Why motherboard during charging giving too low voltage to batery

Because you damaged something in the charging circuit---first rule out parts. Follow an iFixit guide to remove the charge port from the other phone, without installing it fully, just overlay and click into place, see if that port will charge this phone. If not, then you have ruled out parts. (FYI, screws are not important for this problem). If your problem is the motherboard then you have likely knocked off some very tiny components near the battery connector or near the dock connector on the board from disconnecting the battery/port---use your fingernail, not any tools.