I have a 1986 F-150 Extended cab with the 302 c.i.d. engine. I have a new alternator and battery already installed but I can only crank the truck 4 times before having to jump it off. I know that the alternator is sending charge to the battery, but after shutting the truck off the battery starts dumping voltage. I do not know what to do to make the battery hold enough charge to continue cranking the truck. What do I do?