Battery voltage drop

After full charge: 12.8v
After sitting before install: 12.5v
After fast idle: 14.3v
Ungrounded tab at normal idle: 10.5v
Grounded tab at normal idle: 2.2mV

Ran the car for about 15 min in total.

The auto store claims the battery is still good. Had it checked a couple days ago. Unfortunately, because this is my daily driver, I took it off the charger at 100% last night before I went to sleep, and checked voltage this morning before I put it back in before going to work...