Dead battery and stuck gate – 12V charging process question

[Sussystorm]

I used my car three weeks ago with a 70% charge, but today, it’s completely dead. I think it’s the 12V battery, as I can't open the back gate to get the charging cable. I know the 12V is under the passenger floor carpet, but I want to make sure I charge it the right way. Should I plug in a trickle charger, or is there a specific process to follow to avoid causing any damage?

 

[Smart42Driver24]

How old is the battery? Is it the original one? Maybe it is time for a replacement.

 

[Sussystorm]

The battery is the original one, and it's about 3 years old. I’m considering replacing it if necessary.