Probably nothing to worry about. For some reason I usually get the red battery light when the temperatures are above 65 degrees. When I check the voltage via the instrument cluster diagnostic test I get an average reading of 13.6v so I know the battery is charging even though the light is on. If the test shows 12v or under the battery is not getting charged.

You can check the voltage using the instrument cluster diagnostic test #9.
You will need to unlock the test by using your VIN number.

All the info is here to access the diagnostic test is here:
BMW X5 - Access the Hidden Onboard Computer (OBC) Functions.

Or here if you prefer video:
https://www.youtube.com/watch?v=dRDBKn4jn5Y