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EV charger types, what is the difference between AC and DC?

One of the biggest fears of electric car ownership is the charging process. Besides the possibility of charging from the home socket, there are two main types of charging stations: DC (Direct Current) fast chargers, and AC (Alternate Current) chargers. We can differentiate two types of chargers from the current type perspective: DC and AC.

AC chargers (also called level 1 for single-phase AC and level 2 for three-phase AC) are the most common ones because they are using the same current type, that is available in the grid. This is directly related to the EV system architecture. As we know, batteries are using Direct Current (DC), but in the grid, the main source of current is Alternate Current (AC).

In the previous blog about electromobility I explained a bit how to calculate energy and how much does EVs need it (https://codetain.com/blog/electric-vehicles-understanding-the-electricity). In this article, I will focus on the charger types and how the charging is handled. 
One of the biggest fears of electric car ownership is the charging process. In conventional cars this is fast and simple: all you need is to drive to the petrol station, fill the tank within 2-3 minutes and pay for the fuel. The situation is not that easy with electric cars. Besides the possibility of charging from the home socket, there are two main types of charging stations: DC (Direct Current) fast chargers, and AC (Alternate Current) chargers.

Quick explanation, what is AC/DC?

If we want to explore the secrets of electric vehicles and all the forces; we need to understand electrical engineering basics. One of the most important basics are the AC (Alternate Current) and DC (Direct Current) characteristics:
Direct Current, as the name suggests, flows in a certain direction. Batteries construction requires choosing the direction of the current. In one direction, the battery is gathering power and charging, in another is giving power and discharging. Car electric equipment is powered by direct power.
Alternate Current, on the other hand, is changing direction all the time. In common electrical grids, it changes its direction 50-60 times per second (50-60 Hz). In this case, the battery will remain in fluctuating charging/discharging state.

AC and DC chargers.

We can differentiate two types of chargers from the current type perspective: DC and AC. AC chargers (also called level 1 for single-phase AC and level 2 for three-phase AC) are the most common ones because they are using the same current type, that is available in the grid. The usual charging power of the AC chargers depends on the usage of single-phase or three phases current. Single-phase chargers can reach up to 7,4 kW (32A * 230V * 1), where 32 A is the usual maximum value of current, 230 V is the voltage, and 1 - number of phases. Triple phase chargers can reach up to 22 kW (32A * 230V * 3)
DC chargers (level 3 chargers) are called fast or rapid chargers. This name is because simply their power is much higher than the AC ones because they can operate on the higher current and voltage. The usual power of the DC chargers starts at 20-30 kW. The most powerful ones reach 350-360 kW. However, none production car at this moment can handle that much charging power. One of the highest possible charging power is capable by Tesla S plaid (up to 250 kW). Important to mention is that this power can be achieved only in the small window: between 10-30 % SoC (state of charge).

Why are DC chargers faster?

This is directly related to the EV system architecture. As we know, batteries are using Direct Current (DC), but in the grid, the main source of current is Alternate Current (AC). The process of transferring electricity from chargers to batteries involves devices and systems like: Battery Management system, protection circuits, DC/DC converter, and rectifier. In a nutshell, AC current has to be converted into DC to be stored in the battery. When the car is charged by the Direct Current, it can bypass the internal charging device and be directly supplied to the battery (of course with the assistance of the safety systems such as the protection circuit). This reduces the bottleneck and the car can receive more power in a shorter time, which means charging power will be significantly increased.

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