As a researcher in automotive engineering, I have dedicated significant time to studying the challenges surrounding electric vehicle (EV) charging systems. One prevalent issue that frequently arises in EV maintenance is the failure of alternating current (AC) slow charging. This problem not only impacts user experience but also poses risks to charging safety. In this article, I will delve into the composition, working principles, common fault causes, and diagnostic procedures of AC slow charging systems, leveraging tables and formulas to enhance clarity and technical precision, with a consistent focus on the keyword “electric vehicle.”
1. Composition of the AC Slow Charging System
The AC slow charging system of an electric vehicle comprises several critical components, each playing a distinct role in facilitating safe and efficient charging. Below is a detailed breakdown:
1.1 Power Supply Units
These provide the AC power source for charging, including:
- Portable Charging Guns: Connected to a 220V household socket, available in 1.8 kW (10A socket, wire cross-sectional area ≥1.5 mm²) and 3.3 kW (16A socket, wire cross-sectional area ≥2.5 mm²) variants .
- AC Charging Piles/Wall-Mounted Charging Boxes: Offer higher power options (3.3 kW and 7 kW). Examples of vehicles supporting 7 kW include BYD Han, Yuan PLUS, and GAC Aion series .
1.2 Charging Connection Devices
These ensure electrical and mechanical connectivity between the power supply and the vehicle:
- Components: Include supply interfaces (plugs/sockets), vehicle interfaces (plugs/sockets), cables, and on-cable control and protection devices.
- Standards: Predominantly follow the Chinese national standard GB/T 20234.2-2015, featuring a 7-pin interface (5 large, 2 small) for slow charging. The pin definitions are summarized in Table 1 .
| Pin Symbol | Pin Name | Function |
|---|---|---|
| CC | Charging Confirmation | Monitors charging connection status |
| CP | Control Pilot | Transmits control signals |
| PE | Protective Earth | Ground connection for safety |
| L1 | Single-Phase AC | Phase line (single-phase) |
| N | Neutral | Neutral line |
1.3 On-Board Charger (OBC)
The OBC converts AC power from the grid to high-voltage DC for the battery. Its hardware includes:
- Input Filter & Rectifier Board: Filters electromagnetic interference (EMI) and rectifies AC to DC.
- Power Conversion Board: Uses a power factor correction (PFC) circuit and resonant converter to adjust voltage and current, ensuring stable DC output .
2. Working Principles of the AC Slow Charging System
The core of the slow charging system lies in its charging control pilot (CP) circuit, which manages connection status, current regulation, and safety monitoring. Below is a detailed analysis of its operational phases.
2.1 Charging Connection Confirmation
The system uses three voltage detection points (S1, S2, S3) to verify the charging gun’s connection status. The relationship between the charging gun’s state and the resistance measured at CC (Detection Point 3) is shown in Table 2 .
| Connection Status | S3 State | Resistance (CC-PE) |
|---|---|---|
| Not connected | Closed | ∞ |
| Partially connected | Open | RC + R4 |
| Fully connected | Closed | RC |
Here, RC and R4 are resistors in the charging gun, with values dependent on the cable’s current rating (e.g., 1.5 kΩ for RC in a 10A cable) .
2.2 Maximum Allowable Charging Current Determination
The vehicle control unit (VCU) calculates the maximum charging current by analyzing the PWM duty cycle at Detection Point 2 (CP signal). The relationship between the duty cycle (D) and current (I_max) is defined by the following formulas and Table 3 :
- For \(8\% \leq D < 10\%\): \(I_{\text{max}} = 6 \, \text{A}\)
- For \(10\% \leq D \leq 85\%\): \(I_{\text{max}} = D \times 100 \times 0.6 \, \text{A}\)
- For \(85\% < D \leq 90\%\): \(I_{\text{max}} = (D \times 100 – 64) \times 2.5 \, \text{A} \, (\leq 63 \, \text{A})\)
| PWM Duty Cycle (D) | Maximum Charging Current (I_max) |
|---|---|
| \(3\% \leq D < 7\%\) | Charging not allowed |
| \(8\% \leq D < 10\%\) | 6 A |
| \(10\% \leq D \leq 85\%\) | \(D \times 60\) A |
| \(85\% < D \leq 90\%\) | \((100D – 64) \times 2.5\) A (≤63 A) |
2.3 System Readiness Check
- Charging Pile Readiness: When the charging gun is connected, the pile sends a 9V PWM signal via CP (Detection Point 1), indicating readiness .
- Vehicle Readiness: The VCU wakes up the battery management system (BMS) and OBC. The BMS checks parameters (e.g., temperature, voltage) and sends feedback. If no faults are detected, the OBC closes switch S2, pulling the CP voltage down to 6V PWM to signal readiness .
3. Common Fault Causes for AC Slow Charging Failure
Understanding the root causes of charging failure is critical for effective diagnostics. Below is a categorized list of typical issues –.
3.1 High-Voltage Power Supply Failures
- Faults: High-voltage interlock issues, leakage/insulation failures, relay malfunctions, control unit (e.g., VCU, BMS) faults, or CAN communication errors.
- Impact: Prevent the vehicle from powering up the high-voltage system, a prerequisite for charging.
3.2 Power Supply Unit Failures
- Household Socket Issues: Lack of grounding or poor grounding (voltage difference >5V between live and ground wires).
- Charging Pile Faults: Damaged internal components (e.g., circuit breakers, fuses, relays) or failed self-tests (indicated by fault lights).
3.3 Charging Connection Device Failures
- Physical Damage: Corroded or burnt pins in the charging port or gun.
- Electrical Faults: High contact resistance in L/N/PE pins, broken internal wires, or faulty lock switches in the charging gun.
- Communication Failures: Abnormal CC/CP signal voltages or resistances (e.g., CC-PE resistance not matching RC/R4 values).
3.4 Other Component Failures
- On-Board Charger (OBC): Blown fuses, damaged MOSFETs, or 12V power supply issues.
- VCU/BMS: Faults preventing communication or improper control signals (e.g., no charging request sent).
- Battery Pack: Low SOC, unbalanced cells, or insulation resistance degradation.
4. Diagnostic Procedures and Troubleshooting Steps
To systematically address charging failures, follow the four-step diagnostic flow outlined below, supported by practical testing methods.
4.1 Step 1: Verify High-Voltage Power Supply
- Check Indicator Lights: If the “READY” light does not illuminate, the high-voltage system has failed to power up.
- Diagnostic Tools: Use a scan tool to read fault codes and data streams, inspecting high-voltage cables, relays, and fuses. If issues persist, check the OBC, VCU, and BMS .
4.2 Step 2: Test Power Supply Units
- Portable Charger Check: Measure live-neutral and live-ground voltages with a multimeter. A >5V difference indicates poor grounding .
- Charging Pile Check: Observe status lights (e.g., steady green for normal operation) and test with another EV to confirm pile functionality. Inspect internal components (e.g., fuses) if faulty .
4.3 Step 3: Inspect Charging Connection Devices
- Visual Inspection: Look for burnt or corroded pins in the charging port and gun.
- Electrical Testing:
- CP Signal: Measure CP-PE voltage (12V indicates a normal gun-side signal).
- CC Signal: Check CC-PE resistance (should equal RC or RC+R4 depending on the gun’s lock state) –.
- Vehicle-Side Check: Measure CC-PE voltage (5V normal) and CP-PE resistance (14.23 kΩ normal) –.
4.4 Step 4: Evaluate Other Components
- OBC Diagnosis: Use a scan tool to monitor OBC data (e.g., AC voltage, DC current, temperature). Inspect internal fuses and MOSFETs if anomalies are detected .
- VCU/BMS Diagnosis: Scan for fault codes and analyze data streams (e.g., VCU’s charging request signals, BMS’s battery parameters). Check wiring and sensor connections for BMS issues –.
- Battery Pack: Test SOC, cell voltages, and insulation resistance. Address balance issues or physical damage as needed –.
5. Conclusion
Diagnosing AC slow charging failures in electric vehicles requires a deep understanding of the charging control circuit and a systematic approach to fault isolation. By following the outer-in, simple-complex principle—starting with high-voltage power supply checks, then evaluating power sources, connection devices, and finally internal components—technicians can efficiently resolve issues. Key takeaways include:
- The CP/CC signals are critical for charging system communication.
- Grounding issues in power supplies are a common culprit.
- Data stream analysis and multimeter testing are indispensable tools.
As electric vehicle adoption grows, mastering these diagnostic techniques is essential for ensuring reliable and safe charging infrastructure, thereby enhancing user confidence in EV technology.