A Hummer EV that dies overnight usually has a 12V parasitic draw or a module that won’t sleep.
Here’s how to find the cause and whether you can keep driving.
First, know which battery is draining
On a GMC Hummer EV, “dead overnight” usually means the 12-volt battery, not the giant Ultium pack. The 12V battery powers computers, door modules, lights, and the contactors that let the high-voltage system come online. If it is low, the truck may not wake up, charge, or start normally. If you are instead seeing the high-voltage range drop overnight, that is a different kind of parasitic draw: modules, thermal management, or scheduled cabin conditioning waking up when they should be asleep.
Read the full breakdown in the Knowledge Base
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