Understanding U.S. And European Standards For Electric-Vehicle Charging

This is an overview of two automotive-industry standards, SAE J1772 and IEC 61851, with some additional information about charging station connectors. The standards define the voltages and currents allowable and the handshake protocol that the charging station and vehicle must follow.

Table of Contents

Standards
For the U.S. auto industry, the governing document for electric vehicle (EV) charging is the Society of Automotive Engineers (SAE) standard J1772. In Europe, the standard is IEC 61851. These documents define the requirements for “Electric Vehicle Supply Equipment” (EVSE). J1772 says EVSE has three functions: ac-dc rectification, voltage regulation to a level that permits a managed charge rate, and physically coupling the charger to the vehicle. It also defines several “levels” of charging. The levels correspond to different voltage levels and current flows (Table 1).

J1772 AC Charging Methods
J1772’s AC Level 1 charging is the simple “plug into the wall socket” case. It assumes the charger electronics are built into the car. AC Level 2 charging also assumes the electronics are in the car. But at this level, the supply is single-phase ac at a nominal 240 V capable of supplying up to 32 A. AC Level 3 charging just means the vehicle has separate charging ports for levels 1 and 2.

Limits Of AC Charging
None of these levels of ac charging is practical for fast charging. Multiply 240 V by 32 A and you get 7.68 kW. The thermal equivalent of a single gallon of gasoline is roughly 35 kWh. The U.S. Environmental Protection Agency (EPA) estimates that 35 kWh will drive a Nissan Leaf approximately 99 miles. Theoretically, transferring the energy needed to drive a Leaf 99 miles with a 7.6-kW electric supply would require four and a half hours of charging time at the highest ac charging rate. (In normal practice, Nissan says, the Leaf and its 24-kWh battery pack take approximately eight hours to recharge using the 3.3-kW charger Nissan installs in the owner’s garage.) *

DC Charging
On the other hand, there’s dc charging. The charging system starts with a three-phase industrial-capacity ac power drop. It can supply up to 600 V at 400 A (240 kW, which could theoretically deliver 35 kWh in less than 10 minutes).

This changes the design of the charging electronics. Batteries require dc for charging. Carrying around the electronics needed to step down, rectify, and regulate 600 V inside every car would be too expensive. No longer would it be practical to just plug the car into the ac mains directly.

For dc charging, J1772 accommodates power levels similar to AC Level 1 or 2, but looks to levels up to 600 V and 400 A, which could be capable of replenishing more than half of the capacity of the EV battery in as short a time as 10 minutes.

Under J1772, Level 3 AC-capable vehicles could also be configured for dc charging with the addition of a serial data interface and some rearrangement of the internal wiring.

IEC 61851 And J1772 Terminology
The IEC61851 standard used in Europe and China was derived from J1772 and has similar requirements, adapted for the European and Asian ac line voltages. Most terminology differences are superficial. Where the SAE standard describes “methods” and “levels,” the IEC standard talks about “modes,” which are virtually the same.

For example, like J1772 Level 1, IEC61851 Mode 1 relates to household charging from single-phase 250-V (maximum) or three-phase 480-V power connections, with a maximum current of 16 A. (This is a little higher current than the North American limit.) There are further unique requirements for grounding.

Mode 2 uses the same voltages as Mode 1, but doubles the maximum allowable current to 32 A (the same as method 2 in North America). Importantly, Mode 2 adds a requirement for a “control pilot function” (more on that below). It also requires an integral ground-fault interrupter (GFI), which Europeans call a residual current detector (RCD). Mode 3 supports fast charging with currents up to 250 A. Above that, as with J1772, IEC61851 switches to an external dc supply that may supply up to 400 A.

Connectors And User Safety
In either standard, the interconnect arrangement is a key element. The standards provide safety requirements, but the exact methods for meeting them are left to the manufacturer of the connector. For charging connectors, safety implies both protection from electric shock by mechanical means and protection of the charging electronics and the traction battery.

With respect to the actual connection between the EVSE and the vehicle, the standards offer functional and safety requirements but decline to prescribe a single physical configuration, which has led to a variety of plug and socket configurations. For high current levels and dc charging, the pin-out functions, contact dimensions, and safety characteristics of a nine-pin interface are prescribed (Table 2).

Control Pilot
The “Control Pilot” function (pin 6) mentioned above is a handshake mechanism that keeps high voltages off the “hot” charging pins in the connector until the charging station connector is mated to the vehicle connector (see the figure).

Using the three data lines (pins 7, 8, and 9), it may also tell the vehicle how much current the charging station is permitted to supply. The data link for those three pins as specified in J1772 is based on three SAE body-communications standards: J1850, SAE J2178, and SAE J2293. J1772 describes the interactions between the EVSE and the vehicle like this:

• Before there is any connection, the EVSE puts 12 V dc on pin 6.

• When the vehicle is first connected, a 2.74-kΩ “sense” resistor aboard the vehicle pulls down the voltage on the pin 6 line in the EVSE to +9 V. When the EVSE senses voltage drop, it starts to generate a 1-kHz square wave on pin 6 that toggles between +9 and –12 V. (A diode in the vehicle clamps this at –12 V. The diode clamping is a safety feature, intended to allow the EVSE to distinguish between a vehicle and some random resistance accidentally bridging the charging line.)

• What happens at this point depends on the vehicle battery’s state of charge, which is a parameter that the vehicle’s computer tracks. If the vehicle requires ac energy transfer, it closes an internal switch that throws a 1.3-kΩ resistor in parallel with the 2.74-kΩ resistor. This reduces the total resistance to 882 Ω and pulls down the positive peak of the square wave to +6 V. The EVSE interprets this as a request for ac power and turns on the charging voltage.

• Adding complexity, there is a feature that accommodates batteries that emit hazardous gasses during charging. (The presence of this type of battery requires the EVSE to turn on an exhaust fan if it is located in an enclosed area.) If the vehicle has this kind of battery, it switches in a 270- Ω resistor, pulling down the positive peak of the square wave to +3 V.

• After a connection is made, the EVSE provides the vehicle with information about its maximum available continuous current capacity by modulating the pulse width of the square wave between 100 and 800 μs. The relation is linear: 100 μs corresponds to 6 A; 800 μs corresponds to 48 A. A pulse width of 900 μs would mean that the EVSE has its own dc charger, in which case the EVSE and the EV would engage in a more complex data exchange on pins 7, 8, and 9.

• When the vehicle detects that its battery has been charged to its maximum allowable state of charge, it causes the positive square-wave peak to rises to +9 V, signaling the EVSE to remove power. The +9-V, –12-V square wave on pin 6 continues until the cable is disconnected, when it reverts to the continuous +12-V state.

Footnote
*Tokyo Electric Power Company has a patented fast-charging technology that uses dc voltage levels up to 500 V and currents up to 125 A. Under TEPCO’s CHAdeMO protocol, the vehicle exchanges battery parameters with the EVSE. According to Wikipedia, a CHAdeMO DC Fast Charge station delivers 62.5 kW (500 V dc at 125 A). The time to recharge the Nissan Leaf to 80% of capacity would be about 30 minutes.

References