AC vs. DC Charging

Understanding charging speeds, vehicle limitations, dwell times and site capacity is essential to building an EV charging strategy that works for your fleet.

As Canadian fleets continue to add electric vehicles, one of the most important infrastructure decisions is also one of the easiest to get wrong: choosing between AC Level 2 charging and DC fast charging.

The fastest charger is not necessarily the best charger for every fleet. In fact, installing more charging capacity than a fleet can use may add unnecessary equipment and electrical costs. Conversely, choosing slower charging for vehicles that need to get back on the road quickly can limit utilization and undermine the business case for electrification.

The pros and cons of AC and DC fast charging were explained in detail by Eduardo Guraieb, Senior Manager, Product Marketing for ChargePoint during a webinar hosted by the company earlier this year.

Start with the basics

Guraieb explained the fundamental difference between AC and DC charging. With AC charging, the vehicle’s onboard rectifier converts electricity from the charging station into DC power that can be stored in the battery. Because much of the conversion equipment is inside the vehicle, an AC charging station is relatively simple and inexpensive.

DC fast charging moves that conversion equipment out of the vehicle and into the charging station. The station converts AC electricity to DC and sends it directly to the battery, bypassing the vehicle’s onboard charger.

That allows much higher charging rates, but it also makes DC charging equipment larger, more complex and more demanding from an electrical infrastructure standpoint.

"The fastest charger is not necessarily the best charger for every fleet."

The vehicle is part of the equation

A common mistake is to only look at the charger’s maximum output when planning an EV charging installation. Charging speed is ultimately limited by the lower of two values: what the charger can deliver, and what the vehicle can accept.

For example, a 400-kW DC charger cannot force 400 kW into a vehicle capable of accepting only 150 kW. The vehicle remains the limiting factor.

The same principle applies to AC charging. A Level 2 station capable of delivering 19.2 kW will not necessarily provide 19.2 kW to every vehicle. Some EVs may only be able to accept a fraction of that, Guraieb explained.

Where AC charging makes sense

For many fleets, Level 2 AC charging will remain the workhorse. AC stations can typically deliver roughly 7.2 to 19.2 kW, depending on the equipment and electrical supply. That may not sound like much compared with the output of a DC fast charger, but it can be more than adequate when vehicles spend several hours parked.

Consider a typical workplace fleet. If vehicles arrive in the afternoon and remain parked until the following morning, there is little value in paying a premium for extremely rapid charging. The same applies to fleet vehicles that return to a depot after a shift and remain there overnight.

The economics are another advantage. Because the charging station does not perform the AC-to-DC conversion, AC equipment is generally less complex and can require significantly less electrical infrastructure than DC fast charging.

When DC fast charging earns its keep

DC fast charging becomes more attractive when vehicles cannot afford to sit still. Fleet depots that require mid-route top-ups are an obvious example. If a vehicle arrives with a depleted battery and needs to return to service quickly, reducing charging time can have a direct operational benefit.

Modern DC fast chargers can provide substantially more power than Level 2 equipment, with some systems capable of hundreds of kilowatts. At the high end, charging can add hundreds of kilometres of potential range in a relatively short stop.

However, this boost in performance comes with additional infrastructure requirements. A fleet may need a larger transformer, upgraded electrical service, new panels, trenching and other site work. DC equipment also generally costs more to install and maintain and can increase electricity demand.

Don't overlook the charging curve

Even when a vehicle and charger are capable of high charging rates, the vehicle will not necessarily charge at that rate from zero to 100%.

EV batteries use a charging curve. A vehicle may accept a lot of power when its battery is relatively empty, but that power generally decreases as the battery approaches a high state of charge.

The reduction becomes particularly noticeable as the battery approaches roughly 80%. For fleet operations, this means charging from 20-80% can often be much more time-efficient than trying to charge every vehicle to 100% during a shorter stop.

Questions to consider

Before selecting charging equipment, fleet managers should answer three basic questions.

First: How much electrical capacity is available? Determine what the existing electrical service, panels and transformer can support. A fleet may want a high-powered DC charger, but that does not mean the site can accommodate it without a significant (read: costly) upgrade.

Second: How long will vehicles be parked? Dwell time is arguably the most important factor. Vehicles parked for two hours or more may be well suited to Level 2 charging. Vehicles that need to replenish significant amounts of energy in less than an hour may require DC fast charging.

Third: What is the business objective? The goal shouldn’t simply be to install the fastest possible charger. The objective might be to minimize infrastructure costs, maximize the number of vehicles that can charge simultaneously, improve vehicle utilization or enable additional routes.

The right solution could also be a combination of technologies, with Level 2 chargers used for vehicles that are parked overnight, and DC fast chargers used for vehicles that need a midday top-up.

Planning for the future

One final consideration is the lifespan of charging infrastructure. EV technology is changing rapidly, and vehicles entering the market several years from now may have significantly higher charging capabilities than today's models. Installing infrastructure that exactly matches today's vehicles could leave a fleet with an undersized system later.

Ultimately, successful fleet electrification is not about having the biggest or fastest charger. The best charging strategy is the one that balances vehicle requirements, dwell time, electrical capacity, installation costs and future fleet needs. For Canadian fleets navigating the transition to electric vehicles, getting that balance right can be just as important as choosing the vehicles themselves.