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This is basic operation of the high voltage system found on some BEVs and HEVs. The circuitry on the left is powered by the low voltage system. The low voltage ECU controls the high voltage battery output to the vehicle via contactors. The HV battery consists of twelve cell modules wired in series to produce about 400VDC. Low voltage power is provided by a 12.6VDC battery and, in reality an HVDC to LVDC charging module (not pictured). The circuits on the right are HV.
Above the battery pack are HV test points. This is a typical location to validate that the HV battery is no longer sending HV on the cables. High voltage only remains in the pack following power down. The two relays represent the contactors for HV+ and HV-. High voltage is insulated from the vehicle chassis and must remain safely insulated, hence the large resistors between HV and chassis ground. This resistance value must remain within specification (8.1Mohms in this case with the load switches off) and must be validated prior to powering the system back up, this insures correct vehicle isolation.
Now for the title. The High Voltage Interlock Loop (HVIL) is a direct access prevention circuit that is constantly monitored by the low voltage battery controller. If this circuit should fault, the ECU "should" deactivate the contactors, removing high voltage from the cables. This circuit is a low voltage pulsed circuit that runs through MOST HV connections. The controller sends the signal out and looks for it to return. This circuit is represented at the top right of the schematic. You can fault it high, low, and open to watch the behavior. Use the voltmeter near the 12V battery to see what the ECU is seeing on the HVIL circuit.
The HV system is reliant on proper operation of the LV system. The hoops in the HVIL circuit represent HV connections at various components. Be sure to open the HVIL circuit near the ohmmeter and power down the LV to the contactor and to the controller prior to taking your measurements at the test points. The voltmeter is provided to show when the cables are powered and not. Reference to chassis as well. The ohmmeter can be used once voltage is removed and prior to powering up to confirm the HV cables are correctly isolated from chassis.
THESE ARE NOT INSTRUCTIONS FOR REAL LIFE!!!!! CERTIFIED HIGH VOLTAGE TECHNICIANS USING CORRECT TOOLS, AUTHORIZED SERVICE PROCEDURES, AND PPE ONLY!!! This is information only.
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