Alternator Magic
 

1999 F350; 7.3 diesel, 2WD 6 speed
While cruising the charging system caution light illuminated.
Battery volt meter read its usual (appox) 15 volts. I shut off all non-essential electrical for my 10 hour drive home. This was yesterday, Easter Sunday.
Watched the volt meter drop about .5 volts per hour and made it home. Left the truck running and checked the voltage at both batteries. Each showed 10.3 VDC which corresponded to the position of the instrument cluster volt meter.
Recharged the batteries overnight and started the truck w/o problems. Volt meter read about 11 volts and after a few minutes displayed its normal position of appox 15 volts and no charging system caution light.

My initial thought was a bad alternator given the 10.3 volt output. But now I have my doubts.
Any thoughts /guidance? Is there a charging system relay that may be dying?

Eric