I wonder if trickle charging with a few calculator-type solar cells on each edge of the frame (not the face of the frame) would be feasible as well to prolong battery life.

"Integrated lithium-polymer battery for up to 7000 image changes over up to one year"

E-ink screens only consume power when refreshing so if you don't change the image a lot you won't need to charge it often

True, if the processor powers down too.

Many have deep sleep features where they sip energy.

But that requires a wire that goes from the outlet all the way to the charge pad though, right? I think the main goal is to have no visible wire when it's on the wall. On the other hand 7k image changes before going out of battery seems quite a lot. Just not sure if that's a representative number.

Even if you completely run out of power, the screen should stay on the last image it was showing. You might just see the same picture for days/weeks/several heat deaths until you decide to charge it again.

If that's the case wouldn't it be easier to just have a power cord?

Sounds like a good idea.

Just anecdotally I found it pretty fascinating that I bought a new Kindle some three months ago and it still hasn't run out of batteries (came with 76% and now at 16%).

Admittedly I haven't been using it a lot though.

My experience with Kindles is that the battery drain is all about wifi - if you keep it in 'airplane mode' most of the time (except to send new content to it / return library books early etc.) it can last for months on a charge.

These only need to be charged a few times a year