Ok, here is a brief description and pics, on how I moved my starter battery to the trunk of my 03 SL55. Pics are attached below, and the 2nd post

- First I pulled the battery from its original location, along with the ground wire, and metal battery try/plate it sits on.

- Second I removed the plastic cabin airbox cover to locate a spot to feed the wire through the firewall. However after searching, I was able to drill a small hole to the "right side of the fuse panel/left of cabin air cover". You can see in the attached pics where it comes through. After doing this, I realized I probably did not need to take out the plastic cabin air cover, because I had room to drill the hole between it, and the fuse panel.

-Third I cut the factory starter wire, and used a 150 amp ANL fuse block to reconnect to the starter wire, and 2ga running to the trunk.

- Fourth I fed the wire down by where the TCU mounts, then underneath the carpet along the inside/bottom part where the floor pan meets the passenger side door seal. Routed it up to the "left" of the middle fuse panel (which is located behind the passenger seat, underneath the storage compartment), I had to removed the panel that covers that area. Then I fed the wire through a small area, that went directly into the trunk, followed some of the factory wiring harness (this part was tricky, I had to use a metal coat hanger to feed it through). You can see in the pics below, where the 2ga wire came out.

- Fifth I was able to mount the battery in front of the passenger side rear tail light area. The OEM battery is small enough to tuck it underneath there. Also keep in mind I still have to find and install a battery box small enough to fit that area. Thinking a Taylor 48300 series battery box might do the trick, and still fit perfectly. You can see in the pics, how I fed the wires through and mounted another 150 amp ANL fuse block before the battery.



For those interested in doing this, this should be enough info for you to complete this project. Here is a list of what all I order to complete the job so far...

- 2ga amp wiring kit, with 1 ANL fuse block, 150 amp ANL fuse, and other misc connectors - I purchased the whole kit on ebay for $37 bucks
- 0/2ga battery terminal - $10 bucks
- 1 extra ANL fuse block - for engine bay/OEM starter wire to 2ga wire connection - $10 bucks
- 1 extra 150 amp ANL fuse - $3 bucks
- some sort of "small" battery box/tray to secure battery in trunk - Cheap plastic ones average about $10 each, nice aluminum boxes run about $80 on average



The reason for this, was to free up space in the passenger side of engine bay to route a custom intake from the throttle body to that area, to keep it separated from engine bay heat. E55 guys have done something similar to this to mount a coolant reservoir for separated IC systems.

 

Remaining pics are attached below.

 

Don't forget to route a vent tube for that battery too.

 

correct, those vent tubes typically come with battery boxes.