Restarting after changing battery

I purchased an ifixit replacement battery and tool kit. Replaced my aging battery. All went smoothly. I turned phone on and it seemed okay, then it restarted by itself. It's usable for a few minutes, then it restarts again. I let it cycle this way till the battery percentage displayed 7% and I put it on the charger.

It stayed at 7% for an hour. I restarted the device while still plugged in, now it's stuck at 31% and not charging higher. It is still randomly restarting, although less frequently than when I first changed the battery. Ideas anyone? I'm new to this stuff.