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.