Originally Posted by Kookz
The energy density of the LiFePO4 batteries is higher than a conventional battery, and the issue you're speaking of really only has to do with the Ah of the battery. You could probably fit 16 cells into a hollowed out battery case and have 30+ Ah vs 16 Ah on a large convential MC battery. Or maybe I'm not totally understanding the problem you're speaking of, but I've had an 8-cell on my Ducati for 3 years and it's been great (unless it's under 40 degrees out, but I'm guessing you don't need to ride the Vincents in that. The best part is that it starts even when I leave it for 2-3 months at a time while I'm at work.
It's an issue of load vs generator output. Much like the Shorai, the battery you reference needs a healthy voltage and wattage supply to maintain its charge. Seeing as the Alton only puts out 100 watts on average (150 peak) it falls on the inadequate side. Your Ducati puts out roughly 500 watts as comparison.
In the end, I temporarily ditched the case and added a similarly sized AGM (14ah) that I painted with black crinkle paint. An hour in the machine shop and I was all set with a suitable mount. The AGM should be forgiving enough for roller coaster loads and charge rates even if it is a little big and heavy.