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DC Fast Charging Working Principle

DC Fast Charging Working Principle

In the realm of electric vehicle charging, there are primarily two types of charging piles: AC and DC. While AC Electric Vehicle charging piles are often referred to as “slow chargers”, their DC counterparts are dubbed “fast chargers”.

When an electric vehicle is connected to an AC charging post, the vehicle’s built-in “on-board charger” (OBC) is responsible for converting the AC power from the grid into DC power to charge the battery. In the Chinese market, the prevalent OBCs come in two power variants: 3.3kW and 6.6kW. Due to their limited output current, these are termed “slow chargers”. On the other hand, DC charging piles are equipped with a high-power DC charging module, allowing them to convert grid AC power directly. This results in an output current that can exceed 100A, hence the moniker “fast charging”.

DC charging piles are diverse and can be classified based on various criteria such as power capacity, number of charging connectors, design, and installation techniques. A popular method of categorization is by design, where DC charging piles are either integrated or split.

Delving Deeper: How Does a DC Charging Pile Function?

A DC charging pile is a marvel of engineering, comprising several components:

  • Power Unit: This is essentially the DC charging module.
  • Control Unit: Often referred to as the charging pile controller, this unit governs the operations of the pile.
  • Metering Unit: Measures the amount of power being delivered.
  • Charging Interface: The point where the vehicle connects to the pile.
  • Power Supply Interface: Connects the pile to the power grid.
  • Human-Machine Interaction Interface: Typically a touchscreen, allowing users to interact with the pile.

The core of a DC charging pile lies in its “DC charging module” and “charging pile controller”. Beyond these, the structural design is crucial for the overall reliability of the pile. The controller, an embodiment of embedded hardware and software tech, and the DC charging module, a pinnacle of power electronics in AC/DC conversion, are the heart and soul of the system.

The charging process is straightforward: The battery is charged with a constant high current until its voltage reaches a certain threshold. Once the battery’s voltage hits its nominal value and its State of Charge (SoC) surpasses 95% (this can vary based on the battery type), the charging switches to a constant voltage mode with a reduced current.

A crucial point to note is that when the battery’s voltage is high but isn’t fully charged, it’s not in a charging state. If time permits, it’s advisable to switch to a low current charging mode. This entire process necessitates a “DC charging module” to supply the DC power and a “charging pile controller” to manage the module’s operations. The user interacts with the pile through a touchscreen, issuing commands that the controller then relays to the charging module.

From an electrical standpoint, a DC charging pile has a primary and a secondary circuit. The primary circuit takes in three-phase AC power, which, after passing through a circuit breaker and an AC smart energy meter, is converted into DC power by the charging module. This DC power is then used to charge the electric vehicle. The secondary circuit encompasses components like the charging post controller, card reader, and display screen. This circuit handles operations like “start-stop” control and provides status indications like “standby”, “charging”, and “full”.

Key Electrical Principles of DC Charging Posts:

  1. A single charging module, typically 15kW, often isn’t sufficient. Multiple modules need to work in tandem, requiring a CAN bus for equal current distribution.
  2. The charging module’s input is a high-power supply from the grid. Given the implications for grid stability and personal safety, protective measures like air switches and lightning or leakage switches are essential.
  3. The output from the charging pile is high voltage and current. Given the volatile nature of batteries, safety measures like fuses are crucial.
  4. Safety is paramount. Beyond input-side measures, mechanical and electronic locks, insulation checks, and discharge resistors are indispensable.
  5. The battery’s BMS (Battery Management System) dictates the charging process. It communicates with the controller, which in turn manages the charging module.
  6. For monitoring and management, the controller connects to a backend system via WiFi or 3G/4G.
  7. Charging isn’t free. Meters are installed for billing, and card readers facilitate payment.
  8. Clear indicator lights on the charging pile shell provide status updates.
  9. Proper air duct design is vital for cooling. Each charging module typically has its own fan, but an additional fan within the charging pile ensures optimal cooling.

In essence, the world of DC charging piles is a blend of cutting-edge technology and meticulous design, ensuring efficient and safe charging for electric vehicles.



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