The TSB involves replacing the battery cables. Make sure there is good clean connections on both sides of the bolts the screw into the battery and there is no paint or corrosion where the other ground connections meets the firewall block and/or frame.
It is normal for it to charge between 12.5 to 15.5 volts.
The alternator is controlled by the computers for fuel efficiency and to keep battery maintenance minimal.
This is straight from the description and operation
The BCM will enter Charge Mode whenever one of the following conditions are met.
•The wipers are ON for more than 3 seconds.
•GMLAN (Climate Control
Voltage Boost Mode Request) is true, as sensed by the HVAC control head. High speed cooling fan, rear defogger and HVAC high speed blower operation can cause the BCM to enter the Charge Mode.
•The estimated battery temperature is less than 0°C (32°F).
•Battery State of Charge is less than 80 percent.
•Vehicle Speed is greater than 145 kph (90 mph)
•Current Sensor Fault Exists
•System Voltage was determined to be below 12.56 volts
•Tow/Haul Mode is enabled
When any one of these conditions is met, the system will set targeted generator output voltage to a charging voltage between 13.9–15.5 volts, depending on the battery state of charge and estimated battery temperature.