Get the battery load tested. Only way to confirm battery is indeed good and not having a weak cell only holding a ''surface'' charge. Confirmation of a good battery is step one and a load test will stress the battery bringing out any weakness in it.
Normal readings.OK, I understand. I have a Chevy Malibu 2013 LT 2.4 eco. The battery is brand new. Car starts and Voltage is 14.1 to 14.4 but will drop to 13.6 to 13.9. Then I will get a drop to 12.5 for a while then it kicks back up to the 13.6 to 13.9.