If anyone has been wondering were a good place to mount a 2nd or 3rd battery on a 06 navi (I'm sure expedition same):I was able to mount 2 of the biggest deep cycle batteries walmart sells behind the rear tires on outside of the frame rails. the battery box I had bought for them was a hair to wide and long so I cut it into 4 peices (cut in half both ways) and snugged it up to the batteries just overlapping the peices then epoxied them together. Same with the lid . To mount the batteries themselves I used (don't laugh) these metal c!other rack/displays components I got For free From someone who bought out a Shopko that closed they are chrome plated steel and the various peices hook to each other they have a rectangle bar 1\2" by 11\2-2" and came in a few different lengths with a lip on the end . Simple bolted 4 of the pieces that slide over the flat bars to frame on either side facing them in opposite directions. So I can put the Bars in at a angle and twist into place under the battery and used a self tapper to hold in place ( I tested 1 bar with 1 mount to 400lbs plus n bounced on it, 2 with 2 mounts is plenty) then used the straps that came with battery box to hold batteries to frame to prevent any sliding around. Has held up just fine over the last 7 months. I'll get some pics once it dries up out here