Home EV Charging Nuts and Bolts (or is it “Volts and Amps”?)

Mar 04, 2015

In general, most electric vehicle charging occurs at home and at night. Night time charging is both convenient and “green” – it occurs at a time when drivers are usually sleeping and the power grid is less stressed, often with a surplus of unused wind energy. What’s behind “home charging” – the regimen that seems like the simplest and most basic form of recharging your EV?


Charging Standards


The Society of Automotive Engineers (SAE) develops North American standards for EV charging and communications interface with cars and equipment. The specific SAE standard for vehicle charging is known by its alphanumeric name: J1772. This standard comprises two types of charging systems that are suitable for home charging -- Level 1 for 120V charging, and Level 2 for 240V charging.  Level 3 (DC “fast charging”) is in the process of being defined.



Level 1 allows for up to 16 Amps of current at 120 Volts (common household outlet), while Level 2 allows up to 80 Amps at 240 Volts (“dryer” outlet). Most charging systems have lower maximum capacity: common capacities include 12 Amps at 120V (1.4kW) and 30 Amps at 240V (7.2kW). A similar set of standards exists in the European Community governed by IEC specifications and describe a similar set of charging “modes,” while China has yet another set of standards.


The Basics


The actual charging process has components both on and off the vehicle. AC power must be converted to DC energy in order to charge the battery. Virtually all EVs convert power using a small charger that lives “on board” the vehicle. Typical power ranges for on-board chargers range from 3.3kW to about 7kW. Charging hardware that provides AC power such as the 120V Level 1 “trickle charging” cord and 240V Level 2 wall-mounted charging stations is called Electric Vehicle Supply Equipment (EVSE). The EVSE includes the cable; a special coupler plug that connects to the vehicle; and electronics that facilitates power flow, communications, and safety functions.



EVSE-enabled EV charging is one of the safest electrical regimens a consumer could encounter. Why? First, there’s no voltage present in the cable and coupler until the EVSE has determined that the cable is safely connected to the vehicle. The EVSE also checks that the ground connection is present. And, in the very unlikely instance that it detects the potential for an electric hazard, it automatically shuts down. On the vehicle side, there is an additional interlock function that prevents the vehicle from being driven or moved if connected to the station.


The National Electric Code (NEC) section 625 also provides key guidance on the design of EVSEs. Guidance includes provisions on installation, cable type and sizing, and safety. NEC625 also calls for EVSEs to be ‘listed’ by a nationally recognized testing laboratory (NRTL) such as Underwriters Laboratories (UL). Finally, building inspectors must sign off on the installation of EVSE equipment, assuring that the installation complies with the NEC, as well as local building and electrical codes.



Paying for Charging


The cost of home charging includes the cost of the charging station, installation, and the cost of the electricity. There are many ways to pay for charging. Equipment and installation costs are usually paid up front, while the electricity cost is variable and recurring. Some payment schemes bundle the two together in a monthly subscription, much like your cable box is bundled into your monthly cable bill.


The price of electricity is expressed as dollars per kiloWatt-hour (or often as cents/kWh). Each electric utility has one or more tariff schedules that are applicable for residential charging. Tariffs vary widely in cost and complexity from utility to utility. The variability in electricity costs is far greater than that of gasoline prices around the country; the utility cost of charging at home can vary from 5 cents/ kWh to 45 cents/kWh. At the national average price of about 12 cents/kWh, the electricity cost of driving a Nissan LEAF is about 4 cents per mile (vs. 8 cents per mile for a plug-in hybrid Prius at $4.00/gallon for gasoline).


In some states like California, residential electricity usage is priced in tiers; the more electricity you use in a month, the higher the price per kWh as you move up the tier structure. This is to encourage households to conserve their energy usage. But such tariffs discourage plug-in vehicle use because daily charging can quickly drive households into the highest tier, resulting in high rates even during off-peak charging. Un-tiered “time-of-use” rates are one solution to this challenge, but this scheme requires the installation of a separate electricity service just for charging plug-in vehicles. In California, the state public utilities commission (CPUC) is developing a plan to allow EV sub-metering without requiring a second electricity service just for vehicle charging.