Join Date: Feb 2004
Location: Lexington, Kentucky USA
Matteo, the battery ground to the car body is basic... essential. Once you have cleaned that thoroughly, including the bolt, get back to us please and let us know your voltage readings again. When you get the ground terminal lug and ground area on the body shiny clean, apply some electrically conductive anti corrosive grease to the area and to the bolt head. Use a toothed lockwasher under the bolt, also.
Go to an electrical supply store, and ask for "Contax" corrosion inhibitor or "Kopr Shield" made by Thomas & Betts. Either one will do a great job of preventing corrosion of your main electrical grounds and enhancing conductivity over the long run.
Lexington, KY USA