battery voltage

[mastaking]

anybody help me out with this when i start my 2006 6.0 the battery voltage starts out at 12.3 then after the normal 30 seconds thing the alt kicks in and the battery voltage goes up to 13.2 to 13.6 then after 10 or so minutes the voltage starts dropping to 13.1 to 12.8. Do i have a bad alt or is this normal i'm seeing this on my cts the truck has 48,000 miles on it.

[JSchuricht]

It's partly normal. The alternator is on as soon as the truck is running, what you see is the glow plugs shutting off up to 2 min after you turn the key to run. As for the voltage dropping again after a few min, I would start by checking the GPR when you see the voltage drop. Also, try using a voltage meter, it might just be the circuit that your cts is on.

[mastaking]

whats the gpr? it seem like after the glow plugs turn off ( im in san diego california) so its only about 30 seconds the voltage goes up to anywhere from 13.2 to 13.7 then after about 10 mins it starts dropping to about 12.8 or so and the battery are brand new i took back my last set of battery after being in for 10 months thinking i had a bad one but with the brand new ones does the same thing so i kind of ruled out batterys.