FYI you can monitor the battery voltage while you are driving using the instrument cluster diagnostic test #9.
You will need to unlock the test by using your VIN number.

All the info to access the diagnostic test is here:
high cluster: http://www.xoutpost.com/1007582-post12.html
low cluster: http://www.xoutpost.com/1007645-post17.html

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