Charging System Failure on 2005 Chevrolet Uplander

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After 20 minutes of running, first time in the morning, I get a charging system failure. It always happens while on a highway at higher speeds. Then the light goes away and does not come back the rest of the day even after letting it sit for several hours. Positive cable to battery has been replaced but did not solve the problem. No other steps has been taken at this time. Any suggestions?
(1) Answer
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Looks like the alternator started to fail. I'd do a charging system test, however if the problem is intermittent the test might show a good alternator.

This only happens in cold weather. I am in Florida and we are now getting some cold nights. The message appears while the vehicle is cold and checing stats on the volts it is at 11.8 which is below the recommended low of 12 volts. Then the system starts charging and it doesn't appear the rest of the day. Any other posible probelm other than the alternator?