Got to be battery, or a poor connection to battery. Take connectors off battery terminals, clean the surfaces of both, and the inner surface of terminals if they are round 'posts' or flat surface of other type. If that doesn't cure fault make sure battery is fully charged up. If not get it charged, try again before laying out for new battery. If still no luck off you go to battery shop. If it was anything else it wouldn't start on jump leads.