High-Voltage Vehicle Research Tips: GM BEV Low-Voltage Battery Disconnection

Details Posted on 28 August 2025 | Share:

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While researching OEM repair information, you may come across specific notes, instructions, or warnings for high-voltage (HV) vehicles. Some general procedures can be easily overlooked, so it’s important to identify these circumstances. Let’s see what unique considerations General Motors (GM) has on EVs during a low-voltage disconnect.

The low-voltage system in GM battery electric vehicles (BEV) is supported by the HV battery. This is done with a Battery Maintenance Mode or Enhanced Battery Support Mode (ESBM) that checks the low-voltage battery state-of-charge (SOC) at specific intervals and determines if the battery needs to be charged. For most GM BEV, special considerations are noted in the low-voltage battery disconnect procedure.

The Battery Negative Disconnection And Connection document can be found in vehicle-specific service information under: Engine/Propulsion ➤ Starting, Charging, And Low Voltage Energy Storage ➤ Repair Instructions.

Per the Battery Negative Disconnection And Connection document:

• "Always ensure the Battery Maintenance Mode is inactive before disconnecting the low voltage 12 volt battery (or batteries)."
• "This mode can be active with the ignition off, regardless of whether the vehicle charging cord is plugged in or not."
• "When this mode is active, the on-board high voltage battery charger(s) will energize the 12 volt battery cables and charge the 12 volt battery (or batteries)."
• “"isconnecting any battery cables while this mode is active may result in an electrical shock or a burn from hot battery cable leads."

To determine if the low-voltage battery is being charged by the HV system, GM states:

• "Check the 12 volt battery voltage with a digital multimeter before disconnecting any battery cables leads."
• "If the voltage is 13.5 volts or above, the Battery Maintenance Mode is active."
• "The technician must wait for the T18 battery charger to deactivate before disconnecting the battery negative cable."

The length of time the vehicle can be in Battery Maintenance Mode is found under: Engine/Propulsion ➤ Starting, Charging, And Low Voltage Energy Storage ➤ Descriptions And Operation.

Per the Charging System Description And Operation document:

"... the T18 Battery Charger to run until the 12 V battery is charged to a value between to 76.5% to 83.5% or the T18 Battery Charger has run for a maximum of 2 hours with the high voltage contactors closed whichever occurs first."

The Hybrid Modes Of Operation Description document is located under: Engine/Propulsion ➤ Hybrid/EV Controls ➤ Description And Operation. It includes information about a service mode that prevents propulsion and limits certain vehicle functions.

Following OEM procedures is critical for the safety of the technician and can easily be overlooked when performing common tasks.

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