Understanding the Differences Between DC and AC Charging Technologies for Electric Vehicles

When it comes to charging electric vehicles (EVs), confusion often arises regarding the differences between DC and AC charging technologies. This article delves into the technical aspects, helping both newcomers and enthusiasts grasp the nuances of these charging methods.

What Is a Charger Exactly?

At its core, a charger is simply a device designed to supply current necessary to charge a battery. The technology behind chargers can vary widely, but they all provide a DC power source tailored to the intended battery type. Understanding this basic principle is vital to comprehending the distinctions between AC and DC chargers.

Types of AC Chargers

Contrary to popular belief, there is no such thing as a pure AC charger for EVs. Instead, the distinction lies in the technologies used to rectify and convert the input power. Traditionally, EV chargers used a transformer to step down the voltage and a rectifier to convert the AC to DC. However, modern chargers are typically switch mode power supplies (SMPS), converting AC to high DC voltage and then switching the current through a transformer at a very high frequency (200,000 to 1,000,000 Hz).

Limitations of AC Chargers

Commonly available AC chargers typically top out at around 10 kilowatts (kW), with no more than 40 amps being compatible with standard home circuit breaker panels (240 volts in the U.S.). The key point here is that the charger is usually integrated into the vehicle, while the electric vehicle supply equipment (EVSE) acts as a fancy extension cord, providing power and monitoring safety conditions.

DC Charging: A Quicker Path to Power

DC chargers, on the other hand, range from 50 to 145 kilowatts (kW). These chargers convert three-phase AC power into DC, bypassing the onboard charger and directly supplying power to the battery. This makes DC charging significantly faster, especially in the "bulk charge" phase (0 to 80% charge), where a DC charger can be anywhere from five to fifteen times faster.

Factors Limiting Charging Speeds

The limiting factor in both AC and DC charging varies:

For AC charging, the maximum safe input current for the battery or the maximum safe output current from the charger (whichever is lower) determines the charging speed.

For DC charging, the speed drops off significantly above 80% of the battery's charge as the cells near full charge and can no longer handle increased current without damage.

Potential Future Developments

While three-phase AC charging could theoretically increase on-board charger power, particularly with redesigned motor and inverter electronics (as seen in AC Propulsion's Reductive charging technology), there is currently no widely accepted standard for this approach.

Understanding the differences between AC and DC charging technologies is crucial for anyone considering EV ownership or interested in the future of electric vehicle infrastructure. With the ever-evolving landscape of charging methods, staying informed will ensure you make the most out of your vehicle's charging experience.