2008 X5 4.8 battery drain.

[simpson_3]

My friend has owned his 2008 X5 4.8 for around 4 years now and within the last month or so, it has developed an extremely bad battery drain. It's been to a couple different dealers/shops and it seems that no one can fix it. Every time it goes to the shop for this issue, they put a new battery in it (its probably had 2-3 battery's installed in it within the last month). One dealer put a new alternator in and it's still having issues. If it sits overnight, the battery will be dead by morning. Any advice? Thanks.