Phone won't recognise ew battery

I tried to replaced the battery in my wife's iPhone 6 - I've successfully replaced lots of iPhone batteries in the past - but when I tried to turn the phone on, nothing happened. I plugged in the USB charger and the Apple logo showed up for about 10 seconds. Then the Apple logo disappeared for 10 seconds and 10 seconds later it reappeared. It just kept cycling like that without end. Clearly, the phone was not recognising the new battery. I removed the new battery and noticed that there was too much wire/lead to cram into the small space between the battery and the board and to properly seat the connector in its receptacle on the board - it never snapped into place like the other connectors. Also, I noticed that the new battery's connector was somewhat thinner than on the old battery and it was probably impossible to seat it into the receptacle on the board in such a way that it would make proper electrical contact with it. I re-installed the old battery and the phone works just fine except that the old battery won't hold a charge for more than about 24 hours, even if Wi-Fi and Bluetooth are off and there are no incoming or outgoing calls made with it. Does anyone have any suggestions? Should I just try and buy a battery from a different source other than IFIXTI? What sources do you recommend? Thx!