iPhone restarting when battery below X% My iPhone restarts "randomly" when the battery is below a certain threshold (which I haven't been able to specifically pinpoint). A couple weeks ago I had the battery replaced with a new one, as battery life wasn't longer than just a couple of hours

The device has iOS 10.2.1, to be upgraded to 10.3 hopefully during the day.

Any ideas of what could be wrong? I've read suggestions about resetting the iPhone, but just sound like a pretty rough/brute solution to implement, although it's an alternative indeed.
Certain iPhone 6S models were known to have this problem.  You can check the serial number at this site to see if it's affected (presuming it's a 6s; you didn't specify the model):

https://www.apple.com/support/iphone6s-unexpectedshutdown/

Mine had this problem, and the guys at the Apple Store repaired it (I don't know if it was the battery or a component on the logic board).

For other devices, you may have to try erasing all content and setting it up as a new device and give that a try for a few days to see if that fixes it.  I want to say that my brother had the same symptom and a reset was the solution for him (it's been a while).

I've had an occasional problem with restoring years of accumulated backups and phone migrations that only a fresh install seems to fix.