replacement battery not working

Hi

I just installed my replacement battery, following your instructions. All went smooth when installing everything and closing up the iPhone.

I then tried to power up the iPhone but the screen remained black. Thinking the new battery might be empty I put in the charger cable. The Apple logo came on but disappeared after a few seconds, then reappeared for a few seconds,... an endless loop.

I removed the new battery and put my old battery back in and the iPhone worked perfectly. I then tried again installing the new battery but had the same problem.