I know that fuel cells are not supposed to be used as the power plants for combat vehicles, but I was wondering if they could be retrofitted in place of an existing ICE engine in a vehicle. If so, would they require any additional tonnage like if you used a fusion engine or is it purely the tonnage of the fuel cell power plant like it is when you use a ICE engine?
There are certainly armed vehicles in the canon TRO's of a variety of types including ships and even Mechs that are powered by fuel cells. For a world that lacks the industry to readily make fusion engines but can make fuel cells, this might be a viable option for most types of vehicles.
