Anything that works like existing rechargeable batteries. Those would be considered conventional batteries. There is basically no path to a high enough energy density for airplanes for those types of batteries.
Things that involve metal-air reactions are basically fuel cells and don't count. If you go down that route, you'll quickly find yourself working with some kind of chemical fuel. They will suddenly look a lot like existing airplanes in terms of basic concept.
> Anything that works like existing rechargeable batteries. Those would be considered conventional batteries.
Ok, that definition works.
I would have gone with 'whatever people (in the future or now) use for powering their phones, laptops and electric cars' is by definition 'conventional' at that point in time.
Things that involve metal-air reactions are basically fuel cells and don't count. If you go down that route, you'll quickly find yourself working with some kind of chemical fuel. They will suddenly look a lot like existing airplanes in terms of basic concept.