No power, replaced battery, battery is not charging when plugged in
I have a Galaxy 10.5 (SM-T800) that I picked up to repair. It had no power when I bought it, battery was below 3 volts so I replaced that first. Left it to charge overnight and found that the battery's voltage had not change. I can't get it to power on at all, tried hard reset with the buttons (Power, Volume up, Home) and no response. Tried multiple chargers and cables they all work, swapped the motherboard with another SM-T800, though I can't know if that one is any good either. What am I missing? Would it not turn on at all if the charging port is bad? I have a new charging port on its way but even so with a good battery at the right voltage it should turn right on. None of the water stickers show any evidence of water damage either. The screen is not cracked and it has basically no marks on it anywhere, so it probably wasn't dropped.
Any help or input would be really appreciated, thank you.