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Got low battery notice . GMC dealer found battery bad, would not hold charge. 41,500 miles 32 months.
New ACDELCO type 49 AGM 950 CCA. When I check the voltage across the battery terminals with an accurate digital voltmeter first thing in morning new battery reads 12.2 to 12.4. Engine starts . It has been driven 500 miles at highway speeds. Drivers Information Screen Shows 14.5, 13.8 etc when driving, alternator appears to be working. Dealer removed battery and tested it with fancy GM battery tester that that shows battery is good ? One weird thing dealer apparently removed battery without connecting 12 volt soorce to OBII connector to keep computer memory intact. I know this because oil life and fuel filter life reset back to 100%

What voltage does anyone get on yours?
 

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Left my switch on for over 24 hours. Battery was completely dead. Charged it enough to jump it off and check engine light stayed on for over a day while charging back up
 

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sounds like you may well have a defective battery coupled with the "unique" charge characteristics that GM uses on this truck. as crawler dude posted, i have been observing (obsessing) over this issue since my truck was new. it now has 27K and i've concluded a few things.

The longer the truck sits, the longer it will spend at high voltages (14.5+) to recharge which is completely reasonable and expected
No matter how long it charges, when it decides it's charged, rather than drop to a healthy float charge of 13.4V or so and keep it there while driving, it drops all the way down to 12.1 or 12.2V and typically stays there until there is another long-term discharge, like not driving for a few days. This is the charge characteristic that I think is unhealthy for a battery long term and the only rationale I can come up with for it is to make slight increase in fuel mileage by lessening the duty cycle on the generator. In effect, the system charges the battery in as short time as possible, then allows the battery to discharge while driving. An AGM battery is better than a flooded lead acid battery at handling deeper discharges and longer periods between charging without sulfation but there is no literature or manufacturer recommendation that I am aware of that would suggest this is the optimal way to get long battery life. Quite the contrary, this charging algorithm runs counter to battery best management practice.
 
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