The battery management system (BATTERY MANAGEMENT SYSTEM), commonly known as battery nanny or battery housekeeper, is an important link between on-board power batteries and electric vehicles. Its main functions include: real-time monitoring of battery physical parameters; battery state estimation; online diagnosis and early warning; charging, Discharge and precharge control; balance management and thermal management, etc. The battery management system (BMS) is mainly to improve the utilization of the battery, prevent the battery from being overcharged and over-discharged, extend the service life of the battery, and monitor the status of the battery.

Battery Management System (BMS) function and role

From the perspective of the whole vehicle, the functions or tasks of the battery management system (BMS) can be detailed as follows:

1. Protect cells and battery packs from damage;

2. Make the battery work in a suitable voltage and temperature range;

3. After keeping the battery running under proper conditions, meet the needs of the entire vehicle;

4. Battery parameter detection: including total voltage, total current, cell voltage detection, temperature detection, insulation detection, collision detection, impedance detection, smoke detection, etc.;

5. Establishment of battery status: including SOC, SOH, SOF.

6. Online diagnosis: faults include sensor faults, network faults, battery faults, battery overcharge, overdischarge, overcurrent, insulation faults, etc.;

7. Battery safety protection and alarm: including temperature control system control and high voltage control. When a fault is diagnosed, the BMS reports the fault to the vehicle controller and charger, and cuts off the high voltage to protect the battery from damage, including leakage protection, etc.;

8. Charging control: BMS slow charging and fast charging control;

9. Battery consistency control: BMS collects cell voltage information, adopts a balancing method to make the battery consistent, and battery balancing methods include dissipative and non-dissipative;

10. Thermal management function: the collected temperature of each point of the battery pack, during charging and discharging, BMS decides whether to turn on heating and cooling;

11. Network function: including online calibration and health, online program download. Usually use CAN network;

12. Information storage: BMS needs to store key data such as SOC, SOH, charge and discharge ampere hours, fault codes, etc.

It can be seen that the battery management system is not only closely related to the battery, but also closely related to the vehicle system. Among all the faults, the battery management system is relatively high and difficult to deal with compared to other systems.

BMS failure analysis method

1. Observation method

When a communication interruption or control abnormality occurs in the system, observe whether each module of the system has an alarm and whether there is an alarm icon on the display screen, and then check the phenomenon one by one.

Failure recurrence method

The failures of vehicles under different conditions are different. When conditions permit, the failures should be reproduced under the same conditions as much as possible, and the problem points should be confirmed.

2. Elimination method

When a similar interference phenomenon occurs in the system, each component in the system should be removed one by one to determine which part has an impact on the system.

3. Replacement method

When a certain module has abnormalities in temperature, voltage, control, etc., change the position of the module with the same number of strings to diagnose the module problem or the wiring harness problem.

4. Environmental Inspection Law

When the system fails, such as the system cannot be displayed, we should not rush to carry out in-depth consideration, because often we will ignore some details. First of all, we should look at the obvious things: if it is plugged in? Is the switch turned on? Are all the wires connected? Perhaps the root of the problem lies in it.

5. Procedure upgrade method

When an unknown fault occurs after the new program is burned, which leads to abnormal system control, the previous version of the program can be burned for comparison to analyze and deal with the fault.

6. Data analysis method

When a control or related failure occurs in the BMS, the BMS stored data can be analyzed, and the message content in the CAN bus can be analyzed.

Analysis of 15 common failure cases

1. The whole system does not work after the system is powered

Possible Causes

Abnormal power supply, short circuit or open circuit in the wiring harness, DCDC no voltage output

Troubleshooting

Check whether the external power supply to the management system is normal, whether it can reach the minimum operating voltage required by the management system, and whether the external power supply is set to limit current, resulting in insufficient power supply to the management system; the external power supply can be adjusted to meet the requirements of the management system. Electricity requirements; check whether the harness of the management system is short-circuited or open, modify the harness to make it work normally; if the external power supply and harness are normal, check whether the DCDC powering the entire system in the management system has a voltage output; if so If abnormal, the bad DCDC module can be replaced.

2. BMS cannot communicate with ECU

Possible Causes

BMU (main control module) is not working, CAN signal line is disconnected

Troubleshooting

Check whether the 12V/24V power supply of the BMU is normal; check whether the CAN signal transmission line is withdrawn or the plug is not plugged in; monitor the CAN port data and whether the BMS or ECU data packet can be received.

3. The communication between BMS and ECU is unstable

Possible Causes

Poor matching of external CAN bus, too long bus branch

Troubleshooting

Check whether the bus matching resistance is correct; whether the matching position is correct, and whether the branch is too long.

4. The internal communication of the BMS is unstable

Possible Causes

The communication cable plug is loose, the CAN wiring is not standardized, and the BSU address is duplicated.

Troubleshooting

Check whether the wiring is loose; check whether the bus matching resistance is correct, whether the matching position is correct, and whether the branch is too long; check whether the BSU address is duplicated.

4. Insulation detection alarm

Possible Causes

The battery or drive is leaking. , The detection line of the insulation module is connected incorrectly.

Troubleshooting

Use the BDU display module to view the insulation test data, check the battery bus voltage and whether the negative bus voltage to ground is normal; use the insulation shaker to measure the insulation resistance of the bus and the drive to the ground.

5. The main relay does not close after power on

Possible Causes

The load detection line is not connected, the pre-charge relay is open, and the pre-charge resistor is open.

Troubleshooting

Use the BDU display module to check the bus voltage data, check the battery bus voltage, and whether the load bus voltage is normal; check whether the load bus voltage rises during the precharging process.

6. The data of the acquisition module is 0

Possible Causes

The collection line of the collection module is disconnected, and the collection module is damaged.

Troubleshooting

Re-plug the module wiring, measure whether the battery voltage is normal at the connector of the collection line, and measure the resistance value at the plug of the temperature sensor line.

7. Battery current data error

Possible Causes

The Hall signal cable plug is loose, the Hall sensor is damaged, and the acquisition module is damaged.

Troubleshooting

Re-plug the current Hall sensor signal line; check whether the power supply of the Hall sensor is normal and whether the signal output is normal; replace the acquisition module.

8. The battery temperature difference is too large

Possible Causes

The cooling fan plug is loose, and the cooling fan is faulty.

Troubleshooting

Unplug the fan plug cord again; supply power to the fan separately, and check whether the fan is normal.

9. The battery temperature is too high or too low

Possible Causes

The cooling fan plug is loose, the cooling fan is faulty, and the temperature probe is damaged.

Troubleshooting

Re-plug the fan plug wire; power the fan separately to check whether the fan is normal; check whether the actual temperature of the battery is too high or too low; measure the internal resistance of the temperature probe.

10. The system reports an error after the relay operates

Possible Causes

The relay auxiliary contacts are disconnected, and the relay contacts are sticking.

Troubleshooting

Re-plug the wiring harness; use a multimeter to measure whether the auxiliary contacts are on or off correctly.

11. Can't use charger to charge

Possible Causes

The communication between the charger and the BMS is abnormal

Troubleshooting

Replace a charger or BMS to confirm whether the BMS is faulty or the charger is faulty; check whether the matching resistance of the BMS charging port is normal.

12. The vehicle instrument has no BMS data display

Possible Causes

The main control module wiring harness connection is abnormal

Troubleshooting

Check whether the wiring harness of the main control module is fully connected, whether there is a normal low-voltage working voltage for the car, and whether the module is working properly

13. The detection data of some battery boxes is lost

Possible Causes

Some connectors of the vehicle may be in poor contact, or the BMS slave control module may not work normally

Troubleshooting

Check the contact of the connector, or replace the BMS module;.

14. SOC is abnormal

Phenomenon: SOC changes a lot during the working process of the system, or repeatedly jumps between several values; during the charging and discharging process of the system, SOC has a large deviation; SOC always shows a fixed value.

Possible Causes

The current is not calibrated; the current sensor model does not match the host program; the battery has not been deeply charged and discharged for a long time; the data acquisition module acquisition jumps, causing the SOC to perform automatic calibration;

Two conditions for SOC calibration: 1) Reach overcharge protection; 2) The average voltage reaches xxV or more. Customer battery consistency is poor, and the second condition cannot be achieved when overcharged. Check the remaining capacity and total capacity of the battery through the display; the current sensor is not connected correctly;

Troubleshooting:

Calibrate the current in the touch screen configuration page; modify the host program or replace the current sensor;

Perform a deep charge and discharge of the battery; replace the data acquisition module and manually calibrate the system SOC. It is recommended that the customer do a deep charge and discharge once a week; modify the host program and adjust the "average voltage above xxV" according to the actual situation of the customer. xxV. Set the correct total battery capacity and remaining capacity; correctly connect the current sensor to make it work normally;

15. BSU voltage collection is inaccurate

Possible cause: The battery pack is not calibrated after PACK