As an automotive technician specializing in electric vehicles, I recently encountered a challenging case involving a 2019 BYD Qin EV that presented with a critical high-voltage system failure. The BYD car, a popular model in the BYD EV lineup, exhibited symptoms where the OK indicator light did not illuminate, the dashboard displayed an “EV function limited” warning, and the vehicle could not engage high voltage or shift into gear for driving. This issue not only rendered the BYD EV inoperable but also highlighted the complexities of modern electric vehicle systems. In this detailed account, I will walk through the entire diagnostic and repair process, emphasizing key insights and technical aspects that can aid in handling similar faults in BYD EV models.
The initial assessment of this BYD EV began with a thorough inspection of the vehicle’s condition. Upon receiving the BYD car, I confirmed the reported symptoms: pressing the brake pedal and start button resulted in no OK light, the persistent “EV function limited” message, and an inability to activate the high-voltage system or change gears. This BYD EV, with a VIN ending in 0055 and only 327 km on the odometer, suggested a relatively new vehicle with potential underlying issues in its power delivery system. The high-voltage system in this BYD EV comprises a lithium-ion battery pack and a battery management system (BMS), which are critical for supplying power to the electric motor and ancillary systems. The battery pack outputs a nominal voltage of 408.8 V DC with a capacity of 130 Ah, providing approximately 53.1 kWh of energy and supporting a NEDC range of 400 km. Understanding these specifications is essential for diagnosing faults in any BYD EV, as deviations from expected parameters can lead to systemic failures.
To systematically address the problem, I first performed a conventional diagnostic approach. This involved checking the low-voltage 12V battery, as it powers the control systems in the BYD car. Using a multimeter, I measured the battery voltage at 11.2 V, which falls within the acceptable range for a BYD EV. Next, I disconnected the negative terminal of the 12V battery, waited 10 minutes to ensure any residual high-voltage energy dissipated, and donned insulated gloves to inspect high-voltage cable connections. No loose, damaged, or improperly installed components were found, and all管路 were correctly positioned. Notably, the state of charge (SOC) was displayed on the dashboard, indicating that the battery pack itself was functional in this BYD EV. However, the persistence of the high-voltage issue pointed toward deeper electrical or control system faults.
Possible causes for the inability to engage high voltage in this BYD car were extensive and included: motor drive system malfunctions, faults in the battery and BMS, network communication errors, high-voltage safety protection system failures, issues with the multi-integrated control unit (MICU), starter system problems, or cooling system anomalies. Given the complexity, I prioritized the diagnostic sequence to avoid unnecessary steps. After reconnecting the 12V battery, I used a diagnostic scanner to interface with the vehicle’s onboard systems. Clearing existing fault codes initially resolved the “EV function limited” warning, but restarting the BYD EV caused it to reappear. Data stream analysis revealed a specific fault code related to the main contactor loop detection, which directly prevents high-voltage engagement in BYD EV models. This fault indicated that the BMS was unable to verify the proper operation of the main contactor, a critical component that connects the high-voltage battery to the vehicle’s powertrain.
The operational flow of the BMS in a BYD EV involves several key steps. Upon ignition, the start network controls the dual-power supply (IG3 power), energizing the BMS. The BMS then communicates via the CAN bus, receives a start notification, and performs self-checks. If successful, it first engages the pre-charge contactor in the high-voltage distribution box to manage inrush currents. After pre-charging, it disconnects the pre-charge contactor and activates the main contactor, allowing battery power to flow to the motor controller and DC-DC converter for low-voltage system support. In this BYD car, the main contactor loop detection fault disrupted this sequence, preventing high-voltage activation. Potential root causes included a defective main contactor, faults in the contactor wiring or本体, or a malfunctioning BMS unit itself.
To isolate the issue, I proceeded with targeted tests. First, I disconnected the 12V battery again and waited for safety. After removing the high-voltage cable plug and BMS connector, I measured the resistance between pins BK45(A)/7 and BK45(A)/22 on the BMS harness. The reading was 26 Ω, matching the standard value for a BYD EV, which ruled out an open or short circuit in the contactor coil. Next, I reconnected the components and measured the voltage between pin BK45(A)/22 and ground while attempting to start the BYD car. The voltage was 0.7 V, whereas it should have been near 0 V if the contactor was properly disengaged. This anomaly suggested an internal BMS fault, as the control signal was not as expected. Further resistance checks with the BMS disconnected showed infinite resistance, confirming no external short circuits.
Given these findings, I focused on the BMS. After removing and disassembling the BMS unit from the BYD EV, I observed severe burn damage on an internal chip. This chip was identified as a dual MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), which is responsible for switching the main contactor coil. The failure of this MOSFET in the BMS of the BYD car prevented the contactor from engaging, leading to the high-voltage fault. To confirm the MOSFET’s role, I analyzed its pin configuration: the drain (D), source (S), and gate (G) terminals. Using a multimeter, I verified the parasitic diode between D and S, which is characteristic of N-channel MOSFETs commonly used in BYD EV systems. The gate controls the switching action, and a burned MOSFET would fail to conduct, keeping the contactor open.
For replacement, I selected alternative MOSFETs based on key parameters to ensure compatibility with the BYD EV’s electrical requirements. The original chip was compared with the IRFB3607 model, as summarized in the table below:
| Parameter | Original MOSFET | IRFB3607 |
|---|---|---|
| Drain-Source Voltage (VDS) | 60 V | 75 V |
| Gate Threshold Voltage (VGS(th)) | 2.1 V | 2 V |
| Continuous Drain Current (ID) | 16 A | 80 A |
| Junction Temperature (TJ) | 25°C | 175°C |
| Output Capacitance (Coss) | 89 pF | 150 pF |
| Drain-Source Resistance (Rds(on)) | 55 mΩ | 9 mΩ |
| Channel Type | N-channel | N-channel |
The IRFB3607 MOSFETs were suitable due to their higher voltage and current ratings, which enhance reliability in BYD EV applications. To proceed, I referenced the BMS control circuit diagram for this BYD car and derived a simplified electrical connection schematic. The MOSFETs act as switches: when the BMS sends a control signal to the gate, it allows current to flow from the drain to the source, energizing the main contactor coil. The relationship for the contactor coil current can be expressed using Ohm’s law: $$I = \frac{V}{R}$$ where I is the current, V is the supply voltage, and R is the coil resistance. In this BYD EV, the measured resistance of 26 Ω implies a nominal current, but a faulty MOSFET would disrupt this, as seen in the abnormal 0.7 V reading.
Soldering the new MOSFETs required careful attention. I first extracted the damaged chip and soldered leads to the BMS board’s base pins. Then, I connected the drain, source, and gate terminals of the IRFB3607 MOSFETs to the corresponding leads, ensuring secure electrical connections. Finally, I mounted the MOSFETs onto the BMS housing for heat dissipation and mechanical stability. After reassembly, I tested the BYD EV: the OK light illuminated, the “EV function limited” warning cleared, and the vehicle could shift gears and drive normally, confirming the fault was resolved.
Analyzing the root cause, the burned MOSFET in the BMS of this BYD car prevented the main contactor from receiving the activation signal. The BMS detected this as a loop fault through its feedback机制, triggering the error code and inhibiting high-voltage engagement. This highlights the importance of robust component selection and thermal management in BYD EV systems. In summary, repairing high-voltage issues in BYD EV models demands a methodical approach. Key lessons include: thoroughly understanding the BMS and power system architecture in any BYD car; leveraging diagnostic tools for data stream analysis; inspecting all connections for integrity; and prioritizing safety with insulated gear when handling high-voltage components. By sharing this experience, I aim to provide a reference for technicians working on similar BYD EV failures, ultimately supporting the reliability and performance of these advanced electric vehicles.
In deeper reflection, the integration of power electronics in BYD EV designs necessitates familiarity with semiconductor devices like MOSFETs. Their switching characteristics can be modeled using equations such as the drain current formula for saturation region: $$I_D = \frac{1}{2} \mu_n C_{ox} \frac{W}{L} (V_{GS} – V_{th})^2$$ where ID is the drain current, μn is the electron mobility, Cox is the oxide capacitance, W/L is the aspect ratio, VGS is the gate-source voltage, and Vth is the threshold voltage. In this BYD car, deviations in these parameters due to component failure led to system-wide issues. Additionally, regular maintenance checks on BYD EV models should include verifying contactor operation and BMS software updates to preempt such faults. As electric vehicles evolve, continuous learning and adaptation are crucial for effective troubleshooting in the BYD EV ecosystem.