AC vs DC charging is not just a choice between a slower charger and a faster charger. The way electricity reaches an electric car’s battery is different with each system, and the car itself can limit how much power it accepts.
A home wall charger may be rated at a certain power level, while the car may accept less. A public DC fast charger may advertise 150 kW or more, yet the car may never reach that figure. Battery temperature, battery charge level, the vehicle’s charging system and the charger’s available power can all change the actual charging rate.
That makes AC vs DC charging more useful to understand than simply looking at the number printed on a charging station.
AC vs DC charging starts with where the power is converted
The primary difference in how AC versus DC charging systems work, from a technical perspective, is that AC chargers convert the incoming electricity to DC prior to reaching your electric vehicle’s (EV) battery.
An electric car’s battery stores energy as DC, or direct current. With AC charging, the electricity enters the vehicle as AC and passes through the car’s onboard charger. That component converts AC electricity into DC power for the battery.
A DC fast charger performs that conversion outside the vehicle. The charger supplies DC power through the vehicle’s charging connection, allowing it to bypass the power limit of the onboard AC charger.
That difference explains much of the speed gap between AC vs DC charging.
The limitation of the onboard charger is a common mistake for many. Even though an electric car with a 7.4 kW onboard charger will be connected to a significantly more powerful AC charging unit, the car will not automatically charge faster than its onboard charger allows.
Why AC charging can be limited by the car
A common mistake when comparing AC vs DC charging is to look only at the charger.
Suppose an AC charging unit can provide 11 kW, but the car’s onboard charger is rated for 7 kW. The vehicle cannot suddenly take 11 kW just because the charging unit can supply it.
The same situation can happen with a more powerful AC installation. The charging equipment and electrical supply may be capable of delivering more power than the vehicle can use.
The amount of time it takes to charge an electric car depends in part on the capacity of the electric car’s on-board charger. Also, this is one reason why two different electric cars can be plugged into the same type of AC charging unit but may not charge at the same rate.
Finally, the size of the battery in an electric car is another factor. In general, larger batteries require more energy (and thus more time) to obtain the same percent increase in charge as smaller batteries.
Therefore, just knowing what level of power a charger has does not provide enough information to determine how much time a specific electric car would take to fully charge.
DC fast charging removes one major bottleneck
The main advantage in AC vs DC charging appears when the car needs a large amount of energy in a short period.
With DC fast charging, the charger converts grid electricity to DC before sending it to the vehicle. The car does not rely on its onboard AC charger for that conversion.
That allows much higher charging power.
Current DC fast-charging equipment can operate at power levels far above typical AC charging equipment. The U.S. Department of Energy’s Alternative Fuels Data Center lists DC fast chargers with outputs reaching hundreds of kilowatts, while AC Level 2 equipment can range up to 19.2 kW in the cited charging infrastructure data.
The important part is that the car has to be capable of accepting that power.
A 250 kW charging station does not turn a 100 kW-capable electric car into a 250 kW-capable car.
The charger rating is not the same as charging speed
This is probably the most useful point in AC vs DC charging.
When an electric vehicle has a battery that is 20% full and it pulls up to a 150 kW DC fast charger, then the vehicle may initially take a lot of power from the charger as long as the battery systems (battery cells), temperature, and charging rates are within limits. As the battery gets fuller, however, the amount of power taken by the battery will gradually decrease.
The electric vehicle’s charging system monitors and adjusts for the voltage, current, temperature, and “state-of-charge” (SOC) of the battery. The vehicle’s battery management system determines how much power the battery can safely handle.
So a charging session should not be judged only by its peak kW figure.
A car that reaches 150 kW for a short period may not receive 150 kW throughout the entire session.
Why DC charging slows down as the battery fills
The charging curve is another major part of AC vs DC charging.
When the battery has plenty of room for incoming energy, the vehicle can accept a higher charging rate. As the state of charge rises, the vehicle can reduce the power going into the battery.
That is why a DC fast charger can add a large amount of range relatively quickly at a lower battery percentage, while the final part of charging can take much longer.
The exact charging curve varies between vehicles and battery systems. There is no single charging speed that applies to every electric car.
This also changes how people should use AC vs DC charging during a road trip. A short DC session that adds enough energy to continue the journey can make more sense than waiting for the battery to reach 100 percent.
AC charging is better suited to long parking periods
The speed advantage of AC vs DC charging does not make DC the right choice for every charging session.
AC charging works well when the vehicle will remain parked for several hours. Home charging is a common example. The U.S. Department of Energy says most EV drivers charge at home using AC Level 1 or Level 2 equipment.
AC Level 1 provides relatively low charging power from a standard household outlet. AC Level 2 provides substantially more power and is commonly used for home, workplace and public charging. The Department of Energy lists roughly 10 to 30 miles of added range per hour for Level 2 in its consumer charging information, depending on the vehicle and setup.
That can be enough to replace the energy used during a normal day’s driving while the car is parked.
The point of AC vs DC charging is not that one system replaces the other. They serve different charging situations.
DC fast charging is built around shorter stops
The strongest case for DC in AC vs DC charging is when the vehicle needs to get back on the road quickly.
The Department of Energy estimates that DC fast charging can add roughly 100 to 200 or more miles of range in 30 minutes, though the actual result depends on the vehicle, charger and charging conditions.
That figure should not be treated as a promise for every car.
A vehicle with a smaller battery, lower DC charging limit or slower charging curve can add less range during the same period. A vehicle with a larger battery and higher charging capability may take greater advantage of a powerful DC station.
This is where AC vs DC charging becomes a vehicle-specific comparison rather than a simple charger comparison.
Battery temperature can change the result
Temperature also affects AC vs DC charging.
The battery has an operating temperature range in which charging can be managed safely and efficiently. When conditions are outside the preferred range, the vehicle may reduce charging power.
Some electric cars use battery thermal management to prepare the battery for charging. The exact system and behavior vary between models.
A cold battery can therefore charge differently from a battery that has reached a suitable operating temperature. The same charger can produce different results on different occasions.
The number shown on the charger is only one part of the equation.
AC vs DC charging also changes the charging experience
There is a practical difference beyond the electrical system.
AC charging usually involves leaving the car connected while it is parked for a longer period. That makes it suitable for places where the vehicle naturally stays for hours.
DC charging is designed around getting more energy into the battery during a shorter stop. Public DC stations are commonly positioned along major roads and in locations where vehicles need faster turnaround.
The equipment is also more substantial because high-power DC charging requires larger electrical infrastructure and additional equipment. The Department of Energy notes that DC charging can require more components, cooling and communications infrastructure than AC Level 2 charging.
Which one is faster?
In a direct AC vs DC charging comparison, DC fast charging is normally much faster.
AC charging is limited by the vehicle’s onboard charger and the available AC power. DC fast charging sends converted DC power directly toward the battery and can operate at much higher power levels.
But the highest charger rating does not automatically produce the shortest charging time.
A better comparison looks at four things:
- The charging power the vehicle can accept
- The charger’s available power
- The battery’s current state of charge
- The battery temperature and charging curve
These factors explain why two cars plugged into the same DC charger can show very different charging speeds.
AC vs DC charging for everyday electric car use
AC vs. DC Charging is often not a binary choice for many electric vehicle owners. AC charging may be used as the primary method to maintain the regular charging needs of the car while it is parked for extended amounts of time.
However, DC fast charging can be an effective way to quickly provide additional power if the car will only be parked for a short period of time. A driver with access to an electrical outlet at their residence may primarily utilize AC charging methods and utilize DC fast charging for longer road trips. A driver without easy access to residential electrical outlets would likely utilize public AC or DC charging stations to meet his/her daily charging requirements.
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The useful comparison is not simply which technology is faster. It is how much energy the car can accept, how long it will remain parked, and how much range needs to be added.
That is the real difference AC vs DC charging makes to an electric car.
When comparing electric cars, checking only the maximum DC charging figure can also give an incomplete picture. The AC onboard charger rating, usable battery capacity and charging curve can be just as relevant to the way the vehicle will be used.
AC vs DC charging is ultimately a question of where the AC-to-DC conversion happens, how much power the vehicle can accept, and how the battery manages that power during the charging session. DC fast charging has the clear speed advantage, but AC charging remains a major part of normal EV charging because most charging happens while the vehicle is parked for longer periods.
