How do I change battery voltage output and make it constant?
Unfortunately an LD33V (real name LD1117V33) is a lowish dropout regulator, not a low dropout regulator. It's advertised as being better than a 7805, which it is, 1v dropout plays 3v, but the 7805 bar is pretty easy to get under. You can get down to 150mV dropout if you look, certainly beat 1v. Go onto CPC, RS or Mouser, and do a parametric search. MIC5205, ADP1752, BU33TD3, the list isn't endless, but goes on and on.