I don't mean to call you out, but I'm fairly positive this is wrong. I ran the baseband software team at Apple and worked on the first two generations of iPhone

I don't mean to call you out, but I'm fairly positive this is wrong. I ran the baseband software team at Apple and worked on the first two generations of iPhones. I'm intimately familiar with that hardware having done reworks by hand myself on the first prototypes and being in all schematic reviews. They definitely didn't have secondary batteries connected to them. While this may exist on some new models of phone, I've never heard of it. This doesn't even make much sense to me. Assuming you're talking about emergency calls, the application processor is necessary for dialing. You'd need to power the entire phone including the screen to make an emergency call.