Seems like you have the old Delta Dometic AC unit. There is NO battery in any RV Tstat. They discontinued that system about 1995. Odds are the upper control board is defective and I do not think they have support or parts for that old a unit. If you go on top and remove the top cover, you may still be able to get the Model number. There are 2 places for the model number. On top under the top cover and inside on the metal mount pan but you have to drop the inside plenum to get that model number. Without the model number, nobody can really help you. Does the wall tstat have some led bulbs in it? Doug