When running at full speed the charging voltage is 18.3v.  At rest I see 12.7v.  I changed out the regulator...same issue.  How can that be?  I'm seeing the voltage changes on my Garmin unit.  I'm concerned this is too high and may damage the battery and/or the Garmin.

First, verify those voltages with a DMM.  The Garmin may not be giving an accurate reading.  I know my Lowrance unit always shows about a full volt lower that reality (measured with a DMM).  

This has come up a lot lately, including myself. I’ve got a couple threads and one video posted on here about it. New regulator fixed mine and one other members, seems like the only other option to replace was the stator. But most often those aren’t the issue, spinning magnets don’t seem to mess up much