I have a craftsman tractor about 8 years old, it has a 15.5 hp kohler engine. Last year, like all the others I clean it, remove the battery and do the basics before storage. I charged the battery prior to the new season and installed. It fired as it should but found after several stops and starts the battery became weaker until finally it just clicks when the key is turned. The battery is about 3-4 yrs old. The amp meter on the tractor was on the positive side at running rpms indicating to me it was charging. I placed it back on my digital charger and it gave me no indication it was bad. Where do I look next?