Chosen Solution
A customer brought in a dead iPhone 6. I replaced the battery and it worked for some time. Then it went black again, and now that battery is dead. Did it two more times and all of the new batteries I put in seem to fry out. No water damage apparent.
Probably charging IC. A common symptom is when the phone refuses to charge a flat battery. Are you sure those batteries are fried? Have you tried to put them into another iPhone 6?
