How the J1772 charging standard for plug-in vehicles works
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 standardized and varies from vehicle to vehicle). This AC-DC converter communicates via the J1772 protocol and commands the EVSE to energize.
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).
Recently added to the J1772 standard is DC fast-charging. In this case the EVSE not only provides an AC connection to the vehicle but also a DC source. This has previously been done on a limited basis by certain manufacturers (such as by Nissan on the LEAF & Mitsubishi on the iMiEV). CHAdeMO is a consortium of Japanese automakers that was previously the only DC fast-charge standard. Now that the SAE has added this, Audi, BMW, Daimler, Ford, General Motors, Porsche, and Volkswagen have all agreed to implement it as their direct DC charging standard.
Direct DC charging has one primary advantage: speed. When you consider the battery pack size (16kWh for the Volt, 24kWh for the LEAF, 85kWh for the top-of-the-line Tesla Model S), it becomes readily apparent that with the maximum 50A, 230V outlet in most homes (the SAE Level 2 standard actually tops out at 63A), it will take a while to charge. In the case of a LEAF, a dead 24kWh battery charging at a Level 2, 30A rate (30A x 230V = 6.9kW) will take about four hours to recharge.
If you need a charge in a hurry (especially important when on the road), high current and voltage are needed. This comes at a price, though. Voltages greater than 400VDC (many packs operate up to this amount) and currents greater than 100A may be needed to fast charge the vehicle. That same depleted 24kWh LEAF battery, though, can be DC fast-charged from nearly dead back to 80% in less than 30 minutes. The still developing “Level 3” DC fast-charge portion of J1772 has not been formally defined, but may provide for as much as 240kW of charging!
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