Playing with my new-to-me '08 and noticed the battery was perfectly sized to sit in the largest under-seat compartment, so moved it yesterday. *The FJR's battery was an AGM that had "Do Not Tilt" on the back, so used the sealed battery from my '06 FZ1 that was considerably thicker, but still fit. A sealed FJR battery would fit even better.
Didn't find any other posts on this, so snapped a few pics.
It was an experiment so I didn't cut cables, just extended them, didn't alter anything permanently. Made sure to pad the battery and extended the stock rubber tie-down strap with an o-ring.
Didn't even remove the tank, just slid the extending wires down the right side. Next step would be to professionalize the connections and wire routing.
Made a fourth plastic "wall" so the stock battery box is now a sealed storage unit, and then replaced the screw and push-pin fasteners on the stock battery box beauty panel with finger fasteners for quicker access...used a 6mm contained nut on the lower push-pin hole.
Won't pretend that I moved it because of "mass centralization" or anything that esoteric...I'm sure it helps but mainly I moved it for easier access and less fiddling with electrical connections. Plus, it's fun, creates an accessible storage unit, and the under-seat tray seemed to scream, "put the battery here where it belongs!".