Differences between AC and DC Charging

After all, the battery is manufactured and installed in a vehicle. But how do you collect the energy inside it? This is where chargers enter the stage. The following article explains different charging techniques and the challenges of building the infrastructure.

Since the first commercial models of electric vehicles entered the automotive market in 2010, the charging process has gradually changed from a model based exclusively on home charging to a mixed model that increasingly relies on public charging infrastructure.

The successful introduction of fast charging, coupled with the growing popularity of workplace chargers, has changed the way we think about about charging and given drivers more flexibility. Although the preferred method of battery charging will continue to be residential, the availability of public charging infrastructure is a positive factor in the uptake of electric mobility. Battery charging is essential both for the operation of electric vehicles and for the distribution network to which the charging network is connected. This is related to several specificities of the application that determine how the problem is addressed.

There are three parameters that characterize charging:

• charging power,
• type of plug
• charging mode.

These parameters imply differences in the grid’s connection, the associated current/voltage levels, the way in which energy is supplied to the vehicle (AC or DC), the communication systems between the vehicle and the charger, and more.

Direct Current (DC)

• Refers to the flow of electric charge in a single direction. In a DC circuit, the electric charge moves from a higher potential (voltage) to a lower potential. The voltage remains constant over time, and the polarity (positive and negative) of the voltage source doesn’t change.
• In DC circuits, the current is relatively stable and doesn’t change direction.

Alternating Current (AC)

• Is a type of electric current in which the direction of the electric charge reverses periodically. This means that the voltage and current change direction regularly, oscillating back and forth. The frequency of this oscillation is measured in Hertz (Hz) and determines how many cycles the AC goes through per second.
• The main advantage of AC is that it can be easily converted to different voltage levels using transformers, allowing for efficient transmission over long distances.

The key difference between DC and AC is in the direction in which the current flows and the way in which the voltage changes over time. DC is unidirectional and maintains a constant voltage polarity, whereas AC changes direction and voltage polarity periodically.

The diagram below shows the use and application of alternating current (AC) and direct current (DC) in electric vehicles. As can be seen, both types of current are used in public chargers, depending more on the type of vehicle or charger. There are two basic types of charging, either alternating current (AC) or direct current (DC). When AC power is used, either from the mains or from an AC charging station, the current passes through the charging cable to the on-board charger, which converts the AC current to DC and sends it to the battery via the Battery Management System (BMS). When the car is charged with DC, the on-board charger is bypassed and the current is sent directly to the battery via the BMS. The on-board charger is therefore not used during DC charging.

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AC and DC charging in Electric Vehicles. PEM Motion (2022)

The international standard ‘IEC 61851-1’ defines four charging modes, each of which differs in terms of power assessment and communication between the grid and the vehicle. These charging modes are described in the following steps, along with two specific charging techniques: pantograph and induction charging. These techniques are particularly important for opportunity charging in public transport.

Conclusion:

In conclusion, the existing charging modes for electric vehicles offer different options to meet different charging needs. Expanding the charging infrastructure, promoting interoperability and investing in advanced technologies will be key to supporting the widespread adoption of EVs and ensuring a convenient and efficient charging experience for all EV owners.

References:

• Chilean Ministry of Energy Electromobility Platform – How to characterize electric car chargers. (n.d.) Retrieved from: Link