The main issue with charging a nearly dead battery using just the alternator is that it isn't a very energy efficient method unless you already had a reason to drive the X5 quite a bit anyway. You still need to check the voltage to make sure it got charged enough.

Hooking up a battery charger is both more efficient and easier to monitor the battery's charge state. Most electronic battery chargers shut down automatically when the battery is fully charged and have some method to tell you this (e.g. lights, meters, etc.). The X5 has none of these nice to have assurances that the battery is charged.

Funf Dreisig