P1689 and strange battery gauge today

Yesterday my Jeep threw the P1689 code which was a new one on me... did some research and found out it could be the TCM.. was weird because the Jeep did not drive any differently on the way home. I cleared the code last night to see if it would come back on. This morning it started fine but noticed the lights were dim... look at the battery gauge and it was mid way between 9 and 14.. I drove it a bit to see if it would go up, but it then just fell to 9 and the check gauge light came on. shut off the jeep restarted battery gauge between 9 and 14, then dropped to 9 again. I drove it home, had it running for a bit watching the gauge between 9 and 14, then it sort of "kicked in"... a bit of a load on the engine and the gauge went right to 14. I drove it for awhile and it stayed right there. Drove it to work fine and it never moved and the lights were bright again.... long story but couple of questions....is the battery gauge with the alternator is sending to the battery or is it the charge of the battery? if it is the charge going do you folks think maybe a bad ground, or bad alternator?... also could this have caused the other code?? sorry for the long story...