Putting it into practice: decoding EV charging
Understanding the difference between charging types removes most of the confusion around EV ownership. This section explains the core distinction between alternating current (AC) and direct current (DC) charging, why the speed of each is limited by different parts of the system, and why the common label for fast charging is misleading.
The Foundation: AC Charging
Household and workplace power is delivered as alternating current. An EV battery can only store direct current, so the power must be converted before it reaches the pack.
For home and most destination charging, that conversion takes place inside the vehicle using the onboard charger. The onboard charger sets the maximum rate for any AC session. A station rated higher than the car’s onboard charger cannot deliver more power than the vehicle is designed to accept.
Two AC levels are defined for North America under the SAE J1772 standard:
• AC Level 1 uses a standard 120-volt outlet. It typically adds roughly 3 to 5 miles of range per hour. Continuous current is limited to 12 A (1.44 kW) on a 15 A circuit or 16 A (1.92 kW) on a 20 A circuit.
• AC Level 2 uses a 208- or 240-volt circuit, the same type used for an electric dryer. It commonly adds 25 to 40 miles of range per hour. The standard allows continuous currents up to 80 A, corresponding to a theoretical maximum of about 19.2 kW at 240 V.
In both cases the onboard charger is the limiting factor. If a vehicle’s onboard charger is rated at 11 kW, that is the highest rate it can accept from any AC source, regardless of the station’s capability.
A couple of recent videos walk through these basics in more detail:
DC Fast Charging
When hundreds of miles of range are needed in tens of minutes rather than hours, DC fast charging is used. In this process the vehicle’s onboard charger is bypassed entirely.
A DC fast charger contains a large AC-to-DC converter. It supplies direct current directly to the battery. Because conversion no longer depends on the relatively small onboard charger, power levels are much higher—commonly 50 kW to 350 kW or more, depending on the station, the vehicle, and battery conditions. Many modern vehicles can move from a low state of charge to roughly 80 percent in 20 to 40 minutes under favorable conditions. Actual rates are still governed by battery temperature, the station’s output, and the vehicle’s acceptance limits.
The fundamental difference is therefore simple:
• AC charging (Level 1 and Level 2): Conversion from AC to DC occurs inside the vehicle.
• DC fast charging: Conversion from AC to DC occurs at the station.
This single distinction explains most differences in charging speed and most troubleshooting situations.
Why “Level 3” Is Misleading
The phrase “Level 3 charging” is widely used to mean DC fast charging. It is not an official term and it is misleading.
The governing North American standard for light-duty conductive charging is SAE J1772 (SAE Electric Vehicle and Plug-in Hybrid Electric Vehicle Conductive Charge Coupler). The current revision is J1772_202401. It defines four charge methods: AC Level 1, AC Level 2, DC Level 1, and DC Level 2. There is no official Level 3. An earlier proposal for an AC Level 3 was never implemented for light-duty vehicles.
Calling DC fast charging “Level 3” implies it is simply a higher version of the same AC process. It is not. Conversion happens off-board, the power path and communication protocols differ, and the connector is usually different. In North America the common implementation has been CCS Combo 1, which adds two high-current DC pins to the J1772 form factor. Newer vehicles are increasingly moving to the SAE J3400 (NACS) connector, which carries both AC and DC on the same contacts.
Relevant SAE Standards
These documents define the technical requirements for the connectors and charging methods discussed above:
SAE J1772 (current revision J1772_202401) — Covers the general physical, electrical, functional, and performance requirements for conductive charging of light-duty EV/PHEV vehicles in North America, including both AC Level 1/Level 2 and the defined DC levels (DC Level 1 and DC Level 2). It specifies the coupler used for AC charging and the CCS Combo 1 extension for DC fast charging.
SAE J3400 (NACS / North American Charging System, current revision J3400_202409) — Defines the physical, electrical, functional, safety, and performance requirements for the NACS coupler, which supports both single-phase AC and DC power transfer through the same contacts. This is the standard increasingly adopted by automakers for North American vehicles.
https://www.sae.org/standards/j3400_202409-north-american-charging-system-nacs-electric-vehicles
SAE J3068 (current revision J3068_202409) — Covers conductive power transfer for electric vehicles using a coupler capable of three-phase AC (as well as single-phase AC and DC). It is primarily intended for medium- and heavy-duty applications and includes digital communication requirements.
An accessible overview of SAE J1772 (including levels tables, pinouts, and history) is available on Wikipedia: SAE J1772.
The practical takeaway is the location of the AC-to-DC conversion step. Once that is clear, the rest of the charging picture—speed limits, connector differences, and what a given station can actually deliver to a given vehicle—becomes straightforward.
