The EFIS converts power internally so that it can charge the internal battery to a peak of about 16.8V regardless of the input power. What you are seeing represents a partially charged battery. Over the course of several hours of externally powered operation the battery voltage should creep up towards its peak. If you suspect that your battery is not charging properly or if the voltage never gets higher than 15.1, give us a call at 425-402-0553.
