The problem with working with gasoline engines indoors is ventilation. When I was 16 years old, I knew some guys that were working on an enigine in the basement of the parent's home, and the fumes were ignited by the water heater. A big whoomp and they got the scare of their young lives. Fortunately they got out and the fire was small. Since them, I've heard about guys that put appliances with pilot lights in their garages, and suffered the same surprise.
I guess lithium packs can burn too. I don't keep mine inside the house.