I think it's a problem with the pcb. I had the same problem as the guy in the video below. I left my battery in a radio and voltage was reading 0 volts on the post, but 4V when i took it apart and measured the main positive and negative. I jumped it like the guy in the video. When it was at 12V from the main positive , the post also showed 12V. From that point it charged it.
You are already at 18V at the main positive and negative, but only getting 6V from the post. I would bet the battery cells are fine, andyour problem is with the something on the pcb.