Charge Port + Battery Replacement Not working

Hi,

I run a small side business fixing iPhones. The other day i replaced an iPhone 7 charge port flex and battery after a client brought his phone to another repair shop. The client said the repair shop replaced the battery but this did not fix his phone because the “charge port is causing a short on the battery” so the battery went dead.

Long story short, I replaced his charge port and battery, the phone worked well for a few hours and then went dead. I replaced the battery again and the same thing happened. I wasn’t able to charge the phone after replacing the flex cable.

It seemed the batteries went dead very quickly, so I’m assuming some type or short happened. Do I just need to try and replace the charge port flex again (faulty part?) or is this an issue with the board-level ICs?

Update: Ive got another charge port coming in a few days so I can try that if the customer still wants to wait. I don’t see any visible damage around the battery connector but I haven’t checked underneath the waterproofing foam inserts so it is still possible that the other shop botched something (they also tore his home button flex so I wouldn't be suprised).

I tested the voltage through the system as described in this guide: https://www.microsolderingrepairs.com/bl... and got a reading of around 0.7v (same as the guide). So, Im pretty certain this is an issue with the tristar and is therefore way over my capabilities of fixing. Thank you both for the helpful answers!