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How J1772 charging standard for plug-in cars works

08 Oct 2013  | Todd Marcucci

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You may have already learned about a few plug-in vehicles. Whether you have heard about the Chevy Volt, Nissan LEAF, Tesla Model S, or a newer Prius that can plug in, all these newer plug-in EVs use the SAE J1772 standard to connect and charge. What does this mean?


Figure 1: The standard J1772 plug and port. (source: zemotoring.com)


The formal title of the SAE J1772 standard is "SAE Surface Vehicle Recommended Practice J1772, SAE Electric Vehicle Conductive Charge Coupler." In short, the standard constitutes a definition of how a charging station (EVSE, or Electric Vehicle Supply Equipment) connects with, communicates with, and charges the vehicle. In this standard, the EVSE manages the link from the grid or household power to the vehicle. Think of it as a smart outlet that communicates with the vehicle to "handshake" and ensure safe charging. While J1772 is not required by any federal agency to sell an EV in the US, it has now been adopted by all the manufacturers of passenger vehicles worldwide.

The vehicle has an intelligent, on-board AC-DC converter that rectifies the EVSE AC output and steps it up (or down) to a level appropriate for charging the on-board battery pack (which is not standardised and varies from vehicle to vehicle). This AC-DC converter communicates via the J1772 protocol and commands the EVSE to energise.

The original standard was written to provide for 80A charging at 240V, although most implementations are 30A or less. "Level 1" indicates 120VAC charging (usually less than 16A), and "Level 2" indicates 240VAC charging (less than 32A). Actual current usage is determined by the vehicle. Most EVs and plug-in hybrids sold today are provided with some form of portable J1772 (Level 1) EVSE that can plug into the wall.

The J1772 standard and connector provides Pilot and Proximity pins that allow for detection of the EVSE plug when connected (even if not live/charging). The pilot tone is used by the EVSE to identify to the vehicle the maximum current that is available. The standard was designed so that if a vehicle requires more current than the EVSE can supply, the vehicle can then choose not to charge. In general, this doesn't happen, as EV manufacturers appear to have all chosen to be compatible with the lowest available (13A) charge current (the Volt, LEAF, and other major supplied EVSEs that come with the vehicles only output this).

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