A BMW 730Li E66 car with a mileage of about 5 km in 2008. User response: the vehicle can not start after parking, and needs external power supply to start the vehicle. Fault diagnosis: 1) according to the driver's description, the fault occurred several months ago, when the battery was replaced, and then used normally until the failure occurred. The battery voltage is 9.3V, and the battery has lost power. 2) start the vehicle with an external power supply, detect the voltage at both ends of the positive and negative electrodes of the battery is 14.7V, and the charging voltage of the generator is normal. 3) after running for a period of time, the battery is checked by a special battery tester. It is shown as "GOOD BETI'ERY", and the battery is normal. 4) connect the BMW special diagnosis instrument, carry on the whole car diagnosis, read the fault code, display the low voltage fault of multiple control modules, the fault description is that the current fault does not exist. 5) the analysis may be a fault of the dormant current. The current clamp is used to measure the dormancy current of the vehicle. After extinguishing the power, waiting for about 3 min, the starting / stopping button lighting indicator is extinguished. After about 16min, the seat adjustment switch lighting indicator is extinguished and the vehicle enters the dormancy state. At this time, the current monitored by the diagnostic instrument is 0.2A and 60min is still 0. 2A and remains unchanged for a long time, exceeding the specified sleep current value of 80mA. So the abnormal phenomenon of dormancy current exists. 6) as the sleep current is monitored by the power supply module and PM, it is operated on the bus side according to its working principle. In the state, the power supply module PM enters the sleep current monitoring function after 60min. If the vehicle is operated within 60min (for example, turning on the car's headlights, trunk lid, etc.), the dormant current monitoring function will be resumed. After this period, the dormancy current should not exceed 80mA. If the sleep current exceeds 80 mA, the power module will send "shut downCounter" (closing the counter) information after 5 min. After 90s, the vehicle electrical system 5S will be disconnected. If the dormant current is still higher than 80mA after re connection, the electrical system of the vehicle will be disconnected again. If the sleep current is higher than 80 mA, the electrical system will be broken off through the power supply module PM. In the fault code memory of the power supply module, the relevant failure codes (including the constraints and reasons for raising the sleep current) will be stored. But there is no fault code in this case. 7) adopt the one by one power cut lookup method to check the location of the abnormal sleep current. As shown in Figure 50, the power supply module PM is powered by the bus terminal 30U (the fuse bracket powered in the trunk) and the bus terminal 30B (power supply for the fuse bracket in the sundries). The bus end 30U is disconnected, the sleep current remains unchanged at 0. 2A, the bus end 30B is disconnected, and the sleep current remains unchanged at 0. 2A. This indicates that the abnormal sleep current has nothing to do with the power supply before and after the fuse box. 8) after excluding the above factors, the fault of the power supply module PM is analyzed, and the PM of the vehicle on the same model is tried to replace the 60min, and the sleep current is 50mA. Troubleshooting: replacement of power supply module PM, trial run for a period of time, failure does not appear again.