The most likely problem is corrosion between the battery cable clamps and the battery posts. Disconnect both battery cables and thoroughly clean the battery posts and the inside of the cable clamps, then reassemble everything and tighten the clamps snugly. THEN- - -after everything is tight, coat the cable clamps with Vaseline or wheel bearing grease to slow down further corrosion. DO NOT do what a friend of mine did and grease up everything before reassembly!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!