Chosen Solution
Curious if the iPhone SE can boot up and function (for testing purposes) without a battery installed.
Just tested it! Looks like it needs a battery plugged in to start, but the capacity and current charge don’t matter. I put a known dead battery in, plugged it in, and it booted. You can also disconnect the battery and it will just run on the incoming voltage. TL;DR It needs a battery, but the condition of the battery doesn’t matter.