For anyone reading this in the future: You just need to replace the main battery.  There is something in the main battery related to the CMOS and the service tag. New battery installed (Dell original ordered from Dell Support) and no issue since.  
Hello all!  I have a XPS 13 7390 2-in-1 here has a CMOS Battery fault. Everything is fine as long as you keep it charged but if the battery dies it will lose all BIOS settings including service tag and boots into maintenance mode until the service tag is re-entered (and it takes several minutes to get the laptop booted up when this happens).  "No problem" you say, "just open it up an replace the coin CMOS battery!" But there is no coin CMOS battery on this 2-in-1 model   I found this link about clearing the CMOS involving removal of the main battery: [URL]  Does this imply CMOS some how uses the main battery to save settings? If so perhaps a new main battery would fix the problem.  Any help would be much appreciated. CMOS battery is usually such a trivial issue but in this case it makes the laptop unreliable.
XPS 13 7390 2-in-1, CMOS Battery fault, no CMOS battery inside?