2002 Chevrolet Venture Starting Difficulty

either the battery is going bad or he cables are loose/corroded. take off the battery cables and pull back the black and red rubber pieces at each cable end to look at the cable ends. if theyre not clean and somewhat shiny, then this is at least some of the problem. have the battery FULLY charged and load tested. a half charged good battery will falsely fail a battery test. then once all that's done check the charging system. no less than 13.5 volts with all the electric lights, a/c, etc., on at idle