The age of the EV has officially come.
Here's an overview of some of the current realities of charging electric vehicles, including AC vs. DC chargers, standard connectors, and the availability of charging stations.
AC vs. DC EV Chargers: Residential and Public Applications
Electric vehicles require a source of AC or DC power to recharge their batteries, which is commonly supplied from the power grid. Residential charging is usually performed at either 120V and 12A (1.5kW) or 240V and 20-50A (5kW to 12kW).
Electric vehicles are usually supplied with an AC EVSE, or electric vehicle supply equipment, that will have a vehicle plug on one end and a residential wall plug on the other. Hard-wired AC chargers are also available for residential installation but are more commonly found at workplaces or in public parking areas.
DC fast chargers, on the other hand, can often be found at dedicated EV charging plazas, such as Tesla’s Superchargers, and often supply 50kW to 400kW or more.
A Tesla Supercharger. Image used courtesy of Jakob Härter
Charging power at a DC fast charger will often be limited by what the vehicle’s battery is able to accept rather than the supply side.
Due to the complexity of the equipment and the amount of power needed, DC fast chargers are rarely if ever found in residential or workplace parking areas.
EV Charging Connectors Standards
In North America, vehicles will often be supplied with a SAE J1772 / CCS combo connector, which incorporates both AC and DC charging into one connector.
A J1772 connector for charging EVs. Image used courtesy of Michael Hicks [CC-BY 2.0]
Additionally, you may come across a CHAdeMO connector.
A CHAdeMO connector at a charging station. Image used courtesy of Kārlis Dambrāns
In the remainder of the world, IEC Type 2/mennekes is the prevailing standard, with CHAdeMO also sometimes found. Despite different physical differences, all charge inlet standards share the same basic features: large-gauge power delivery wires, a protective earth connection, some sort of bi-directional signaling system, and a latch to secure the connector in the port.
Both the IEC and SAE standard connectors use a pair of wires known as Charge Pilot and Proximity Pilot for communication and signaling, whereas the CHAdeMO standard uses a more complex set of signals as well as a CAN bus for communication between the supply equipment and vehicle.
Communication between the power source and the vehicle’s battery and on-board charger is critical to ensure the safety of the user and the longevity of the battery and the charging connectors. Allowing the vehicle to detect when the plug is fully inserted allows the on-board charger and supply equipment to ensure that both sides of the connection are always safe to touch. The vehicle must detect when the latch is pressed on the connector to allow the vehicle to stop drawing power before the connector is unplugged to prevent arcing.
The vehicle must be able to communicate what voltages and currents it can accept and whether ventilation is required to prevent overheating. Additionally, the supply equipment must constantly monitor for ground faults to ensure that the vehicle’s chassis remains isolated from the battery.
On-Board Vehicle Chargers
The vehicle’s on-board charger handles all AC charging, and will also handle DC charging if voltage conversion is needed (for example when charging an 800V car from a 400V charger).
It will work in conjunction with the battery management system to rectify and boost the supply voltage to the necessary voltage for the battery, perform constant-current and constant-voltage charging, and stop charging if any fault conditions are detected in the battery or supply equipment.
An example of a BorgWarner onboard battery charger. Image used courtesy of BorgWarner
The on-board charger will also offer some scheduling system which allows the driver to plug in but defer charging until a specific time. This is useful when electricity rates vary throughout the day, to allow charging to occur mostly or entirely when rates are lowest (often late at night and in the morning).
Charging Station Infrastructure
Charging networks are the electric vehicle’s equivalent of gas stations, and are what enable owners of electric vehicles to travel large distances beyond what they can cover on a single charge. Charging standards allow most vehicles to charge at most public chargers.
Tesla currently uses proprietary charging connectors and protocols to prevent others from using their public chargers, although this has the possibility to change at some point in the future since an adapter could easily be created by Tesla to allow other vehicles to charge. Charging is fastest between 20% and 80% state of charge, so recharging times are often quoted in terms of this.
Optimally, you would find a charging station along your route each time you approached 20% remaining charge and then proceed on your way once you had reached 80% charge. Most electric vehicles have the capability in their infotainment navigation system to account for this, and will plan stops along the route at charging stations that have been pre-programmed by the manufacturer.
Altitude changes and weather are often taken into account, as well as ambient temperature and the use of the heating and air conditioning. All of these factors will have a significant impact on the total range of the vehicle.
The only downside of this system is the need to keep the relevant databases updated and to ensure that all major charging networks are included. In North America, some of the most extensive charging networks are operated by ChargePoint, Blink, and EVgo. Each requires the driver to carry an RFID membership card, which unlocks the charging station and allows the charger to bill the driver for energy and/or time used at the station.
Since demand for EV chargers has outpaced the growth in charging infrastructure, most public stations also charge a penalty fee if the vehicle remains plugged in after completing a charge to encourage drivers to move their vehicle to allow another EV to use the station.
Residential Charging: V2G Technologies
Residential charging offers unique challenges and opportunities to utility providers.
EV charging mostly occurs at home and puts a large, constant draw on the power grid for many hours. To help encourage EV owners to charge in the lowest-demand times, utilities commonly offer a steep discount on power during the off-peak hours (often from 11pm to 9am), in exchange for a higher rate during the peak times, usually the mid-afternoon.
However, an enticing prospect for the next generation power grid is called "vehicle-to-grid" or V2G. A V2G setup acts as a buffer for energy in the grid, storing it during off-peak hours and returning it to the grid during peak times. This allows EV owners to leave their vehicles plugged in when not in use and reduce their energy bills.
V2G technologies are currently being developed using powerline communication protocols and bi-directional chargers in next-generation EVs.
Image used courtesy of Motor Verso
Currently, long-range travel in an EV does require some careful planning and longer stops than would normally be taken by a gasoline-powered car to refuel.
Battery technology, however, is constantly progressing and each generation of new EVs can charge faster than the ones before. It’s only a matter of time before ‘filling up’ an EV is just as fast as fueling with gas.
What information about EV charging would you most like to know as an EE? Share your questions in the comments below and we may be able to develop resources for you!