As a researcher in the field of electric vehicle maintenance, I have encountered numerous cases where electric cars fail to charge via AC slow charging systems. This issue is particularly prevalent in the rapidly growing China EV market, where the convenience and cost-effectiveness of slow charging make it a popular choice for private owners. However, when an electric car does not charge properly, it can lead to significant user frustration and potential safety hazards. In this article, I will delve into the intricacies of AC slow charging systems, analyze common failure causes, and present a detailed diagnostic methodology based on my practical experience. The goal is to provide technicians with a reliable framework for quickly identifying and resolving these faults, thereby enhancing the overall reliability of electric cars, especially in regions like China where EV adoption is accelerating.
The AC slow charging system in an electric car is designed to utilize standard household power sources or dedicated single-phase AC charging equipment. It typically involves converting 220 V AC power to high-voltage DC through an onboard charger, which then supplies the power to the battery pack. Charging times can range from 5 to 20 hours, depending on the battery capacity and charging rate. Understanding the components and operation of this system is essential for effective fault diagnosis. In my analysis, I will break down the system into key elements, including the power supply devices, charging connections, and control units, all of which play a critical role in ensuring seamless charging for electric cars. The increasing complexity of these systems in modern China EV models underscores the need for robust diagnostic approaches.
To begin, let me outline the fundamental components of an electric car’s AC slow charging system. The system primarily consists of the AC power supply device, the charging connection apparatus, the onboard charger, the power battery pack, and associated control units such as the battery management system. The power supply can be a standard 220 V AC socket paired with a portable charging cable or a dedicated AC charging station. Portable chargers, often referred to as “portable EVSE,” typically have power ratings of 1.8 kW or 3.3 kW, requiring specific socket types and wire cross-sections to handle the current. For instance, a 1.8 kW charger uses a 10 A socket with a conductor cross-section of at least 1.5 mm², while a 3.3 kW charger requires a 16 A socket and a 2.5 mm² cross-section. In the China EV context, charging stations with powers of 3.3 kW and 7 kW are common, supporting models like those from BYD and other local manufacturers. The charging connection includes interfaces and cables, with standards such as GB/T 20234.2-2015 being prevalent in China EV models, featuring a 7-pin configuration for safe and efficient power transfer.
| Terminal Symbol | CC | CP | PE | L1 | L2 | L3 | N |
|---|---|---|---|---|---|---|---|
| Terminal Name | Charge Connection Confirm | Control Pilot Signal | Earth Line | Single/Three-Phase AC Power | Three-Phase AC Power | Three-Phase AC Power | Neutral Line |
The onboard charger is a critical component that converts AC to DC. It comprises input filtering and rectification circuits, power conversion stages, and control logic. The process begins with EMI filtering to eliminate interference, followed by power factor correction to optimize voltage and current alignment. Then, a resonant circuit with switches and transformers steps up the voltage, and finally, rectification and filtering produce the high-voltage DC output for the battery. In electric cars, this charger must handle varying input conditions while maintaining efficiency and safety. The control units, including the vehicle control unit and battery management system, coordinate the charging process by monitoring parameters and ensuring compatibility. For China EV models, adherence to local standards is crucial, and failures here can lead to charging interruptions.
Now, let me explain the working principle of the AC slow charging system, focusing on the charge control guide circuit. This circuit is pivotal for establishing a safe and reliable connection between the electric car and the charging infrastructure. It involves multiple detection points, resistors, and switches that communicate the charging status and capabilities. The circuit includes detection points 1, 2, and 3, which monitor voltages and resistances to determine connection integrity, cable capacity, and maximum charging current. For example, detection point 3 measures the resistance between the CC and PE terminals to confirm whether the charging gun is fully connected. The resistance values vary with the connection state: when not connected, it is infinite; in a semi-connected state, it is the sum of RC and R4; and when fully connected, it equals RC. This mechanism ensures that the electric car only charges when a secure connection is established, reducing risks in the China EV ecosystem.
| Parameter | Not Connected | Semi-Connected | Fully Connected |
|---|---|---|---|
| S3 State | Closed | Open | Closed |
| Resistance Between Detection Point 3 and PE | Infinite | RC + R4 | RC |
The vehicle control unit uses detection point 3 to ascertain the cable’s rated capacity by measuring the resistance, which depends on RC and R4 values. These resistances correlate with the cable’s current-carrying capability, as shown in the table below. For instance, a 10 A cable has RC = 1500 Ω and R4 = 1800 Ω, while a 32 A cable has RC = 220 Ω and R4 = 3300 Ω. This information helps the electric car’s system determine the maximum allowable input current, ensuring that charging does not exceed the cable’s limits. In China EV applications, this is vital for preventing overheating and damage.
| Parameter | 10 A | 16 A | 32 A | 63 A |
|---|---|---|---|---|
| RC Resistance (Ω) | 1500 | 680 | 220 | 100 |
| R4 Resistance (Ω) | 1800 | 2700 | 3300 | 3300 |
Additionally, the PWM signal at detection point 2 defines the maximum charging current based on its duty cycle. The relationship is given by the formula: for duty cycles between 10% and 85%, the maximum current Imax is calculated as Imax = (D × 100) × 0.6, where D is the duty cycle. This can be expressed mathematically as:
$$ I_{\text{max}} = (D \times 100) \times 0.6 $$
For example, if D = 50%, then Imax = 30 A. The table below summarizes this relationship, which is standardized for electric cars to ensure interoperability across different charging stations, including those in the China EV network.
| PWM Duty Cycle D | Maximum Charging Current Imax (A) |
|---|---|
| D < 3% | Charging Not Allowed |
| 3% ≤ D ≤ 7% | Requires Digital Communication |
| 7% < D < 8% | Charging Not Allowed |
| 8% ≤ D < 10% | 6 |
| 10% ≤ D ≤ 85% | (D × 100) × 0.6 |
| 85% < D ≤ 90% | (D × 100 – 64) × 2.5, ≤ 63 |
| 90% < D ≤ 97% | Reserved |
| D > 97% | Charging Not Allowed |
Once the connection is verified, the vehicle and charging station engage in a handshake process. The onboard charger activates, waking up control units like the VCU and BMS. These units perform self-checks to ensure conditions such as battery temperature and high-voltage interlock are within safe limits. If no faults are detected, the S2 switch closes, pulling the voltage at detection point 1 from 9 V PWM to 6 V PWM, signaling the charging station that the electric car is ready. The station then closes its relays K1 and K2 to initiate power flow. Throughout charging, the system continuously monitors the connection and adjusts the charging current based on PWM signals. In China EV models, this process is optimized for local grid conditions, but faults can arise from various sources.
Based on my experience, common causes of AC slow charging failure in electric cars can be categorized into several areas. First, high-voltage system issues, such as faults in the high-voltage interlock, insulation failures, or relay malfunctions, can prevent the vehicle from powering up. Second, power supply problems, including faulty 220 V sockets without proper grounding or defective charging stations, are frequent culprits. In the China EV market, where infrastructure is still evolving, these issues are more pronounced. Third, connection apparatus failures, such as high contact resistance in charging port terminals, damaged charging guns, or broken internal resistors, can disrupt the control guide circuit. Lastly, component failures in the onboard charger, VCU, BMS, or communication errors can halt charging. For instance, if the onboard charger’s low-voltage supply is interrupted due to a blown fuse, the entire charging process fails. Similarly, communication faults between control units can lead to mismatched parameters, common in complex electric car systems.
To diagnose these faults, I have developed a systematic approach that proceeds from external checks to internal components. The first step is to verify if the electric car can achieve high-voltage power-up. If the OK or READY light on the dashboard does not illuminate, it indicates a high-voltage system fault. Technicians should use diagnostic tools to read fault codes and data streams, focusing on high-voltage relays, insulation resistance, and control unit communications. In China EV scenarios, this often involves checking for localized issues like battery management system errors. The second step involves inspecting the power supply. For portable chargers, test the 220 V socket for proper grounding using a multimeter. Measure the voltage between live and neutral, and between live and earth; a difference of less than 5 V indicates good grounding. For charging stations, check status indicators or test with another electric car to confirm functionality. If the station fails, internal components like circuit breakers or relays may need examination.
The third step is to examine the charging connection apparatus. Visually inspect the charging port and gun for signs of damage or corrosion. Then, use a multimeter to measure voltages and resistances. For the charging gun, set the multimeter to DC voltage mode and check the CP to PE voltage; it should be around 12 V. For the CC to PE resistance, it should match RC (e.g., 1500 Ω for a 10 A cable). Pressing the lock button should change the resistance to RC + R4. On the vehicle side, with the ignition on, the CC to PE voltage should be 5 V, and the CP to PE resistance should be approximately 14.23 kΩ. Deviations indicate faults in the gun or wiring, which are common in electric cars due to frequent use. In China EV models, ensuring compliance with local standards is key here.
The fourth step involves checking other components. Use diagnostic tools to monitor the onboard charger’s data streams, such as input voltage, current, and temperature. If anomalies are found, inspect internal fuses and MOSFETs. For the VCU and BMS, scan for fault codes and verify communication via CAN bus. Check battery parameters like voltage, state of charge, and insulation resistance. For example, the battery’s health can be assessed using the formula for state of health (SOH), often derived from capacity measurements:
$$ \text{SOH} = \frac{C_{\text{current}}}{C_{\text{nominal}}} \times 100\% $$
where C_current is the measured capacity and C_nominal is the original capacity. If SOH drops below 80%, it may affect charging. Additionally, inspect battery connections for looseness or corrosion, as these can cause intermittent failures in electric cars.
In conclusion, diagnosing AC slow charging failures in electric cars requires a methodical approach that combines theoretical knowledge with practical skills. By understanding the charge control guide circuit and following a step-by-step diagnostic流程, technicians can efficiently locate and resolve issues. This is especially important in the China EV market, where reliability and safety are paramount for consumer confidence. As electric car technology evolves, continuous learning and adaptation will be essential for maintaining these systems. I hope this article serves as a valuable resource for professionals working with electric cars, enabling them to enhance user experiences and contribute to the sustainable growth of the China EV industry.