iPhone 7 randomly turns off

Hi I recently bought an cracked iPhone 7, in hopes of resseling it, note it was my first time fixing an iPhone, I bought the replacement screen, and put the bluetooth home button, the first time i encountered the problem was when I finished it all, it started turning off and then on until it drains a battery which happens really fast, it usually happens on 20% and 50%, I tought it was due to battery which was replacement one even though it was on 100% health, so i replaced it with the original one, and when it is about to turn off it becomes very hot under the rear camera, I cleaned all the connectors with PCB cleaner with awarenes of not damaging it, I also have to menition the backplate on battery and the lcd connectors has only one screw holding the backplate, also the display backplate had one bruised screw which I had to cut off and bruise that backplate side, but the display is fine.

Any help would be appreciated, thanks in advance!

It was due to battery, I replaced the replacement battery with the original one, after that it shut down just once and it drained my battery from 52% to total 0, after that It didnt shut off, I can also use various apps it wont turn off, thanks everyone!