In 2021, my 2014 Malibu's battery was testing at about 50% with my battery tester. Walmart has an AGM
battery for it, but only a few stores carried it. I was told that more of the stores would have the battery.
Link to H8 AGM battery at walmart: EverStart Platinum AGM Battery
Looking online, it appears that it's available at even my local Walmart: $169.xx
Plus the Walmart I drove to that had it at the time, installed it in about 20 minutes
(had to wait turn for other customers).
Warranty was 3 years total replacement; prorated up to 5 years IIRC.
I replacement my trunk battery a few weeks later, even though it tested fine.
It's a small AGM battery available at Oreilly store locally (Checkers). My car worked
just fine with the 7-year old battery in the trunk, but I felt it better to replace it after 7 years. Oreilly guy tested it when I took the core back and offered to refund new battery since the old one tested good on their tester. It must be unhooked completely from the before testing.
Be aware the main battery showed various voltages as the system adjusted to the
new battery. Because it was new, the system sometimes acted like it was overcharged
and showed 12.4 volts or so as the car reduced charging so it can determine the
ability of the new battery to power the car (my interpretation of what was happening).
I think the car sees how long the battery can handle the load before dropping to a
voltage too low and then the car starts recharging.