Laptop Battery has 13V, reports 0% and is not discharging nor charging

I recently bought a used Lenovo ThinkPad T480. When it first arrived both the internal as well as the external battery where in a bad state of health. The seller took care of that problem by replacing both batteries with new replacement units. Now after four charge cycles the external battery (which is a 24Wh Sanyo) only reports 0% charge although it has (measured) 13V which, to my knowledge means it's fully charged. This battery is now constantly displayed as charging which causes the internal battery to not being charged at all. I already tried the pinhole reset and discharge and recalibrate

Question by KingWarin on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.