I have a 2009 GMC Acadia SLT with 250k miles on it. I have experienced a charging system warning and replaced the battery, alternator, serpentine belt, and cleaned all of the posts under the hood. I still, occasionally, will get the same warning when starting the car after it has been sitting for a while. What would cause an "occasional" problem that appears to go away by itself?

Could be a few things.

I'd start be getting one of those power-port volt meters so you can look at it instantly when you get the warning: https://www.walmart.com/ip/Lighter-T...SABEgKs2_D_BwE

Then you can begin to see if it is actually a low-voltage situation or something else.

Hey CathedralCub,
Thanks for replying. I have only experienced the waring when first starting the car. I have checked the voltage at the alternator and at the battery. This confirms the 12 volt reading on the dash gauge (I'm using an analog meter). If I turn the engine off and let it sit for a while, when I restart it,sometimes it all seems to be working great and it doesn't fail again, as long as the engine stays running. I have also noticed that when I'm experiencing this problem, if I disconnect the battery and let all systems drain and then reconnect, the voltage is back in the works properly.
The intermittent part is only when I start the engine. Sometimes I get an alternator response and sometimes I don't. I don't experience any changes while the engine is running. It'* all determined at the initial startup

I see what you are saying, but the dash gauge might be moderated a bit so as to not panic most drivers if they see it move for normal activity. The gauge in the accessory port will show you exactly what is seen moment by moment.

How about the battery cables? You didn't mention those. I've seen some anecdotal evidence on other forums where the newer GM battery cables are prone to failure, especially the negative cable leading to all kinds of electrical gremlins. And no, you can't tell if they're bad with a cursive visual inspection.

Thanks for the suggestion. Easy and simple enough to do.

What we do is allow the battery to be drained a bit (unplug or pull a fuse to keep from starting) then start the vehicle. With engine running we measure the voltage drop across the cables for both sides of the battery and the charge line from the alt. If it drops more than a tenth or two the cable may need to be replaced. They will develop resistance if they have large amounts of current flowing through them. If the engine has issues with starting the extra time pulling 200 plus amps will add resistance much faster than normal starting will do.