Turbo Trainer Advice

I think the Tacx Neo is the only smart trainer that does that

"The trainer can be used with and without a plug. When riding on the trainer without a plug, it functions like a wireless motor brake (like the Bushido). The permanent motor and electronics are set to generate energy from your movement, enabling the trainer to communicate with apps and control the resistance unit. It features all Smart functionalities, but you will have to keep cycling to maintain the communication and control of the trainer.

For an optimal cycling experience we advise you to use the plug. Without one, it cannot simulate descents and when you stop pedalling the trainer will soon come to a stop. Road feel is always available."

I'd expect fluid and magnetic resistance units to stay at the same resistance setting when you disconnect power, so you ought to be able to continue using them as a dumb trainer.

I doubt manufacturers encourage using them like this though.

4iiii fliiiight claims to work for over 2 hours from an internal battery.