Here's a fuel-cell powered aeroplane with a 5-hour endurance. OK, it's a tech. demonstrator but it does work. It's based on a self-launching glider which has the option of Lithium cells in the wings and an
electric propulsion system.
This may be a stupid question, but if we do end up flying battery powered aircraft, will we be able to rely on the "fuel gauge".
I'd say "yes", much more than we do now. When you think how inaccurate/unreliable fuel gauges are on most GA aircraft (I don't trust 'em) and compare that to a battery pack where each individual cell is monitored, along with the environmental conditions... The control unit knows exactly how much charge has gone in and out, what the efficiency of the battery is at various temperatures, how it degrades with time & cycles, etc. Not to mention the voltage gives a bit of a clue when running low.
Something like
this, perhaps?