The hybrid drive motor battery contains 3 relays that allow the high voltage (HV) DC batteries to be connected to the vehicle. The three HV relays are a positive HV contactor relay, negative HV contactor relay and current limit relay, also known as the precharge relay. These relays close and open in a certain sequence controlled by the battery energy control module (BECM). The control circuit for the HV relays use a common ground and the BECM provides a voltage signal to turn them on and off individually. During power up and power down the circuits are monitored by both the BECM and the hybrid powertrain control module (HPCM) located inside the power inverter module (PIM). The BECM measures this voltage on the terminal voltage sense wires. The HPCM monitors this voltage from the HV DC bus inside the drive motor generator control module.

To start the sequence of power up for the HV DC system the HPCM sends a contactor command signal to the BECM, the HV negative and HV precharge current limit relay turn on first. This allows the HV DC circuit to limit current through a precharge current limit resistor. The voltage then travels through the 300 volt DC cables up to the PIM. The BECM then closes the HV positive contactor relay after it measures voltage within 95 percent of the calculated pack voltage and then opens the precharge relay. The sequence times and voltage levels are monitored by both the BECM and HPCM. If the time limit or voltage value during a particular sequence is out of the specified range, the BECM and/or HPCM will set DTCs.

To disconnect the BECM will open both the HV positive and HV negative relays at the same time when the system conditions are ready. If a HV relay fails a system check during precharge or power down, the high voltage interlock circuit (HVIC) will not source HVIC current until the fault is repaired and the DTC is cleared by a scan tool.