I would stay away from the lithium batteries for this application. they can be very touchy. I they are of the smart pack design they can turn them self off to prevent damage, charge them wrong and they may catch on fire, go below 3.3 volts per cell and they may be damaged, and you can not fast charge them, one hour is about as good as it gets. Nicads may not be as modern, but they work, and take a lot of abuse, and in a 18 - 20 lb model how much weight are you really saving. Lithiums are great for park fliers and the like where energy density is everything, but you will not see any advantage in you Bobcat.

Steven