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.
Please read this post and share more info...I am having this same issue. Brand new battery. Is the alternator going bad.
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.