You need to see if there is a parasitic current drain on the battery. An Ammeter is placed in series with the positive battery cable and the current measured after allowing all modules to go to "sleep" should not be greater than 100 mA. If this is not the case, remove each fuse in the 2 fuse boxes, one at a time, until the current reduces.
If the battery drains that quickly, I would expect a very large current drain.
I like to keep it simple in the beginning. My new battery kept draining like that and it turned out the technician had stripped the battery sidepost so it was making very little contact, thus not charging sufficiently. Bought a set of side lugs at Autozone and it fixed the problem. Hope your problem is this simple to fix.
When I first got the vehicle, it ran fine for a couple months then the battery began to wear down, which wasn't new. So I swapped it with a battery I already had which is 3 years old. Again, everything was fine but only for a couple of weeks this time. The same symptoms as previous battery. It charges when running but after 2-3 hours of sitting the multimeter reads 10.26 and doesn't drop anymore than that. My lights have never dimmed while driving with either battery. I read about parasitic draw and took out the OnStar and the instrument panel fuses though it made no difference. That's as far as I've got.
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