Today I'll install a Shorai LFX09A2 battery on the Duke. It's an equivalent amp hour rating, but less than half the size and will make a bit more room under the seat. Also it will not self-discharge like all lead-acid batteries do. I put an even smaller one in our MuZ Skorpion (660 Yamaha single powered), left it sitting with no charger for six months, and it fired up on the button no problem. I've got them in every bike we have except the Harley. Well, there's a Ballistic four cell in our '56 Velo...all that would fit in the fake case. It's kick start, so no problem. I'll need to run the lights this July in Oregon.
But the real reason I got it was the weight savings...almost six pounds (2.66 kg to be exact) from under the seat. That's nearly 2% of the bike's dry weight!
I did the same thing to my Buell XB12, with about the same % weight change, and the difference in handling was noticeable. Like the opposite of putting a big, heavy Givi box on the back...
Lighter makes it righter!