Sudden battery power drops with new battery

Hi,

I bought a new battery from iFixit to replace my fast draining old battery.

The new battery discharges much much more quickly but mostly it has very sudden power drops. For instance when the screen is off and I am using it to listen to music, it may drop 30% throughout the course of ten minutes.

The power drops even when charging it. If I charge it from off/red battery splash screen and when it gains enough charge to boot the phone, I've seen it immediately switch to the shutdown spinner during the apple boot logo when the power drop happens, and then return to the red battery splash picture.

Would it be possible that I made some common mistakes while replacing the battery and is there anything I should look for or check for?

Thanks