I have solved the mystery at least as far as my truck is concerned. It was rain water getting into the vent of my battery. If you battery has caps, pop them off. If the back 3 cells are to the top and front 3 cells are normal, then you may have the same problem I had. To prove it, open the hood, spray the windshield with an open hose and watch where the water goes. Mine went right on top of the back 3 cells vent cap. I lowered the acid level, replaced the tray, and installed a piece of plastic over the battery with double side tape. Problem is fixed!!!!