Know battery health of all iphones iPhone battery retain it capability up to 80% when it is new(with 500 charging completely cycle) So when you charge your iphone,it counts the number of charging attempted so when it reaches 500 times your battery performance decrease. So change your battery because it's health is low/
The issues is “your phone is shorted" no matter how much battery you buy will be drained to 0.01V or to 0V so when the battery is less than 2.6V will not charging symbol but sometimes will show apple logo then black screen repeatedly once connected to charger…..so let the short fixed first YOU WILL ENJOY THE
Sorry for late reply first check the original back housing using 3u tool if changed or not because sometimes network issues is caused by back housing change. In other hand you said battery draining is huge but you didn't say if the phone is overheating or not….if the battery draining faster that means that there is short circuit in baseband circuit/RF circuit which leads to unstable network so do the following to know the cause of your problem. (1) Dial *#06# to get IMEI number (2) Setting>>general>>about then check for IMEI number, model firmware and ICCID(when sim card inserted) if you miss one of mentioned above in either method the problem is related to Baseband circuit if no information missing then the problem is related to RF circuit. After confirmation what you have to do is to separate logic board then connect it with iSocket then test to know if the problem was caused by bonding pads or not.if problem not solved do cosmetic inspection on RF circuit the do diagnosis by measuring bonding pads.if...
Iphone has 4 microphones 2 near charger port “for normal “ 1 near front camera “for free hand call/loudspeaker/video call/video recording using front camera” your problem is related to this so you need to change front camera 1 near back camera “for video recording using back camera”
Sorry for late reply but as others said about back housing…I agree with them….you can then try to unscrew all screw that connect logic board and back house you can disconnect back camera too.then turn on the phone and check. You can ask why? The reason is screw can short to ground and cause some issues like phone turned on but no display,phone turned on but dark camera and so