The only iPhones that have had the same battery connector are the 5C and 5S. In the early days it was acceptable to use vice versa batteries for these two but now I do not recommend as you will have issues
The problem is the batteries . I buy from a different company and had the same issues , contacted them via live chat and were told the new batteries would be updated to include the health. received a new batch with a later date and all good. Therefore the issue lies within the battery and some sort of software programming they must have to do to the battery itself.
I would be sure this is a problem with the touch ic , not the screen .It is commonly known as touch disease. try lifting screen up while still on and connected and putting a fair amount of pressure on the main board (at the black area on shield below the screen connectors )with your thumb and see if the touchscreen responds while pressure is on. if so then afraid you would need to have the touch ic replaced . It is expensive and requires micro soldering skill. Best left to professionals that have carried out this particular repair before .
Are you trying to test the screen without the home or proximity cable attached? If so it will actually boot to home screen fine after about 4 mins 10 seconds . If all the proximity cable and home cable are attached then it should be booting in the normal around 30 seconds .
I was actually just testing this last night as i own a repair business. iphone 7 and iphone 8 both original screens . my way of testing iphone 7 screens (mine all use the original apple lcd and touchscreen) is to attach the screen only , no front camera or home button. It will appear to boot loop but it takes 4 mins 10 seconds to get eventually to home screen . During some of this time i use it to check for any visual defects on LCD. now my findings were the iphone 8 screen booted up in the same time 4 mins 10 seconds on the iphone 7. LCD fine but touchscreen not working at all. HOWEVER........ it has only came to light that ios 11.3 is preventing iphone 8 users that have had a COPY screen on their iphone 8 now have no touch. Copy screens worked fine on previous ios. i am wondering if the iphone 8 touchscreen would work on my iphone 7 if the 7 did not currently have ios 11.3 installed . I would bet my bottom dollar the touchscreen would operate as normal.
i am just after repairing a water damage iphone 7 after swimming pool incident . From what i seen the entry point appears to be via face on, bottom left mesh of your phone. i detached the haptic connector and seen slight corrosion. i cleaned with 99% isopropyl, back together and working perfect now . also someone had mentioned loud speaker (bottom right of phone) not working correctly after water . try this . Download an app called Sonic (app is orange with a black wavy line through it ) set to about 450hz for 5 minutes say, then that will physically push moisture out of the mesh. try it even now and you will feel air com out if held to your lips
ill be quick with this one. suspect power ic which is main board fault so fairly expensive to have repaired . HOW YOU CAN TEST 1. disconnect battery 2. plug phone into mains with battery disconnected if apple logo does not appear after a few seconds then suspect power ic , changing battery will not cure this as main board fault. i would start with that first before exploring other avenues
possible power ic issue (simple way to put it) which would be repairable by a main board specialist. quick test is open phone, disconnect battery , plug phone to mains and see if apple logo appears after a few seconds (it will appear for a few seconds , go off and repeat) if the apple logo appears then its a good indication you have a power ic issue . if it doesnt then could just be a defective battery . aftermarket batteries are pants tbh
i would bet it is poor quality screens . there are many different factories making/replicating iPhone 6 screens now. finding a good supplier and paying the premium is the only way forward. Iphone 6 screens are very expensive for good screens. To be truthful there's more profit in an iPhone 4 repair currently than an iPhone 6 repair (at least if you are using good screens)