I think 20000mAh is achievable now. I have a 20k battery pack on my desk and it’s pocketable, if heavy. Much thinner than the Energizer phone. I’m picturing your phone design as an original Galaxy Fold, but solid instead of the hinge/internal screen. The chassis could fit more than a 10k battery easily, and the shape is simple to ruggedize.
> Avenir Telecom wanted €600 for a phone with the internals of a €200 phone.
This is the big issue. Avenue’s not to blame here. It’s unavoidable, the design is just too non-standard.
If you could get a military contract then it might work.