The battery on my 2003 Suburban keeps being drained. I charge it up to 12+ volts and overnight it drains to about 5-6 volts. I believe I have ruled out a bad battery as I have switched it with another vehicle and the problem stays with the Sub. I removed the wire that attaches to the alternator and the problem doesn't go away so I think it isn't the alternator, plus the battery reads at about 14.75 when the Sub is running so the alternator seems like it is doing its job??

What else would be draining the battery that fast? What else do I check and how?

 

Welcome to the forum.

You need to monitor current draw while pulling fuses one at a time to isolate the circuit that's drawing current. If the battery is dropping down to five or six volts, there's a sizeable current draw.