OX-34 you're hitting on some valid points, also I ride solo a lot and would love to have a second battery as backup since there is literally almost no way to bump start this monster.
That said I now need a battery guy to tell me if I can mix two identically rated but different chemistry batteries. The guy in this vid
https://www.youtube.com/watch?v=dZzinsfI2Y0 states that one battery will discharge another if they are different capacities and i'm wondering if different chemistries might have some sort of hang-up as well since the alternator may charge one quicker than the other?
BUT despite my initial statements, ideally I would have actually wanted to run a small lead battery as the back up/aux and also to warm up the lithium in the occasional extreme cold startup (been camping in sub 20F a few times) if I can't for example mix a small 6ah lead acid (YTZ7S) with a much larger capacity 18ah lithium I may yet again be out of luck.
Hope all that was clear enough.