If it's a 2015 car, and battery - I think the battery is sealed.
In cause the place they are mounted is still in technical sence "not a nice place for a long life". So now it's cold outside and in the garage. I think it's reached the end of it life.
Good chargers and conditioners shows it if the battery has a problem. And: with the most good conditioners you can also charge such a small battery like fitted in the morgan.