Direct current (DC) fast charging is critical for long-distance electric vehicle (EV) travel and for the continued growth of EV adoption, but how does it work?
EV batteries store what’s known as DC power, while the electric grid supplies alternating current (AC) power. When you use Level 1 or Level 2 charging, your EV receives AC power that must be converted to DC before it can be stored in your car’s battery. To do this, your EV has an onboard charger.
DC fast charging, though — as its name implies — provides DC power straight to your EV’s battery; the AC-to-DC conversion happens in the charging station before the electrons enter your vehicle. That’s why DC fast charging is able to provide a much faster charge than Level 1 or Level 2 charging.
There are currently three DC fast charging connectors used in the U.S.: CHAdeMO, CCS (Combined Charging System) and Tesla Supercharger. Your brand of EV will determine which connector you can use, though in some cases adapters are available that allow charging with a different plug.
A given DC fast charging station may have only a single connector (Tesla Superchargers only offer Tesla’s proprietary plug, for example) or multiple, such as a station with both CHAdeMO and CCS connectors (think of a gas pump with hoses for both gasoline and diesel fuel). CHAdeMO, however, is becoming less popular among manufacturers in the North American market.
Below, we go into detail about a few additional aspects of DC fast charging.
Pricing for public DC fast charging will vary based on the location, station and network provider (e.g., ChargePoint, Electrify America, EVgo), among other factors. (Because of its power requirements, DC fast charging is not suitable for residential settings.) As an example, charging at an Electrify America station will cost 31 cents or 43 cents per kilowatt-hour (kWh), depending on membership. A ChargePoint DC fast charger in Tarboro, North Carolina, costs 27 cents per kWh.
To put this in more concrete terms, let’s say your EV has an efficiency of 3.5 miles per kWh (or 28.5 kWh per 100 miles) and you stop to charge at a station with one of the pricing structures mentioned above. You add 50 kWh — good for 175 miles of range — and will be paying between $13.50 and $21.50. In contrast, filling up a gas car that gets 30 mpg at a price of $3.10 per gallon will cost approximately $18 for those same 175 miles.
Other charging station providers base their pricing on time spent charging. EVgo, for instance, has rates in North Carolina between 24 and 30 cents per minute. Per-kWh charging is generally considered more equitable, however, since different EVs charge at different speeds (see discussion below), meaning some vehicles will receive more electrons for the same price.
DC fast charging is more expensive than charging at home (which in North Carolina is the equivalent of paying less than a dollar per gallon of gasoline) for a few reasons. One is that there are more infrastructure and capital expenses associated with installing and maintaining the energy-intensive equipment. Site hosts — the organizations that host a charging station on their property — may also be on the hook for demand charges, which are fees added by power companies that are based on the highest amount of power drawn in a given period.
When charging, your EV is in constant communication with the DC fast charging station to determine how much power to draw. Ultimately, how fast your EV charges is influenced by several variables, including the ambient temperature (extremes are generally worse), battery temperature (a warm battery will be able to accept the most power), the current state of charge of the battery (lower is better), the charging rate of the station and the acceptance rate of your vehicle. We’ll primarily focus on the latter two variables in this article.
A DC fast charging station’s charging rate is measured as its maximum output in kilowatts (kW). For light-duty vehicles, you’ll find stations ranging from 50 kW to as high as 350 kW, and generally speaking, the lower the kW, the slower the charge. However, choosing a higher-powered DC fast charger over a lower-powered one does not guarantee that you’ll charge more quickly. That’s where your EV’s acceptance rate comes into play.
An EV’s acceptance rate is the maximum amount of power it can take, also measured in kW. Consider the Chevy Bolt EV. The Bolt EV has an acceptance rate of 55 kW (considered quite slow today). That means that its peak charging rate would be approximately the same whether you stopped at a DC fast charger rated at 62.5 kW, 150 kW or 350 kW. The inverse is true as well. Even though a Porsche Taycan can charge at up to 270 kW (its acceptance rate), if you plugged in at a station rated at 150 kW, the vehicle wouldn’t be able to reach its peak.
Here are the acceptance rates of some other EVs:
Another important concept related to an EV’s charging speed is its DC fast charging curve. Every EV model has its own charging curve, which is basically how much power it pulls (and how many miles it adds) over time as it charges. Knowing your EV’s charging curve can be a big help, especially on longer trips, when deciding for how long to stay at a station. In many cases, it’s smarter to hop between chargers — stopping only briefly at each one — rather than lingering for a prolonged period at a given location.
Typically, an EV will charge at its maximum rate for only part of a charging session, usually in the lower half of the battery pack. For how long specifically depends on the vehicle. The one general constant across charging curves is a ramp down of charging speed at approximately 80% charge, which occurs to protect the battery.
Let’s take a look at the charging curve below from InsideEVs for the Volkswagen ID.4. The vertical axis shows the power being drawn by the EV in kW, and the horizontal axis is the current percent charge of the vehicle’s battery.
Almost immediately, the ID.4 reaches its acceptance (maximum) rate of 125 kW, and it holds that rate until it charges to approximately 30%. At that point, it begins to steadily draw less power, pulling 100 kW at 45% and 80 kW at around 60%. A steeper drop-off occurs when the ID.4 hits about 80% charge.
Another way to visualize this same charging session is by time. You can see that it takes about 20 minutes for the ID.4 to go from 0 to 50% (adding more than 100 miles of range), another 20 minutes to move from 50 to 80%, and then 25 minutes to add that final 20%.
For comparison, below are charging curves from InsideEVs for the Tesla Model 3 and Ford Mustang Mach-E (note the different kW values on the vertical axis). Out of Spec Reviews’ YouTube channel also contains great videos on the charging curves of various EVs.
Keep in mind that acceptance rates and charging curves don’t automatically go hand in hand. In other words, an EV with a higher acceptance rate does not necessarily have a “better” or “more desirable” charging curve. So, it is important to do your research on the topic, particularly thinking about your long-distance routes and needs, before making a purchase decision.
Most EVs have battery management systems to try to blunt any negative effects of DC fast charging on their battery. However, compared to Level 1 and Level 2 charging, DC fast charging does put more strain on batteries in the form of heat buildup.
The jury is still out on how much repeated DC fast charging will impact the health of your battery, but it is generally recommended to avoid using it exclusively. If you have access to and time to use slower charging, it’s probably safer than solely relying on DC fast charging. EV charging is an ecosystem, and charging levels should be selected based on your driving needs for that day. Another reason to save DC fast charging for only when you truly need it is to leave stations open for drivers who might be in a bind.