Why does it only stay on when plugged in?

Hello. I bought an iPhone 6s from ebay, that was water damaged. According to the description it sounded like all I needed to do was replace the screen and battery. I bought both parts online, the screen from amazon and batteries from ebay. As I waited for the parts to come in, I took out the old battery and lcd and dipped the logic board in isopropyl alcohol, then the entire phone each for an hour and dried it with a hair dryer.

After gettingthe first battery, the screen hadn't came in yet so I plugged the phone to my laptop and itunes detected it so I knew it was charging. I left it plugged in overnight to the adapter and in the morning I disconnected the battery to have it ready and charged when the screen came in. 2 days later the screen came in and I replaced the screen. I turned it on and it showed at 46 percent, I found it kinda odd. Left it plugged in again overnight and in the morning the battery was all the way low, but the phone seemed to detect it when it was plugged in but wouldn't charge.

Thought it was the battery because itunes still detected it so I ordered another from another seller off ebay. When it came in , I swapped batteries and it turned on so I left it plugged in again and didnt charge but was still on. I plugged it in my laptop and now it doesn't detect the phonet. Odd thing is if I unplug it from the power, it turns off and won't stay on . Unsure if it could be the charging port because it was previously working and all the water damage was to the top components because the main camera doesn't work, or flash. I was able to test the mic, headphone jack, and speakers and worked fine when I got the first battery.