Since the last IOS update, my phone repeatedly turns off and restarts, when not connected to a power source.

I’m not sure how an IOS update could effect my battery, but 2 years ago on an older iPhone I updated my phone and the phone would not even turn on past the Apple screen. I ended up having to do a factory reset, via a computer, and lost important information. Now, my phone is useless when not plugged in. I’ve just performed the network reset to no avail. Any guidance here would be a blessing.