Dead battery, sometimes
This has been a head scratcher for me and everyone I have asked about it.
The first time I had this problem was back in November. I went out to start the truck and it cranked, but just didn't have enough juice to fire. I let it sit for about 20 minutes, that sometimes lets the battery recover. I go back out and the remote won't unlock the doors. Completely dead. I had to hook up jumper cables for about 20 minutes to get it to fire. No problems until about March. I drove to the store, parked and left the radio on for about 15-20 minutes. Go to start and nothing, just a click. After that the radio won't turn on, interior lights are dim. This time I disconnect the battery, jump directly to another car then reattach the battery. Maybe not the safest thing, but it was cold.
The battery is fairly new, bought it back some time around September. Every connection to the alternator and battery is tight and free of corrosion, same is true for every ground I can find. I have occasionally saw the volt meter drop down to 12 while driving, making me think it's a charging issue. But what's the cause? It's so infrequent I can't pin anything down.
The truck is a 2007 chevy 1500, new body style. 5.3.
I'm about to put another battery on with a relay between the two so I can run off either battery or both just so I don't get stuck somewhere, but that's just a bandaid to the problem.
For parasitic drains, you might have to get an ampmeter in line with the battery cables and start pulling fuses looking for large drains. Any weirdness with tail lights, brake lights, etc?
I had the same problem with my 99 that turned out to be the stock radio slowly draining the battery. I can't remember exactly how I found the problem but it seems like I
connected my 12v tester light to the negative battery cable and then to each fuse under the hood and in the cab, if the light glowed dimly it was a parasitic drain. Maybe someone else here can give you more details...also, mine has a ground to the block and to the frame, the one at the block comes loose about once a year (even with loctite) and
gradually makes it harder and harder to start until I remove the bolt and clean the terminals (at the block).......May have nothing to do with your issue, just my two cents.
I don't think it's something drawing power. Reason I say that is there shouldn't be anything on the truck that can drain the battery in 15 minutes. Maybe if every light was on, every pump, motor etc was running then I could see a large draw, but I'm confident that wasn't the case. Contributing factor? Absolutely possible, but I don't think that's the cause.
I'll check the grounds again, that sounds like it could be the problem.
Any other ideas? I'm stumped.