An on-board charger , also known as an AC charger, is installed in an electric vehicle. It connects to an AC outlet via a socket and cable, providing charging power to the electric vehicle using three-phase or single-phase AC power. The advantage of an on-board charger is that it can charge the electric vehicle anytime, anywhere, as long as there is an AC outlet with the charger's rated voltage. The disadvantages of an on-board charger are that its power output is relatively low due to space limitations in the electric vehicle, resulting in a smaller charging current and a longer charging time. The connection between the on-board charger and the charging power supply is shown in Figure 6-1.
Principles and characteristics of on-board chargers for electric vehicles
On-board chargers are mainly used to replenish the power of small electric vehicles. They have a relatively small charging power and can be used to charge electric vehicles by utilizing AC charging piles built on roadsides and in residential areas, making full use of off-peak hours for charging.
1. Principle and Structure
The on-board charger input uses an active power factor correction circuit, the electrical principle of which is shown in Figure 6-16 and its appearance is shown in Figure 6-17.
2. Functional Features
(1) It has the ability to safely and automatically fully charge the power battery of electric vehicles. Based on the data provided by the battery management system (BMS), the charger can dynamically adjust the charging current or voltage parameters, perform corresponding actions, and complete the charging process.
(2) It has the function of high-speed CAN network and BMS communication, judges whether the battery connection status is correct, and obtains the battery system parameters and real-time data of the whole group and individual batteries before and during charging.
(3) It can communicate with the vehicle monitoring system through the high-speed CAN network, upload the working status, working parameters and fault alarm information of the charger, and receive the start charging or stop charging control command.
(4) Comprehensive safety protection measures.
1) AC input overvoltage protection function.
2) AC input undervoltage alarm function.
3) AC input overcurrent protection function.
4) DC output overcurrent protection function.
5) DC output short circuit protection function.
6) Output soft-start function to prevent current surges.
7) During the charging process, the charger can ensure that the temperature, charging voltage and current of the power battery do not exceed the allowable values.
8) It has a single-cell voltage limiting function and automatically adjusts the charging current dynamically based on the battery information of the BMS.
9) Automatically determines whether the charging connector and charging cable are correctly connected. The charger will only allow the charging process to start after it is correctly connected to the charging station and battery; if the charger detects an abnormal connection with the charging station or battery, it will immediately stop charging.
10) Charging interlock function to ensure that the vehicle cannot start before the charger is disconnected from the electric vehicle's power battery.
11) High voltage interlock function: When there is a high voltage that endangers personal safety, the module will lock and have no output.
12) It has flame retardant properties.
3. Technical Specifications
(1) Environmental conditions.
1) Operating temperature: -30~70℃ (above 50℃, the output power is limited to 50% of the rated power).
2) Relative humidity: 5%~95%.
3) Altitude: Not higher than 2000m.
(2) Communication input.
1) AC working voltage: 220V (1±20%) (single-phase three-wire).
2) AC operating frequency: (50±1) Hz.
3) Full-load power factor: not less than 0.99.
4) Total harmonic current distortion rate: not greater than 5%.
4. DC output
(1) Current stabilization accuracy: not exceeding ±0.5%.
(2) Voltage regulation accuracy: not exceeding ±0.5%.
(3) Ripple coefficient: not less than 0.5%.
(4) Full load efficiency: not less than 94%.
(5) Voltage range: 140～350V.
(6) Current range: 1 to 8A.
(7) Maximum power: 2.5kW (output can be controlled by constant power).
5. Structural protection
(1) Fully enclosed structure, with an enclosure protection rating of IP54.
(2) The metal casing and parts of the charger are treated with double-layer anti-rust treatment, and the non-metallic casing has an anti-oxidation protective film or is treated with anti-oxidation.
(3) The printed circuit boards and connectors inside the charger are treated to prevent moisture, mold, and smoke.
6. Mean Time Between Failures (MTBF)
Mean Time Between Failures (MTBF): Not less than 50,000 hours.
Classification and Requirements of Electric Vehicle On-board Chargers
(1) Vehicle-mounted DC-DC converter
A DC-DC converter is a technology that converts direct current (DC) into another type of DC, primarily transforming voltage and current. It plays a crucial role in energy conversion and transfer in electric vehicles. DC-DC converters are divided into unidirectional and bidirectional types. Unidirectional DC-DC converters allow energy to flow in only one direction, while bidirectional DC-DC converters maintain the same DC voltage polarity across the converter but change the direction of the current as needed, thus achieving bidirectional energy flow. Bidirectional DC-DC converters can also recover energy, broadening their application scope.
Electric vehicles include three main categories: pure electric vehicles, hybrid electric vehicles, and fuel cell electric vehicles. Onboard bidirectional DC-DC converters are a key technology for electric vehicles.
Hybrid fuel cell vehicles: Due to the current level of fuel cell development, fuel cells generally serve as the primary power source, while supercapacitors (UC) and high-voltage batteries (HVB, typically 100~500V) provide auxiliary power, forming a hybrid fuel cell vehicle, as shown in Figure 6-14. The bidirectional DC-DC converter acts as the power management unit for the supercapacitor and battery, providing auxiliary energy during vehicle acceleration, peak power during hill climbing, and energy recovery during deceleration/braking, thereby effectively improving energy utilization efficiency.
Pure fuel cell electric vehicles: The bidirectional DC-DC converter serves as the power management unit for the on-board battery (Low Voltage Battery, LVB, typically 12V or 24V). Its functions include: cold start of the fuel cell, providing power to the fuel cell air compressor, assisting the vehicle during acceleration, recovering energy during deceleration/braking, and improving the vehicle's acceleration and deceleration performance, as shown in Figure 6-16.
Pure electric vehicles: AC motor drive system: The bidirectional DC-DC converter regulates the DC-side voltage of the inverter, making field weakening speed regulation and regenerative braking easier to achieve, expanding the motor speed range, and improving system energy utilization efficiency, especially for the low-inductive reactance motors commonly used in electric vehicles. The bidirectional DC-DC converter directly drives the DC motor to operate in four quadrants.
Automotive electronics: The electrical systems in modern cars consume increasingly more electricity, with vehicle electrical systems including both 12V and 42V levels. A bidirectional DC-DC converter can be used for 12~42V dual-power conversion systems.
(2) Vehicle Charger
An on-board charger is a technology that converts alternating current (AC) to direct current (DC), transforming grid power into electrical energy for an electric vehicle's battery. Installed in the electric vehicle, the on-board high-voltage charger connects to an AC outlet via a plug and cable; therefore, it can also be called an AC charger. The advantage of an on-board charger is that it allows charging whenever the battery needs it, as long as there is an available power outlet. Its disadvantage is that it is limited by space constraints within the vehicle, thus limiting its power handling capacity and allowing only low-current, slow charging, resulting in a generally longer charging time.
A charger's basic components include a power unit, a control unit, an electrical interface, and a communication interface. The electrical interface includes the charger's power supply cable and connecting devices, the charging cable, and the charging connector. When charging an electric vehicle, the electric vehicle and the charging equipment must be correctly connected to ensure the safe transfer of electrical energy from the charging equipment to the electric vehicle under normal conditions. Even with some negligence during normal use, it should not pose a danger to the surrounding environment or people (especially the charging operator). Basic functional requirements include:
1) The charger should be able to charge one or more of the following types of batteries: lithium-ion batteries, nickel-metal hydride batteries, lead-acid batteries, etc.
2) During the charging process, the charger dynamically adjusts the charging parameters and performs corresponding actions based on the data provided by the battery management system to complete the charging process.
3) The charger should have the ability to communicate with the electric vehicle or battery management system. The purpose of this communication is to: determine the battery type; determine if the charger is correctly connected to the electric vehicle's battery system; obtain parameters of the electric vehicle's battery system, and the battery's status parameters before and during charging; the charger should also have the ability to communicate with the charging station's monitoring system.
Human-machine interaction (HMI) functions allow charging personnel to obtain information about the charger. The charger should display the following information: battery type, charging voltage, charging current, and energy metering information; appropriate prompts should be provided in case of malfunction; and battery temperature and charging time should be displayed. Tables 6-1 and 6-2 outline some technical requirements for on-board chargers.
