As a technician specializing in electric vehicle diagnostics, I recently encountered a challenging case involving a BYD EV model E5 that was unable to perform AC charging after a front-end collision repair. This issue is critical because, as the number of BYD cars on the road increases, charging failures can lead to stranded vehicles or inability to power on once the battery depletes below a certain level. In this article, I will detail my first-person experience in diagnosing and resolving this AC charging fault, focusing on the systematic approach that combines theoretical understanding with practical troubleshooting. I will extensively use tables and formulas to summarize key concepts, ensuring clarity for professionals working with BYD EV systems.
The fault manifested as an intermittent inability to initiate AC charging. When connecting the AC charging gun, the dashboard sometimes failed to display the charging connection indicator, while at other times, it showed the indicator but with a charging power of 0 kW. This inconsistency pointed to underlying issues in the charging system, which I hypothesized could involve components like the high-voltage control unit, battery management system (BMS), or related circuits. To begin, I verified the fault by testing with different charging stations and replacing the AC charging port, but the problem persisted. This ruled out external factors and directed my attention to the vehicle’s internal systems, particularly those unique to BYD EV models.
Understanding the AC charging system’s operation is essential for diagnosing such faults in a BYD car. The process involves multiple stages, from connection confirmation to power delivery, and relies on precise voltage and resistance measurements. For instance, the AC charging control circuit includes components like resistors and switches that facilitate communication between the charging station and the vehicle. A key aspect is the detection points, where voltages are monitored to confirm connections. For example, the voltage at detection point 1 can be modeled using a voltage divider formula. If we denote the supply voltage as $$ V_{supply} $$, and the resistances as $$ R_1 $$ and $$ R_3 $$, the voltage at detection point 1 ($$ V_{DP1} $$) is given by:
$$ V_{DP1} = V_{supply} \times \frac{R_3}{R_1 + R_3} $$
In a typical BYD EV setup, $$ R_1 $$ is approximately 1000 Ω and $$ R_3 $$ is 2740 Ω, so with a 12 V supply, $$ V_{DP1} $$ should be around 9 V when the connection is proper. Deviations from this indicate faults. Similarly, the resistance between detection point 3 and PE (protective earth) is critical for gun connection confirmation. The resistance value $$ R_{total} $$ can be expressed as:
$$ R_{total} = R_c + R_4 \quad \text{(when switch S3 is open)} $$
$$ R_{total} = R_c \quad \text{(when switch S3 is closed)} $$
where $$ R_c $$ is the cable resistance and $$ R_4 $$ is 2740 Ω. In the BYD car I worked on, these values were used to verify the integrity of the charging path.
To systematically analyze the fault, I broke down the AC charging process into steps and summarized them in a table. This helped in identifying where the failure might occur.
| Step | Description | Key Parameters | Expected Values |
|---|---|---|---|
| 1 | Charging station and gun connection | Voltage at detection point 4 | 0 V (ground) |
| 2 | Vehicle charging port and gun connection | Resistance at detection point 3 | $$ R_c + R_4 $$ or $$ R_c $$ |
| 3 | Full connection confirmation | Voltage at detection point 1 | 9 V |
| 4 | Vehicle readiness | PWM signal at detection point 2 | Pulse width modulation |
| 5 | Charging initiation | Current calculation | Min of station, cable, and charger limits |
In the specific BYD EV E5 model, the slow charging system integrates components like the high-voltage control unit, BMS, and body control module (BCM). When the AC charging gun is inserted, the high-voltage control unit communicates with the external charger and sends signals to BMS and BCM. The BCM then activates the dual-power relay, which supplies power to modules such as DC/DC converter, BMS, gateway, and instrument cluster. The BMS, upon waking, checks the charging connection signal and controls contactors for AC charging. The current $$ I_{charge} $$ allowed is determined by the minimum of the station’s capacity, cable limit, and onboard charger rating, which can be represented as:
$$ I_{charge} = \min(I_{station}, I_{cable}, I_{charger}) $$
where $$ I_{cable} $$ is derived from the resistance $$ R_c $$, often using a relation like $$ I_{cable} = \frac{k}{R_c} $$ for a constant k. In BYD cars, this ensures safe charging operations.
Moving to the diagnostic phase, I used a multimeter and diagnostic tools to check voltages and resistances in the charging circuit. The initial symptom—intermittent charging indicator—suggested a loose connection or faulty component. I focused on the dual-power relay circuit, as it controls power to critical modules. The relay’s operation can be modeled with Ohm’s law; for instance, the coil current $$ I_{coil} $$ is given by:
$$ I_{coil} = \frac{V_{supply}}{R_{coil}} $$
where $$ R_{coil} $$ is the coil resistance. In this BYD EV, the dual-power relay 1 (IG3 relay) should receive 12 V from the BCM to close the switch and supply power to F2/32 fuse, which powers BMS and VTOG. However, my measurements showed 0 V at the relay coil input, indicating a break in the control path.
To organize my findings, I created a table summarizing the key measurements and their implications.
| Component | Measurement | Expected Value | Actual Value | Interpretation |
|---|---|---|---|---|
| Fuse F2/32 | Voltage | ~12 V | 0 V | No power to BMS/VTOG |
| Dual-power relay coil | Voltage input | ~12 V | 0 V | BCM not outputting control signal |
| BCM G2P-5 to G77-1 | Resistance | <1 Ω | <1 Ω | Wire intact |
| VTOG to BCM signal line | Resistance | <1 Ω | Infinite (intermittent) | Loose connection |
| BMS BK45(B)-18 | Resistance to high-voltage control | <1 Ω | Infinite | Disconnected terminal |
Further investigation revealed that the VTOG sends a wake-up signal to the BCM via a specific wire, which in this BYD EV was found to have high resistance due to a loose connector at GJB05 pin 26. This explained the intermittent charging indicator. Additionally, the BMS charging sense signal line was open-circuited at terminal BK45(B)-18, likely damaged during the collision. The resistance here should ideally be low, as per the formula for continuity:
$$ R = \frac{V}{I} $$
where a high R indicates an open circuit. Repairing these connections restored the charging functionality.
In summary, the fault in this BYD car was multi-faceted: a loose connector disrupted the BCM’s control output, while a disconnected terminal prevented the BMS from sensing the charging signal. This case underscores the importance of a methodical approach in diagnosing BYD EV charging systems, combining circuit analysis with practical checks. The integration of formulas and tables, as shown, can streamline troubleshooting and ensure reliable repairs for the growing fleet of BYD cars on the road.