Charging systems for electric vehicles
Charging stations provide power to electric vehicles at multiple levels.
A charging station is designed to charge an electric vehicle (EV) or plug-in hybrid electric vehicle (PHEV). Typically, a charging station includes a connection to building or premises electrical power, disconnect, cabling, and the EV connector plug. NFPA 70: National Electrical Code (NEC) refers to a charging station as EV supply equipment. Charging stations require an ac power source and must include safety interlocks—regardless of the station type.
Currently, EV charging stations are classified into three charging levels with provisions for future applications. Level 1 and Level 2 charging stations provide ac to EVs that have battery chargers—including rectifier, controls, and protection circuits—within the car. Level 3 charging stations provide regulated dc to EVs with controls and protection circuits located within the car, as well as control feedback from the car to the charger.
Level 1: A Level 1 charging station is inexpensive to install and requires minimal electrical renovation. Not surprisingly, Level 1 is the slowest charging method. Typically, for an EV (or PHEV with 40 miles of electric-only driving) it takes 8 to 10 hr to reach a full charge. The circuit required is a standard 120 V, 20-A power receptacle with a load of 1,200 W. A charge cord with an SAE J1772 plug is needed and is provided as standard equipment with the Mitsubishi I, Toyota Plug-In Prius, Nissan Leaf, and Chevy Volt (see Figure 1).
Level 2: Level 2 charging also requires a charging station to be installed with an SAE J1772 plug (see Figure 2). This plug configuration has been standardized on U.S. production EVs and PHEVs. The J1772 has five pins: ac Line 1, ac Line 2, ground, proximity detection, and control pilot. Proximity detection prevents the vehicle from moving while connected to the charger. Control pilot communicates charging level information from the car to the charging station. A Level 2 charging station requires a 208 to 240 V, single-phase circuit ranging from 30 to 80 A. The charging time is 2 to 3 hr. The load is approximately 4,000 W.
Level 3: Level 3 dc fast charging is not currently standardized in the U.S. An example of a typical Level 3 charging station installation is a 480 V, 3-phase, 125-A circuit capable of delivering a full charge in 30 min. Installation costs can be quite expensive. Concerns from the EV and battery industries have prompted caution for this higher level charging because of diminished battery life and recommend Level 3 charging only occasionally.
Of these charging systems, Level 2 charging is likely to be the most common form of charging for the foreseeable future.
Charging station selection
Charging station selection criteria depend on location, cost, and return on investment. The location of the charging station may be determined by time of use, convenience, available time for the charge, and the quantity of users in the area.
Home is where Level 1 charging mainly occurs. There is no installation cost as long as a 120 V, 20-A receptacle is located near the vehicle. Some vehicles are supplied with a Level 1 charger. However, many buyers opt for 240 V, 16-A Level 2 chargers.
The office is a logical charging location because of the amount of time the EV or PHEV is likely to be parked there. Level 2 charging makes the most sense. Also, buildings could get LEED credits for installing charging stations.
Retail businesses have mixed reasons for installing charging stations including getting customers to use their services or purchase their product, or to add an environmental or green aspect to their businesses. Typical customers could spend 45 to 90 min in their establishment, thereby giving the Level 2 charging station time to provide a half-charge or topping-off charge for the customers’ EVs and PHEVs. These businesses could include malls, grocery stores, and some forward-thinking downtown locations.
The current range and charging characteristics of EVs on the market with varying electric motor and battery size coupled with the individual’s driving and charging habits determine whether travel is mainly local or includes longer range trips. Trips approaching more than half the vehicle’s range will require advance planning and attentive use of the on-board electronics to adjust driving speed, driving mode, and use of the vehicle’s electrical accessories.
Even with excellent planning and attention to detail, drivers may experience a new form of malady associated with EVs called range anxiety. The concern is that range anxiety may distract from the important task of safely operating the car, obeying the traffic laws, and observing the road.
NEC EV charging system requirements
The following list provides a brief overview of applicable NEC requirements for EVs:
NEC Article 625: Electric Vehicle Charging System defines an EV as an automotive-type vehicle for on-road use, such as passenger automobiles, buses, trucks, vans, and neighborhood electric vehicles. Article 625 is not applicable to electric motorcycles or off-road electric vehicles such as industrial trucks or golf carts.
NEC 625.5 requires all electrical materials, devices, fittings, and associated equipment relating to electric vehicle charging systems to be listed or labeled.
NEC 625.15(A) requires electric vehicle supply equipment to be marked by the manufacturer with: “For use with electric vehicles.”
NEC 625.29 covers interior sites for charging stations including mechanical ventilation requirements for permanently mounted supply vents and exhaust fans. Positive-pressure ventilation systems with supply fans and exhaust vents are allowed for areas or buildings specifically designed and approved for that application.
NEC 625.29(B) states: “Where electric vehicle non-ventilated batteries are used or where the Electric Vehicle Supply Equipment is listed or labeled as suitable for charging electric vehicles indoors without ventilation and marked in accordance with NEC 625.15(B), mechanical ventilation shall not be required.”
Enclosed garages and parking structures without adequate ventilation shall be subject to NEC 625.29.
Stu Sutherland is an electrical project coordinator with exp U.S. Services Inc. His expertise is electrical design for public and private buildings.
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