontrol.

To find out more about how Ampcontrol can optimize your AC and DC fleet charging operations, get in touch today.