As always, this is my understanding.
Practically, you can’t write a policy excluding some power chairs, not others - it will just lead to confusion and mistakes.
Regarding batteries, the size of battery in a power chair is considerably larger than a phone. Yes, both pose a risk, but the power chair is greater. I believe there are containment devices that fit mobile phones where it gets dropped in with, put the lid on and then it burns out. Because of the physical size of powerchair batteries, that’s not possible.