High-Voltage Power-On Strategy and Fault Diagnosis for New Energy Vehicles

The high-voltage power-on process in new energy vehicles involves the traction battery supplying high-voltage electricity to vehicle high-voltage electrical equipment, including the high-voltage control box, motor controller, drive motor, etc. This critical process requires precise control strategies and robust fault diagnosis systems to ensure safety and reliability.

High-voltage system schematic in electric vehicle

Table of Contents

High-Voltage Power-On Process

MSD: Manual Service Disconnect - A safety switch that physically disconnects the high-voltage system for maintenance.

VCU: Vehicle Control Unit - The central controller managing vehicle functions.

BMS: Battery Management System - Monitors and manages the battery pack.

The high-voltage power-on process can be understood through the monitoring of three voltage points:

Voltage Monitoring Points:

V1 Monitors MSD connection quality and complete series circuit of traction battery

V2 Monitors voltage after precharge resistor

V3 Monitors precharge voltage to the load

By comparing V1, V2, and V3 voltage values, the connection status of various contactors can be determined.

Driving Mode Power-On Sequence:

VCU controls the closure of the negative contactor

BMS controls the precharge contactor

When precharge voltage reaches target value, precharge is deemed successful

Positive contactor closes

Precharge contactor opens

Driving mode high-voltage power-on process is complete

Voltage Sequence Analysis:

t1: MSD properly connected, battery modules in series correctly, V1 = battery rated voltage 500V

t2: Negative contactor closes, V2 is in series with precharge resistor, V2 voltage < V1

t3: Precharge contactor closes, battery begins precharging external high-voltage equipment, V2 and V3 are parallel, V2 voltage drops then both V2 and V3 rise together

t4: Precharge complete (V2 = V3 ≥ 90% V1), positive contactor closes

t5: Precharge contactor opens, power-on complete

Control Strategy

When the key is turned to ON position:

VCU wakes up and completes self-check

VCU sends first CAN message requesting closure of high-voltage interlock loop enable

MCU and BMS are awakened

BMS self-check completes normally

BMS monitors interlock loop signals

BMS detects high-voltage loop insulation condition

BMS checks battery SOC status, individual cell voltages, and battery temperature

System determines whether vehicle is in charging or driving mode

If all conditions are met, power-on program executes

Insulation Detection Method:

The traction battery high-voltage power source serves as the detection power source. A bridge impedance network is established between the positive/negative terminals of the traction battery and the vehicle chassis. By controlling the on/off states of electronic switches T1 and T2, the equivalent resistance between points A and B is changed. BMS calculates the insulation resistance value and determines insulation performance.

Fault Diagnosis

Common high-voltage power-on failures in new energy vehicles include failure to achieve high-voltage power-on, READY light not illuminating, etc. These issues occur across various brands and models with numerous potential causes.

Fault Categories:

Category	Description	Potential Causes
Category 1: Initialization Stage	Controllers not completing self-check	Low battery SOC, excessive cell voltage difference, battery over-temperature/under-temperature
Category 2: Safety Checks	Safety parameter violations	Insulation resistance too low, incomplete interlock circuit, missing low-voltage electrical signals
Category 3: Execution Stage	Contactors not operating correctly	Precharge failure or timeout due to abnormal contactor operation

Fault diagnosis example - precharge contactor adhesion

Fault Diagnosis Example 1: Precharge Contactor Adhesion

Fault: Occurs after negative contactor closes at time t2.

Detection: After negative contactor closes, V2 voltage < 50% of V1 voltage, but before t3 (precharge contactor should not yet be closed), V3 voltage gradually increases, indicating precharge has begun.

Diagnostic Condition: Negative contactor closed with V2 ≤ 50% V1, and 120-150ms later, V2 voltage reaches 80% of V1 voltage → Precharge contactor adhesion fault.

Fault diagnosis example - positive contactor open circuit

Fault Diagnosis Example 2: Positive Contactor Open Circuit

Fault: Occurs at time t4 after precharge completion.

Detection: Positive contactor fails to close normally. After 100ms, precharge contactor opens, precharge capacitor discharges through discharge resistor, V3 voltage decreases.

Diagnostic Condition: After closing positive relay and opening precharge relay, V3 voltage does not reach ≥95% of V1 voltage → Positive relay open circuit fault.

Other Potential Fault Points:

MSD not connected or fuse blown
Negative contactor adhesion
Precharge resistor burned out
Positive contactor adhesion

Conclusion & Recommendations

For resolving high-voltage power-on failures in new energy vehicles, based on analysis of power-on processes and control strategies:

Check controller self-completion status: Verify "ON", "CHG", or "WAKEUP" signals and CAN communication between controllers.

Address insulation faults: Inspect high-voltage wiring for damage, check high-voltage connectors for debris, separately measure insulation resistance of high-voltage components and wiring to chassis ground.

Resolve high-voltage interlock faults: Follow vehicle manual to check interlock circuit connections, verify circuit continuity and electrical signals.

Analyze execution stage faults: Collect and analyze V1, V2, V3 voltage data to determine if corresponding instructions execute at set times, thereby deducing fault location.

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Frequently Asked Questions

What is the purpose of precharging in NEV high-voltage systems?

Precharging gradually charges the capacitance in the high-voltage circuit before full power is applied. This prevents large inrush currents that could damage components, extends contactor life, and provides a soft-start for sensitive electronics like motor controllers.

What is a typical insulation resistance threshold for safe operation?

Most NEV systems require minimum insulation resistance of 500Ω/V for the high-voltage system. For a 400V system, this would be at least 200kΩ. Systems will prevent high-voltage power-on if insulation drops below these thresholds for safety reasons.

How does the high-voltage interlock (HVIL) system work?

HVIL creates a continuous low-voltage loop through all high-voltage connectors and components. If any high-voltage connector is disconnected, the loop opens, and the BMS detects this as a fault, preventing high-voltage power-on. This ensures all connections are secure before energizing the system.

What are common symptoms of high-voltage power-on failure?

Common symptoms include: vehicle won't go into READY mode, warning lights for high-voltage system, reduced power mode activation, inability to charge (for charging-related faults), and diagnostic trouble codes related to high-voltage contactors, insulation, or interlock circuits.

What safety precautions should technicians take when diagnosing high-voltage systems?

Always: 1) Wear appropriate PPE (insulated gloves, face shield), 2) Disconnect 12V auxiliary battery, 3) Wait specified time for capacitors to discharge (typically 5-10 minutes), 4) Verify no voltage present using approved voltage testers, 5) Properly isolate the high-voltage system before any disassembly.

How can I differentiate between a BMS fault and a contactor fault?

Use voltage measurements at key points (V1, V2, V3 as described). If voltage measurements follow expected patterns but contactors don't operate, suspect control/communication issues. If voltages deviate from expected patterns even with correct contactor operation, suspect measurement/sensing issues or actual electrical faults in the high-voltage circuit.