AC vs DC Electric Vehicle Charging Infrastructure.
Electric vehicles had existed for a long time, even back when horses were the primary mode of transportation between 1828 and 1835. Hungary, the Netherlands, and the US had people who thought ahead and developed small-sized EVs. The first crude electric vehicle was built in 1832 by Robert Anderson, but a successful one was not made until the 1880s when William Morison built one.An electric vehicle (EV) needs to be charged and recharged just like your smartphone. To achieve this, charging stations, also known as Electric Vehicle Supply Equipment (EVSE), supply power for plug-in EVs. They are as old as electric vehicles and have improved over the years. EV charging can be done with AC or DC supply.
Comparison Between AC And DC Charging Stations
Despite being the more popularly used, AC charging stations have their peculiarities, while DC charging stations have new introductions and drawbacks. I have compared these two kinds of charging stations based on the following subject:
Nature of direction of AC and DC Charging
Alternating currents (AC) are generally popular for changing direction within short periods, and the grids produce AC typically. It is easy to transmit AC from one place to another, so it is usually supplied to the EV chargers (charging stations). However, the batteries in electric vehicles can only store electric power in direct current (DC).How then do you charge a DC battery in your car with the AC power supplied by the grid? To charge these vehicles, supporting infrastructure like AC and DC charging infrastructure have been put in place to provide electric power. The AC charging stations supply power as AC, but the onboard charger inside the electric car converts the AC to DC to be accepted by the batteries.The difference between an AC charging station and a DC charging station is how power finds its way to the batteries. For the AC charging station, the onboard charging circuitry is directly connected to the AC supply. Then, it makes use of a converter that converts AC to DC before transmitting it to the car's battery. Meanwhile, DC charging stations give direct current to the electric vehicles, without converting in the onboard charger; instead, the AC to DC conversion is done in the station before transmitting through the EVSE.
Charging Speeds and Voltages of Electric Vehicle Charging Infrastructure
Electric vehicle charging infrastructure has various levels based on speed and capacity. The current classification of charging speeds is that of levels 1, 2, and 3. Levels 1 and 2 are for AC charging, while level 3 is known as DC fast charging. For level 1 AC charging, it would take an overnight charge to add 50-60 miles, by an output of 1.3kW to 2.5kW to a 120V household outlet. Meanwhile, level 2 charging involves improving to about 208V-240V, charging at the rate of 4kW to 18kW (equivalent to 12 to 54 miles per hour).With the use of DC charging, which bypasses the use of a converter, charging is faster. This speed is expected since the time taken for converting AC to DC is part of what contributes to the slow charging experienced at AC charging stations. DC fast chargers can charge at an output rate of 50kW to 350kW. A vehicle, depending on the type, can be fully charged within 15 - 45 minutes.
More about DC fast charging operation
Each DC charging station has a unique port connector. This uniqueness limits the type of vehicles that you can charge in a DC charging station. So, for example, a Tesla supercharger cannot charge vehicles other than Teslas. The other two kinds of DC chargers available include Combined Charging System (CCS) and CHAdeMo, and they are relatively common and can be adapted to different electric vehicles.More so, various electric vehicles have different battery capacities and power acceptance rates. The amount of power that a battery can accept at a time determines its rating. At least I can tell of newer EV models that can accept up to 270kW hourly, though the common models have been known to accept power at a rate of 50kW per hour. As EV batteries' ratings continue to increase, the rating of charging stations keeps growing as well. As of 2020, the highest EV charging station I have heard of is 350kW. So how can these two (the capacity of the chargers and that of the batteries) match when chargers are going faster? The good thing is that it does not even have to match. All that needs to happen is a communication between the vehicle and the charging station on how much power is required and it would be dispatched.
Pros and Cons of the Electric Vehicle Charging Infrastructure Available
AC charging is still very popular, especially level 2 charging. So, you cannot always expect to drive into a DC charging area, even if you wanted to, because the closest one may be far away. While AC charging is still safe and great, it is slow compared to DC charging and requires more hours of charging.Interestingly, the use of DC fast charging has some disadvantages too. The case of thermal issues is one of them that brings concern. Prolonged charging with DC fast charging heats up the EV batteries, and this slightly degrades the batteries over time. While the long-term effects are still a debate, it calls for more heat control technologies.Furthermore, It is expensive to install, and this is a significant downside. For DC charging stations, a higher voltage is needed. They need the 480-volt level to operate most of the time, and the cost for this voltage requirement is relatively higher compared to the AC counterpart. So, on the side of the installation companies and the EV users, there would be cost implications.AC ChargingDC Charging1Conversion to DC is done inside the car.Conversion is done outside the car in the charging station.2The charging curve is a flat line; that is, it just charges at a continuous rate throughout.The charging curve is a degrading curve as the rate of charging reduces with time. This implies that the initial fast rate at which the EV battery accepts power reduces as it approaches total capacity.3The charging speed is usually around 22kW-43kW per km/h.Can charge up to 50-100kW per km/h.4AC stations are popular and more.The latest DC charging stations exist in Europe but are not as popular as AC charging stations.5Uses a limited onboard charging converter.Uses a larger converter outside the EV for fast and bidirectional charging.Generally, AC and DC charging infrastructure are developing fast to facilitate the better use of electric vehicles. Yet, DC charging still stands out because of its speed. It also has its levels of capacities in terms of current, voltage, and power. Therefore, while we can use what we have, it is better to explore better options like DC charging and improve efficiency.This blog is part of a V2X series. Continue to the other blogs in the series.