went to start my 87 3800SC today, at first it started right up, drove it outside & shut it off, hr later went to start it and it was like the battery was almost dead, about a half a revolution and then "Click". checked the battery with a multi meter, says 12 volts. cleaned the Pos and Neg terminals, started but still sounds like a half dead battery . WHAT else to check ? Complete engine and cables were new a year ago, and it hasn't been driven at all.