I just brought a brand new 1/6 Stuart from a friend but the 12 volt battery is dead and doesn't hold a charge. I attribute this to being stored for over 7 years w/o being charged. I went to RadioShack and purchased a Enercell 12 volt. How long should I charge this battery for its first charge? The instructions on the RC Stuart says 18 hours but this battery didn't come with the tank. I would hate to damage it. Please advise?