Don’t these Dreamliners also have back batteries?

Even on light GA aircraft the radio will run for an hour on the starter battery. This narrative doesn't sound realistic.

Yes and many pilots being walkie talkies in GA as backup. Not sure if airline pilots do this though. And its kinda hard to root around for it and fiddle with it while trying to keep an unpowered jetliner in the air. They're more for emergencies where the radio is the only problem.

By the way, the age old rule is "Aviate, Navigate, Communicate" in that order of priority. So it could be they just had their hands full with the Aviate part.

They have a RAT (ram air turbine) that deploys automatically under specific conditions. It’s basically a turbine providing electric and hydraulic power. It was almost certainly deployed on the accident flight. It will only power the most critical equipment, though. Possibly, that does not include the ADS-B transmitter (which broadcasts position and related data).

This analysis is convincing about being able to hear the sound of the RAT from the crash video: https://www.youtube.com/watch?v=SbDJjgN7Xbo

Yes, lithium ones, those were the ones that kept catching fire then the 787 was just out.

I guess they're just for the time until the RAT kicks in. Or to augment it.

Ps there's also the APU, a small turbine in the tail for generating electricity.