I've only driven my 5 twice, and both times the battery has drained to no output at all, I can charge the battery in full and leave it for a week and it does not discharge at all, but after driving it 10 miles to the bodyshop it's totally dead again, almost like the alternator is not charging it at all, so it's gone from full charge, to ten minutes later, dead as a door nail. Incidently the alternator is brand new, as is starter motor (if that makes a difference) Any ideas please?