How Does DC Fast Charging Work?


DC Fast Charging (DCFC) systems convert the current from AC to DC outside the car and feed it directly to the EVs battery.

This allows electrical power of up to 350kW to be input into the battery and speeds up charging time by a multiple of 12. The key things to remember about how fast charging works are:

At a Glance:
1️⃣ DCFC charge rate is up to 12 times faster than level 2 charging.

2️⃣ Conventional wisdom states that DCFC chargers should not be used regularly; however, recent studies dispute this.

3️⃣ DCFCs only supply the charge level dictated by the EVs acceptance rate.

4️⃣ DCFCs have limitations; however, DC fast charging is essential when going on a long trip as it is the only acceptable charging solution.

To fully understand how DC fast charging works, it’s essential to know what DC fast charging is. The scope of this article is a detailed description of what DCFC systems are.

Key Parts Of a DC Fast Charger (DCFC)

A DC Fast Charging system converts the AC current to DC outside of the EV. It is not restricted by the limitations of the EVs own rectifier, which due to cost, size, and weight restrictions, is substantially smaller.

The result is that the charging speed is from a maximum of 22kW to 350kW (charging station dependent).

DC Fast Charging systems are capable of charging at a substantially faster rate, as shown below:
🎓 Source: US Department of Transportation)

Table 1: Level One Charging

DescriptionLevel One
Input120V AC 12 (16 amps)
Typical Output1kW
Estimated PHEV Charge Time5 – 6 hours
Estimated BEV Charge Time40 – 50 hours
Charger Cost+-$100
Installation CostFree
Typical Range per Hour of Charging2 – 5 miles

Table 2: Level Two Charging

DescriptionLevel Two
Input208 – 240V (80 amps)
Typical Output7kW to 19kW
Estimated PHEV Charge Time1 – 2 hours
Estimated BEV Charge Time3 – 8 hours
Charger Cost$300 to $3,500
Installation CostFree (for plug-in units) to a couple of thousand dollars if the premises requires an upgraded electrical system.
Typical Range per Hour of Charging10 – 20 miles
UsesHome, Workplace, and Public

Table 3: DCFC Charging

Input200 – 500VDC 350 amps
Typical Output50 to 350kW
Estimated PHEV Charge TimeMany DCFC systems won’t service a PHEV
Estimated BEV Charge Time20 minutes – 1 hour
Charger Cost$10,000-$40,000
Installation CostDepending on how far away the electrical supply is and if there is sufficient capacity, the prices range from$4,000 to $51,000.
Typical Range per Hour of Charging180 – 240 miles

The Key Components Of A DCFC System

DCFCs use a 3-phase 480VAC circuit that delivers the vehicle’s direct current (known as DC) rated to 350kW. The parts of a DCFC system in America are as follows:

  • The utility power supply
  • The DC Fast Charging station mounted on a pedestal or wall.
  • An AC-to-DC rectifier and a DC-to-DC converter
  • DC car supply system
  • Charge controller
  • A payment system
  • The charging cord includes a cable management system (coil, retractable, etc.)
  • The connector at the end of the charging cord

The Fast DC charging process is described below

  1. The payment is made through a credit card or phone.
  2. The EV’s system communicates with the charge controller setting the EVs acceptance rate.
  3. The AC current passes through the “AC to DC” rectifier and converts AC power to DC.
  4. The DC current is stepped down to the level that the EV requires.
  5. The EV advises the state of charge and stops accepting once the battery is at the desired level.

Types of DC Fast Charging Connectors

There are four standards of DC fast charging connectors used:

  • CCS Combined Charging System Coupler
  • Tesla Connector.
  • CHAdeMO (Japanese EVs)
  • GP/T (Chinese cars)

The CCS and the Tesla connectors are primarily used in America. 

Previously non-Tesla vehicles could not use a Tesla supercharger; however, the company has recently agreed with the Federal government to begin removing the restrictions. Shortly, all cars will be able to recharge at Tesla superchargers.

AC And DC Fast Charging

In 2001 the J1772 connector was developed as part of California’s requirement to have a standard charging interface for all EVs.

In 2010 it was modified into the present design. The J1772 is standardized for Level 1 and Level 2 chargers (AC) and is one of four possible DCFCs (DC) connectors. 

Tesla models can use the J1772 plug by using an SAE J1772 Charging Adapter. Comparing AC and DC charging provides a more comprehensive understanding of how DC fast charging works.

Limitations Of DCFC

DC Fast Charging units provide the fastest charge and are ideal when positioned on major routes enabling EVs to charge quickly.

But there are several limitations or challenges with the technology that constrain them.

DCFC Systems Are Expensive To Install

Fast DC Charging Stations require 200 – 500VDC and up to 350 amps or input current. This means they need to be near a capable electrical power source, failing which, one must be installed.

You Can’t Use DC Fast Charging Continually

Manufacturers state that using a DCFC to charge an EV regularly can significantly reduce the battery’s life expectancy, which will result in its early degradation.

DCFSs Are Limited To 350 – 400 kW

Although technology is being explored to increase the charge limit of DCFCs, the present battery chemistry used in EVs cannot accept a higher charge. 1,000 kW chargers are in development; however, their usefulness will be determined by the EV systems and battery types.

The EVs Systems have restrictions

Each EV recharging system allows a certain level of charging current or amount of power. This is called the acceptance rate.

This may be substantially below the capacity of the DCFC system, and the EV will throttle the charge rate to fall in line with its capacity.

DCFCs Are Available On A First Come, First Served Basis

DCFCs are public chargers, so there may be a long queue to await their turns before a charger becomes available. 


Does DC Fast Charging Affect Battery Life? 

To protect the EV batteries, EV systems slow the charge rate after it reaches 80%. 

The Ford manual (EV) recommends limiting DCFC charges and ending at an 80% state of charge for two reasons.

  • Beyond 80%, the charge rate slows, and because it takes longer, the charging cost increases.
  • Frequently charging your electric vehicle using DCFC may cause the batteries’ efficiency and lifespan to be reduced.

This has been disputed by an Ohio National laboratory study which showed the same battery as AC systems over 50,000 miles.

How Often Should I Use A DC Fast Charger? 

The accepted wisdom is that DCFC systems should not constantly charge EV batteries.

The study above showed that daily DCFC charging over 50,000 miles resulted in the same degradation of AC charging systems.

Key Takeaways

Now that you’re up to speed on how DC fast charging works, here are the key things to remember.

1️⃣ DC Fast Charging (DCFC) systems offer a much faster charging rate than Level 1 and Level 2 chargers, with electrical power input of up to 350kW, making them ideal for long trips.

2️⃣ There are four types of DC fast charging connectors: Combined Charging System (CCS) Coupler, Tesla Connector, CHAdeMO (Japanese EVs), and GB/T (Chinese cars).

3️⃣ Although DCFC systems provide fast charging, they have limitations such as high installation costs, potential battery life impact, and restricted charging speeds based on the EV’s acceptance rate.

Understanding EV charging in general, will give you greater context into how DC fast charging operates.