If the power battery management system malfunctions, it loses monitoring of the battery and cannot estimate the battery's state of charge. This can easily lead to an increase in battery overcharging, over-discharging, overload, overheating, and inconsistency issues, affecting battery performance, lifespan, and even driving safety.
The following lists common fault types in the power battery management system of electric vehicles, provides a simple analysis of their possible causes, and offers common analytical approaches and troubleshooting methods for reference.
Common Fault Types and Troubleshooting Methods in Power Battery Management Systems
Common fault types of the power battery management system (BMS) include: CAN system communication failure, BMS malfunction, abnormal voltage acquisition, abnormal temperature acquisition, insulation failure, internal and external total voltage measurement failure, pre-charge failure, inability to charge, abnormal current display failure, and high voltage interlock failure.
1. CAN communication failure
A detached CAN cable or power cable, or a disconnected terminal pin, can cause communication failures. With the BMS powered on, set the multimeter to DC voltage mode. Touch the red probe to the internal CANH and the black probe to the internal CANL, and measure the output voltage of the communication line, specifically the voltage between CANH and CANL. A normal voltage value is approximately 1.5V. If the voltage value is abnormal, it indicates a BMS hardware failure, requiring replacement.
2. BMS is not working properly.
When this phenomenon occurs, the following aspects should be considered:
(1) BMS power supply voltage: First, measure the power supply voltage of the vehicle to the BMS at the vehicle connector to see if there is a stable output.
(2) Unreliable CAN cable or low-voltage power supply cable connection: An unreliable CAN cable or power output cable connection will cause communication failure. The communication cable and power cable from the motherboard to the slave board or high-voltage board should be checked. If any loose or disconnected cable bundles are found, they should be replaced or reconnected.
(3) Removal or damage to connector pins: Removal of pins from low-voltage communication aviation connectors will cause no power to the slave board or data from the slave board to be unable to be transmitted to the motherboard. The connectors and plugs should be checked and replaced if any pins are found to be removed or damaged.
(4) Control motherboard: Replace the board for monitoring. If the fault is resolved after replacement, it is determined that the motherboard is faulty.
3. Abnormal voltage acquisition
When abnormal voltage acquisition occurs, the following situations should be considered:
(1) The battery itself is undervoltage: Compare the monitored voltage value with the actual voltage value measured by the multimeter, and replace the battery after confirmation.
(2) Loose bolts on the acquisition line terminals or poor contact between the acquisition line and the terminals: Loose bolts or poor contact between the terminals will cause inaccurate acquisition of individual voltages. In this case, gently shake the acquisition terminals to confirm poor contact, then tighten or replace the acquisition line.
(3) Damaged fuse on the acquisition line: Measure the resistance of the fuse. If it is above 1 S2, it needs to be replaced.
(4) Slave board detection problem: Confirm that the collected voltage is inconsistent with the actual voltage. If the collected voltage of other slave boards is consistent with the battery voltage, the slave board needs to be replaced and field data collected. Historical fault data should be read and analyzed.
4. Temperature acquisition anomaly
When abnormal temperature acquisition occurs, the following situations should be considered:
(1) Temperature sensor failure: If a single temperature data is missing, check the intermediate connector. If there is no abnormal connection, it can be determined that the sensor is damaged and can be replaced.
(2) Unreliable connection of temperature sensor harness: Check the intermediate docking plug or control port temperature sensor harness. If it is found to be loose or detached, the harness should be replaced.
(3) BMS hardware failure: If monitoring shows that the BMS cannot collect the overall temperature and it is confirmed that the wiring harness from the control harness to the adapter to the temperature sensor probe is normal, it can be determined that there is a hardware problem with the BMS and the corresponding slave board should be replaced.
(4) Reload power after replacing slave board: Reload power after replacing faulty slave board, otherwise the monitoring value will show abnormality.
5. Insulation fault
In the power battery management system, short circuits between the inner core and outer casing of the connector in the working harness, or short circuits between a damaged high-voltage wire and the vehicle body, can lead to insulation failures. Similarly, short circuits between a damaged voltage acquisition wire and the battery box can also cause insulation failures. For these situations, the following methods should be used for analysis, diagnosis, and repair:
(1) High-voltage load leakage: Disconnect DC/DC, PCU, charger, air conditioner, etc. in sequence until the fault is cleared, and then replace the faulty parts.
(2) Damaged high-voltage line or connector: Use a megohmmeter to measure and replace it after inspection and confirmation.
(3) Water ingress into the battery box or battery leakage: Treat the inside of the battery box or replace the battery.
(4) Damaged voltage acquisition line: After confirming that there is leakage inside the battery box, check the acquisition line. If it is found to be damaged, replace it.
(5) False alarm of high voltage board detection: Replace the high voltage board. If the fault is resolved after replacement, it is determined to be a high voltage board detection fault.
6. Nesab total voltage detection fault
Causes of total voltage detection failures can be categorized as follows: loose or detached connection between the acquisition wire and terminal, leading to total voltage acquisition failure; loose nuts causing arcing and total voltage acquisition failure; loose high-voltage connectors causing arcing and total voltage detection failure; and the maintenance switch being pressed, causing total voltage acquisition failure. In actual testing, the following methods can be used for repair and handling:
(1) The connection between the terminals at both ends of the total voltage acquisition line is unreliable: Use a multimeter to measure the total voltage at the detection point and compare it with the total voltage under monitoring. Then check the detection line and find that the connection is unreliable. Tighten or replace it.
(2) Abnormal high voltage circuit connection: Use a multimeter to measure the total voltage at the test point and compare it with the total voltage under monitoring. Then check and repair switches, bolts, connectors, fuses, etc. from the test point in sequence. If any abnormality is found, replace them.
(3) High-voltage board fault detection: Compare the actual total pressure with the monitored total pressure. If the total pressure returns to normal after replacing the high-voltage board, it can be determined that the high-voltage board is faulty and should be replaced.
7. Pre-charge failure
The causes of pre-charge failure can be categorized as follows: loose or detached external total voltage acquisition terminals leading to pre-charge failure; lack of 12V voltage on the main board control line causing the pre-charge relay to fail to close; and damage to the pre-charge resistor leading to pre-charge failure. Based on actual vehicles, the following categories can be checked respectively.
(1) External high voltage component failure: When the BMS reports a pre-charge failure, disconnect the main positive and main negative circuits. If the pre-charge is successful, the failure is caused by the external high voltage component. Check the high voltage junction box and PCU in sections.
(2) The motherboard cannot close the pre-charge relay: Check if the pre-charge relay has 12V voltage. If not, replace the motherboard. If the pre-charge is successful after replacement, the motherboard is confirmed to be faulty.
(3) Damaged main fuse or pre-charge resistor: Measure the conduction status and resistance value of the pre-charge fuse. If abnormal, replace it.
(4) High voltage board external total pressure detection failure: If the pre-charging is successful after replacing the high voltage board, the high voltage board is confirmed to be faulty and can be replaced.
8. Unable to charge
Charging failures can generally be summarized into two situations: First, the CAN bus pins at both ends of the connector may be retracted or detached, preventing communication between the motherboard and the charger, thus preventing charging. Second, a damaged charging fuse may prevent the charging circuit from forming, thus preventing charging from completing. In actual vehicle inspections, if a charging failure is encountered, the following aspects can be considered for troubleshooting:
(1) The charger and the motherboard do not communicate normally: Use an instrument to read the working data of the vehicle's CAN system. If no charger or BMS working data is found, immediately check the CAN communication harness. If there is poor contact of the connector or interruption of the line, repair it immediately.
(2) The charger or motherboard is faulty and cannot start normally: Replace the charger or motherboard and then reload the voltage. If it can charge after replacement, it can be determined that the charger or motherboard is faulty.
(3) If the BMS detects a fault, charging is not allowed: The fault type is determined by monitoring, and then the fault is resolved until charging is successful.
(4) The charging fuse is damaged and cannot form a charging circuit: Use a multimeter to check the continuity of the charging fuse. If it cannot be connected, replace it immediately.
9. Abnormal current display
Loose terminals or bolts in the control harness of the power battery management system, or oxidation on the surface of the terminals or bolts, can all cause current errors. When abnormal current display occurs, the installation of the current acquisition line should be thoroughly checked.
(1) The current acquisition line is not connected correctly: This will cause the current to be reversed. Replace the line.
(2) Unreliable connection of current acquisition line: First, ensure that there is a stable current in the high voltage circuit. When the monitoring current fluctuates greatly, check the current acquisition lines at both ends of the shunt. If the bolts are found to be loose, tighten them immediately.
(3) Check the oxidation of the terminal surface: First, ensure that there is a stable current in the high voltage circuit. When the monitored current is much lower than the actual current, check whether there is an oxide layer on the surface of the terminal or bolt. If so, treat the surface.
(4) Abnormal current detection of high voltage board: If the monitored current value is above 0 or 2A after disconnecting the maintenance switch, the current detection of high voltage board is abnormal and the high voltage board should be replaced.
10. High-voltage interlock fault
When the ON position is engaged, measure whether there is high voltage input, check whether the four terminals are securely connected, and measure whether there is 12V voltage at the drive terminal (the thin wire is the voltage drive wire). Depending on the specific situation, it can be divided into the following three categories:
(1) DC/DC fault: Measure the DC/DC high voltage input connector. If there is a short-term high voltage when the ON position is turned on, it is determined to be a DC/DC fault and should be replaced.
(2) DC/DC relay terminals are not securely connected: Check the high and low voltage terminals of the relay, and reconnect any unreliable terminals securely.
(3) Faulty motherboard or adapter board, causing DC/DC relay to not close: Measure the voltage drive terminal of DC/DC relay. If there is no 12V voltage for a short time when the ON position is turned on, replace the motherboard or adapter board.
