I don't know if this is relevant to younger XKs but on my 2008 4.2 XKR, you need to apply the parking brake to completely shut down the electronics. Otherwise, as I was told, more 12v systems stay active and the battery drains quicker. It's like telling it to go to sleep.
A CTEK charger should come as standard equipment with these cars. I always hook one up if I can when the car is left for 3 days +