Leone55

As title says my battery keep going flat on my e55 when the car is left for a few days or more, the battery was replaced in 2011 it's a yuasa battery my concern is there is also another battery in the boot? I've done a search and can't seem to find why it is in there? Could this be what is draining my battery? Here is a picture of the one in the boot

01BlackE320

Wow that's different. The prior owner either had some monster sound system or just wanted a backup battery. I would check out how it's wired in. It is either just connected to the other battery in parallel or there's a switch over somewhere (battery 1 - battery 2 - both). If you disconnect the main battery does the car start and run off the other one? If the car is flat in a few days then I guess both batteries are discharging. I would disconnect the rear one and start troubleshooting one at a time