As an automotive technician specializing in electric vehicles, I recently encountered a challenging case involving a BYD EV that was unable to initiate high-voltage power. This BYD car, a 2020 BYD Qin EV model, exhibited normal low-voltage system operation but failed to engage the high-voltage system, with the dashboard displaying an “EV function restricted” warning. The vehicle had accumulated approximately 47,030 km and was equipped with a 100 kW permanent magnet motor. In this detailed account, I will walk through the diagnostic process, repair procedures, and key insights gained from resolving this issue, emphasizing the intricacies of BYD EV systems. Throughout this article, I will refer to the vehicle as a BYD EV or BYD car to maintain clarity and focus on the brand’s technology.
The initial step in diagnosing this BYD EV involved connecting a diagnostic scanner to perform a comprehensive system scan. The Battery Management System (BMS) stored a critical fault code: P1A3400, indicating a pre-charge failure. This code suggested that the high-voltage circuit could not complete the pre-charge sequence, which is essential for safely energizing the system in a BYD car. To gain deeper insights, I examined the data stream during attempts to activate the high-voltage system. The data revealed that the front drive motor bus voltage fluctuated cyclically, starting at 6 V, rising to 137 V, and then dropping back to 6 V, repeating this pattern four times. Such behavior pointed to an underlying insulation issue or a short circuit within the high-voltage components of this BYD EV. Insulation failures are particularly critical in electric vehicles like the BYD car, as they can compromise safety and system integrity.
To isolate the faulty component, I conducted a module-wide scan using the diagnostic tool. The scan identified the Positive Temperature Coefficient (PTC) heater module as the source of the problem, with two specific fault codes: B121209 (PTC drive component fault) and B121D09 (No. 4 insulated gate bipolar transistor drive chip functional failure). The PTC module in a BYD EV is responsible for cabin heating and relies on IGBTs to regulate power flow. By disconnecting the PTC high-voltage connector from the charging and distribution unit and short-circuiting the high-voltage interlock terminals, I simulated a bypass scenario. Upon pressing the start button with the brake pedal engaged, the BYD car successfully initiated high-voltage power, confirming that the PTC was the root cause of the failure in this BYD EV.
Further investigation involved resistance measurements across the PTC’s high-voltage terminals. Using a multimeter in ohmmeter mode, I measured a resistance of approximately 193.1 Ω between the positive and negative terminals, which was abnormal; under normal conditions, the resistance should be infinite, indicating no electrical path. This finding suggested an internal short circuit within the PTC of the BYD car. Given the cost implications of replacing the entire PTC assembly, the owner requested a repair instead. I disassembled the PTC unit and inspected its control board, which housed four IGBTs, each controlled by an independent drive chip managed by a central processing unit (CPU). The circuitry in a BYD EV is designed for precision, but faults like these can disrupt the entire high-voltage system.
Based on the resistance value and the fault codes, I hypothesized that one of the IGBTs had failed. IGBTs are crucial in BYD EV power electronics, as they switch high currents and voltages efficiently. Using the multimeter, I tested each IGBT’s terminals—gate, collector, and emitter—for resistance. The No. 4 IGBT showed measurable resistance between the gate and collector, as well as the gate and emitter, indicating a breakdown. In a functional IGBT, the gate should be isolated from the other terminals, with resistance approaching infinity. This failure aligned with the symptoms observed in the BYD car, where the pre-charge voltage was being pulled down prematurely.
To quantify the electrical relationships, consider the basic principles governing IGBT operation in a BYD EV. The voltage-current relationship for an IGBT can be described by: $$V_{CE} = V_{GE} – I_C \cdot R_G$$ where \(V_{CE}\) is the collector-emitter voltage, \(V_{GE}\) is the gate-emitter voltage, \(I_C\) is the collector current, and \(R_G\) is the gate resistance. In this BYD car, the faulty IGBT likely caused \(V_{CE}\) to drop abnormally, disrupting the pre-charge process. Additionally, the power dissipation in the IGBT can be modeled as: $$P_{loss} = I_C \cdot V_{CE} + \frac{1}{2} \cdot f_{sw} \cdot E_{sw}$$ where \(f_{sw}\) is the switching frequency and \(E_{sw}\) is the switching energy. Excessive power loss due to the short circuit could have contributed to the failure.
After replacing the No. 4 IGBT with a compatible component, I proceeded to test the associated drive chip. The drive chip in a BYD EV generates the gate signals for the IGBTs. I measured voltages between specific pins of the No. 4 drive chip: 5 V between pins 18 and 20, and 15 V between pins 6 and 1, which were within expected ranges. However, when I applied a 5 V square wave signal from a signal generator to pin 13 of the drive chip and used an oscilloscope to monitor the gate waveform of the new IGBT, the output was a flat line instead of the expected 15 V square wave. This indicated that the No. 4 drive chip was also defective in this BYD car. The drive chip’s role is critical in a BYD EV, as it ensures proper IGBT switching, and its failure can result from voltage spikes caused by the IGBT short circuit.
Replacing the No. 4 drive chip resolved the issue. I then performed an offline test to verify the repair. By connecting the PTC control board to a 12 V power supply and integrating it with a universal gateway and CAN bus system, I cleared the stored fault codes. For functional testing, I used a CAN replay device and connected load bulbs to the PTC board, with a high-voltage power supply set to 400 V. The bulbs illuminated correctly when toggling between 50% and 100% heating levels, confirming that the PTC was operational. This step is vital in BYD EV repairs to ensure components meet specifications before reassembly.
Upon reinstalling the PTC into the BYD car, I tested the high-voltage system: it engaged seamlessly, and a follow-up diagnostic scan showed no fault codes. The successful repair underscored the importance of systematic troubleshooting in BYD EV systems. To summarize the diagnostic data and measurements, I have compiled key findings into tables below, which highlight the parameters and outcomes relevant to this BYD car case.
| Module | Fault Code | Description |
|---|---|---|
| BMS | P1A3400 | Pre-charge failure |
| PTC | B121209 | PTC drive component fault |
| PTC | B121D09 | No. 4 IGBT drive chip functional failure |
| Component | Measurement Point | Resistance (Ω) | Normal Value |
|---|---|---|---|
| PTC High-Voltage Terminals | Positive to Negative | 193.1 | Infinite |
| No. 4 IGBT | Gate to Collector | Measurable | Infinite |
| No. 4 IGBT | Gate to Emitter | Measurable | Infinite |
The repair process for this BYD EV highlighted several technical aspects. The pre-charge sequence in a BYD car involves charging the high-voltage bus capacitors through a pre-charge resistor to limit inrush current. The voltage during pre-charge should follow: $$V_{precharge}(t) = V_{battery} \cdot (1 – e^{-t / \tau})$$ where \(\tau = R_{precharge} \cdot C_{bus}\) is the time constant, \(R_{precharge}\) is the pre-charge resistance, and \(C_{bus}\) is the bus capacitance. In this case, the faulty IGBT in the BYD EV caused \(V_{precharge}\) to deviate, leading to repeated failures. Moreover, the insulation resistance, which should be high to prevent leakage, can be calculated as: $$R_{insulation} = \frac{V_{test}}{I_{leakage}}$$ where \(V_{test}\) is the test voltage and \(I_{leakage}\) is the leakage current. The measured 193.1 Ω indicated severe insulation degradation in this BYD car.
In conclusion, this case demonstrates the complexity of high-voltage systems in BYD EV models. The failure of the No. 4 IGBT and its drive chip in the PTC module caused a cascade of issues, including pre-charge failures and insulation faults. Regular maintenance and advanced diagnostics are essential for BYD car owners to prevent such problems. By applying methodical testing and leveraging formulas for analysis, technicians can efficiently resolve issues in BYD EV vehicles, ensuring reliability and safety. This experience reinforces the value of understanding BYD EV architecture, particularly in high-power applications where components like IGBTs play a pivotal role.
| Test Step | Parameter | Measured Value | Expected Value |
|---|---|---|---|
| Drive Chip Voltage | Pins 18-20 Voltage | 5 V | 5 V |
| Drive Chip Voltage | Pins 6-1 Voltage | 15 V | 15 V |
| IGBT Gate Waveform | Output with Signal Input | Flat Line | 15 V Square Wave |
| Offline PTC Test | Load Bulb Illumination | Functional at 50% and 100% | Functional |
Reflecting on this repair, I emphasize that BYD EV systems require meticulous attention to detail. The integration of high-voltage components in a BYD car demands rigorous insulation checks and component-level repairs to avoid costly replacements. As electric vehicles like the BYD EV evolve, technicians must stay updated on diagnostic techniques and theoretical foundations, such as those involving IGBT behavior and pre-charge dynamics. This case not only resolved the immediate issue but also provided valuable insights for future encounters with BYD car models, underscoring the importance of a proactive approach in automotive electrification.