Ihave an older Troy-Bilt riding lawnmower that stopped running for no apparent reason. When we checked the battery it was dead and couldn't be charged. Got a new battery and the lawnmower started and ran for awhile (couple of days) then would not start and the same story again. Took it to the only person around here who will work on Troy-Bilts and he "fixed" it . He actually "fixed " it 3 times always with the same outcome. He finally said that he probably wasn't able to repair it because it must have a short and the wireing harness was no longer available. I didn't mention, every time this happened the battery was then unable to take a charge. HELP.. Buteo

You could also put some sort of quick disconnect on it so that when you are not using it the battery is disconnected.

I would look at the possibility of a bad battery again. Some of the batterys available for lawn and garden are less than perfect and can fail. Try as the previous poster said and disconnect the ground cable when you out it away. This will tell you if there is a drain on the system.. . Not likely.
Don