iPhone 6 charging incredibly slowly

I fitted new iFixIt batteries to two of our iPhone 6's. Everything has been fine until today when I noticed that my iPhone wasnt charging when plugged in. I would see the battery charging symbol and my PC would give the tone to say it was connected, but it just didnt charge. I watched it slowly drop down to 3% (even whilst plugged in!) and then decided to switch off, checked the lightening socket was clean and checked the cable (used the other iPhone and it worked perfectly). I plugged it back-in and switched it on to find it now reported 14% and over the course of the following 30mins or so, it crept up to 18%. The charge percentage seems to be climbing very, very slowly so I'm wondering what could be the issue. Do I have a dodgy battery? I guess I'm not going to know unless I change it (which means buying another :-( ) but I would welcome any other ideas. I've checked the FL11 and C279 capacitors from another thread to make sure they are still there (they are!) so I'm at a loss.