I searched this out, and aside from threads mentioning that it has been done, the technical details are few.

I am going to be relocating the battery to the trunk to create room up front for a water tank in my coupe. As I have seen with other installations, the battery will go in the space of the right side of the trunk, near the CLP (PSE). That's the easy part. The part I am wondering, from those who have done it, is how did you route your cables? Did you drop down under the car, route through the inside of the car, etc? Did you ground to the frame, or did you run a discrete ground to the engine bay?

I will have a high capacity fuse on the positive terminal. I bought an assortment of amperages. If you did the same, what amperage did you wind up using, and why? If you have done this, please share some pics.

 

I can[t speak to coupes, but in the Roadster this is easy, and you can take an inside route. I realized this when I put my 50 watt GMRS radio in the top storage area, behind the seat. While that cable is only a #8, there was a lot of room to run something bigger. I ran it thru the passenger compartment all the way to the fuse block next to the battery. The one questionable part would be the passage from cabin to ECU/BCM/RCM box, my wire was small enough to fit - but a #0 probably won't be. I dont see a BIG deal with going under the car along the chassis - but have never considered it, does it not stand to reason that such a path would be protected and safe in most any circumstance?

I'd be uncomfortable with a fused lead, as your battery is your surge filter and part of the voltage regulation.

I'v done the James1549 grounding mod to my last two Roadsters - therefore, I'd run a #0 or so to the block and a #6 or #4 to the chassis, right in the area around the new battery location.

You sure mod the hell outta your cars, guys talk a big game, but YOU on the other hand.............................

 

Thanks, Mark, I appreciate the words! I do my best to maintain a certain level of quality, as I want the mods to fit the aesthetic of the car, and reliability is paramount to me. As an engineer, I tend to obsess about using best practices. I have always enjoyed projects that provide modern technology and increased power, but maintain the look of the older vehicle. There are some major changes coming up in the car this summer, but all fit this model. More power, efficiency, technology, but it needs to look as untouched as possible from the outside. These cars will never be considered sleepers, but if they were.... This project makes room for a water tank of a bit over 2.5 gallons, right where the battery went. I considered putting the tank in the trunk, but the hoses would be much larger than the battery cables, I would lose thermal efficiency, and the benefit of putting the much heavier battery in the trunk will net about a 30# weight advantage in the right rear of the car, versus putting the water tank in the same place. I have considered going small battery - I have a 21# Li-Ion battery from my last FWD car project, but I have found that while it will start the car OK, it has no reserve capacity to just sit with the windows down and the radio on. I do not want a mod to take away from the enjoyment of the car with oddball compromises.

I will be using 2/0 cable. Inside the cabin is my preference, and if I can get it up to the passenger footwell, then I think I can get it passed through to the engine bay, especially when I have the engine out of the car here in a week or two. Underside of the car exposes cables to potential for damage from rocks/debris and getting snagged. They will also deteriorate more quickly there. I may have to come out of the trunk, over the bulkhead, down to the floor, then move to the middle of the car, down the right side of the transmission tunnel, then over to the ECU feedthrough area in the passenger footwell. I think the cables may be too big to manage to just go down the right side of the car, below the door sill area. I doubt the carpet would cover it properly there.

 

first u might look at the bat on the left side (drivers side, if u remove all that styrofoam u can weld in the factory batt tray & tie down) I did use that as my ground also added ground straps going forward to the ground under engine left side .
I used a 1/2" dia stranded cable , right near the fuse flap you know that little one next to the water tank I added a hot post with batt and other small things and my rear batt cable
I ran it down the front r side wheel well cover into car(there are already holes there then under the carpet (without removing carpet) down the inside of the door Covered by the door sill
Lifted up the carpit goinh over the rear tire hump (there is an opening in the corner next to the gas ) into the trunk cable on right side ,,,under the tail lights to the batt ,,already have my ground welded in connected positive, then my ground FIRED IT UP that was my set up for 12 years in my first V8 I did use a 900cca batt. jim
NOTE run cable front to back not from back to front