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| Fig. 1: A plug-in hybrid charging at a public EV charging point in Fremont, California (Source: Wikimedia Commons) |
A vehicle charging station provides electric energy for recharging electrically-powered vehicles, including hybrid electric vehicle, plug-in hybrid electric vehicle, extended-range electric vehicle, battery electric vehicle. The development of charging infrastructures has played an essential role in supporting the growing population of electric vehicles (EVs). This essay discuses various types of commercially-available charging stations seen in public areas and residential homes, and it also brings up the the issue of grid impacts exerted by charging station and the corresponding solutions.
A charging station is composed of three main elements - a supply device, a power cord, and a connector. A supply device is the core of a charging station, providing electricity and shock protection. Some information systems such as meters for measuring the amount of energy delivered while an EV is charging may also be contained in the supply device. A power cord transmits electrical current and communication signals between the supply device and the connector. A power cord can either carry alternating current (AC) or direct current (DC), depending on the power supply levels. A connector, also known as coupler, is a plug on the power cord that connects the supply device to the charging outlets on the EV. Because of the wide range of EV types, several electrical energy supply options are required to meet their varying charging needs. As can been seen in the following section, three power levels have been widely defined in the EV community.
Level 1 charging method uses a standard single phase alternating current (AC) of 120V at up to 20 amp branch circuit, which is the lowest common voltage level found in both residential and commercial buildings in the United States, to deliver around 2 kW of power. [1] Level 1 carries the actual charger along with the power cord on board with the EV. Level 1 charging is mostly seen at EV owner's home and the charging is typically conducted overnight. Charging time for Level 1 can be up to 8-14 hours to fully charge an EV, depending on its battery capacity. [2] Due to the small amount of power Level 1 provides, Level 1 charging time is longer than other charging types, which can have as short as 30 minutes for one charging cycle. The need for fast EV recharging prompted more solutions involving applications of high voltage and direct current, and these technologies are discussed in the following paragraphs.
Level 2 delivers up to 20 kW of power from either single or three Phase AC of 208-240V at up to 80 amps. [1] The J1772 standard has been adopted by the Society of Automotive Engineers (SAE) to regulate the connector and power cord used in Level 2 charging. The power cord for level 2 is different from that of level 1 in that the cord of Level 2 is permanently fixed to the supply device rather than having it with the vehicle. Level 2 is a popular residential charging option since it takes a lot less time than Level 1 to fully charge an empty battery (around 4 - 8 hours). [2] Some commercial buildings and public parking areas also deploy Level 2 charging option for a fee or free of charge if they wish.
Level 3 charging, or fast charging, delivers a maximum power of 240 kW, supplying very high currents of up to 400 amps at voltages up to 600 volts DC. [1] Due to the deployment of DC voltage, Level 3 allows direct and fast charging to the vehicle battery, bypassing the vehicle on-board charger. Level 3 is a highly-powered charging option, with the potential to fully recharge an EV in less than 30 minutes. [2] Standards for Level 3 have not yet been finalized; two major competing standards for Level 3 charging include J1772 Combo and CHAdeMO. Despite the amount of time that Level 3 charging saves, most residential electrical service does not provide this capacity of electrical power that Level 3 delivers, thus making Level 3 infeasible for implementation at home.
As EVs vehicle gains its popularity and vehicle charging station becomes more widely-distributed, concerns about the impact of EV charging on the power grid have arisen. Load characteristics of the original power supply and the planning and operating of the power grid can all be influenced by the increasing number of charging stations and amount of charging demand. [3] One promising solution for minimizing the downsides of EV charging has emerged. Smart grid, made possible by two-way communication technology and computer processing, is the key to smart EV charging. A smart grid enables utilities to manage when and how EV charging takes place, collect power delivery data and apply specific rates for EV charging. [4] A EV recharging session can be initiated during times of low demand and be avoided during periods of peak load with the help of smart grid communication. Furthermore, in a vehicle-to-grid scenario EVs can supply their battery energy back to the grid while the grid is on peak load.
Three power levels for EV charging have been defined to accommodate various power grid standards of utilities, varying types of EVs and different needs of charging time. Though EV charging might increase the load burden of power grid, technologies such as smart grid allows utilities to inform EV owners to pick off peak periods for recharging to alleviate grid load and receive reduced cost incentives. However, uniform standards for connectors, power cord and supply device across different levels of EV charging still need to be defined. Many companies are also looking into more advanced EV charging technologies such as inductive charging and battery swapping, which might eventually stop the debate over what standards to follow to satisfy charging station specifications.
© Hsiao-Hsuan Lin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] K. Morrow, D. Karner. and J. Francfort, "Plug-in Hybrid Electric Vehicle Charging Infrastructure Review," Idaho National Laboratory, INL/EXT-08-15058, November 2008.
[2] L. Dickerman and J. Harrison, "A New Car, a New Grid," IEEE Power Energy Mag. 8, No. 2, 55 (2010).
[3] "How the Smart Grid Enables Utilities to Integrate Electric Vehicles," Silver Spring Networks, July 2013).
[4] J. Taylor et al., "Evaluation of the Impact of Plug-In Electric Vehicle Loading in Distribution System Operations," IEEE 5275317, 26 Jul 09.
