I think my car's battery is dead

After my little story yesterday about how I almost got stuck in the parking area with my car, I started my day a bit unusually. After I finished my morning routine, I walked to my car to remove the battery and bring it home. Before removing it, though, I was curious if I could start the engine again after the night, especially since it was cold enough, below 10 degrees. Luckily and surprisingly, the engine started easily without any worries. So, at least at that moment, everything seemed to be fine, but I still, decided to charge the battery again just to be sure.

During the charging process, I noticed some interesting things too. When I first connected the charger, it showed me that the battery had 12.9 V. In my opinion, and according to Google, that's a normal voltage for a car battery. So, it made me wonder if I really needed to charge it or not. But I had already brought it home, so I decided to start charging it anyway, as it couldn't hurt.

The charging didn't take long, though. It took only about 20 minutes, and the tool showed me that the battery was full. Of course, this wasn't really anything new, as I already knew that the battery was full, so this behavior was pretty predictable. But right after charging, it showed about 13.1 V. After some time, though, it went back to the same 12.9 V again.

Also, I have never changed the battery since I bought the car in 2019. So, that battery has already been in use for about 7 years without being replaced. I only brought it home on some very cold days when the temperature dropped below 25–30 degrees. I guess it was a pretty successful and original battery that came with my car.