I want to make sure that I understand everything.
First, on the "bad" battery, which is now in a different EFIS:
1) Did the battery stop charging at 14V, or is that as far as you've observed it so far? It might take as long as a full day for the battery to reach its peak charge of over 16V.
Next, on the the other, good battery that is now in the suspect EFIS:
2) Was 15.8V also the charge level before it was put in the suspect EFIS?
3) Is this EFIS connected to external power?
4) Has the "I" voltage gone up, down, or remained the same over the course of a few hours?