In have a 2003 Hummer H2 on a long trip my display started reading battery not charging randomly came on and went off than my battery light started coming on and off. I noticed my volt guage started dropping than all my system warning lights came on I lost power before complete shutdown. Today I used a battery boost charger it started up fine I have no warning lights voltage reading about 14 drove it around for about an hour and starts up fine any ideas what happened? I was told about a fuse in engine compartment that prevents alternator from charging battery" Can anyone narrow it down to a number or position in fuse block"

 

MOVED to the H2 Section which is where all H2 questions go. Crazy like that............

 

Do this first....Check all grounds! Next, If you have a multimeter, set it to AC, yes AC, voltage and see what the reading is across the battery terminals with the engine running and under a load (blower on full blast and headlights on bright). Report back what the reading is. When the diodes start to die in the alternator they start producing AC current. Next, turn meter to DC Volts and measure across the battery terminals. Report reading. Then put a lead in the large nut of the back of the alternator, and the other to a ground. Report reading. They should be very similar. If they are not similar (within about a half a volt), you likely have a loose connection and its creating a voltage drop. There is a junction block that goes from the battery to the alternator and starter as well as a fusible link.

 

In case you don't have them, see attached PDF for circuit details.