A Technical Analysis of Charging with the J1772 Connector

The behind-the-scenes of EV charging

Before getting into the technical details, a few terms must be defined.

1. SAE J1772 — This is the “standard” connector in North America and is maintained by the Society of Automotive Engineers (SAE). It is critical to note that this covers not only the recommended practices for the physical connector, but also the electrical, functional, and performance requirements.
2. EVSE — The Electric Vehicle Supply Equipment, as the name implies, is the unit that is responsible for supplying an electric vehicle with power. Contrary to popular belief, this is not the charger, but simply the unit that allows the vehicle to access grid power. More on this later.
3. BMS — In order to ensure the safety and longevity of a battery pack, they are coupled with a Battery Management System. The BMS serves a number of functions such as regulating battery charging/discharging, cell balancing, and temperature monitoring.

What is a “Charger” Anyway?

Imagine charging a phone. This would typically look like a phone connected to a USB Cable, which connects to a charging brick, that plugs into an outlet.

In this case, the brick can be considered a charger. That is, it converts AC from the outlet to a predetermined DC voltage using (typically) a switch mode power supply.

As per the USB C power delivery protocol, the phone “informs” the brick which voltage it requires, and the brick sets its output appropriately. The protocol supports a number of voltages, namely 5V, 9V, 15V, and 20V. This should not be confused with the USB type C connector as it is simply a physical plug that can interface to a variety of devices.

How does this relate to EVs?

Electric vehicles using the J1772 connector conduct charging very similarly to the above example, with one major exception.

The EVSE acts more like an outlet than a charging brick. It allows the vehicle to connect to the grid (or other supplies such as a battery backup), but does not handle charging. Since electric vehicles use such a large variety of different battery configurations (which require different voltages to charge), the actual charger is on the vehicle itself.

That being said, the EVSE serves an important safety role. First, the building’s electrical wiring is limited by the amount of current it can safely deliver. The EVSE helps regulate the amount of power the vehicle is allowed to draw. Second, the EVSE acts as a switch. When the J1772 connector is not plugged into the vehicle, the EVSE disconnects power to the connector through a contactor (similar to a relay). It also checks for a number of faults, which would also result in the EVSE terminating the connection.

Once the EVSE allows current to flow through to the vehicle, the onboard charger converts the inputted AC (120/240V AC depending on if the EVSE is configured for level 1 or level 2 charging) to the appropriate DC voltage. This is then fed to the battery through the BMS.

The EVSE/Vehicle “Handshake”

The EVSE and vehicle communicate through a “handshake”. The diagram below shows the pins on the J1772 connector.

L1 and L2/N supply the power (the latter is either L2 or N depending on what AC voltage the EVSE is supplying), ground is a connection to earth, and CP and PD are communication pins.

First, the PD, or proximity detection, pin allows the vehicle and EVSE to know that they are connected. The ground pin and PD pin are connected through a resistor on the EVSE side; this allows the vehicle to detect when it is plugged in. A practical reason for this is to prevent users from shifting from park with a charger plugged in.

Next, the CP, or control pilot, pin is what allows the vehicle to inform the EVSE that it is ready to charge. The EVSE outputs a 1 kHz square wave on the CP pin, and the vehicle applies a resistance between the CP pin and ground. The vehicle nominaly applies about 2.7 kΩ of resistance across the pins and lowers that to about 880 Ω to signal that it is ready to charge. The EVSE can then close the contactor, allowing current to pass through the hot pins. Furthermore, the square wave output from the EVSE informs the vehicle of the maximum current it is allowed to draw through Pulse Width Modulation (PWM).

Another interesting point about the signal lines involves the physical connector. Note the button and the latch on the connector. Both the button and the latch are mechanically linked such that when the button is pressed, the latch goes up (and vice versa).

There is a small microswitch inside the connector that gets actuated when either the button is pressed or the latch is actuated, prompting the connector to apply a resistance across the proximity pin and ground. This informs the EVSE that the cable is about to be plugged in or unplugged, allowing it to disconnect power to the hot pins to reduce the risk of arcs.

What Does This Mean for the Consumer?

For the average consumer, this protocol allows for convenience and safety. Any electric vehicle with the J1772 could be safely plugged into any J1772 EVSE without having to consider the specifications of the EVSE or charger. Furthermore, the communication between the EVSE and vehicle helps prevent many faults that could otherwise result in fires or injury. Evidently, the elegant yet powerful charging protocol is critical for the successful deployment of mass electric vehicle adoption.

Further Readings

This article only serves as a surface level overview of the protocol; for more details, take a look at IEEE’s PowerPoint on EVSE basics or Technology Connection’s EVSE explanation video.