Why my phone constantly rebooting after battery replacement?

I have just replaced my iPhone 6s battery. I had to repair it because although my battery could hold a charge for a few hours, it wasn't long enough for me.

Immediately after replacing the battery and turning the phone on, it said I had 1% charge, so I shut it down and started charging it.

After 15 minutes of charging, I turned the phone back on and it said I had 55% of charge, lasted 30 seconds, then shut right off.

It does charge and will work without being plugged in, but every 20 seconds it will

reboot CONSTANTLY.

The battery percentage jumps from 1% to random numbers and Battery Health does not recognize the battery or give any reading.

The 3rd party app "Battery Life" (which is very accurate and I use often) tells me the the battery is "PERFECT" or "VERY POOR" . It changes it's answer every time I reboot.