Just to be devil's advocate here - and maybe save you some time, money and grief -
No part of ANY turntable's signal path is on the grid. The current is generated purely by the movement of the magnets and coils versus one another. So all you would be doing is using a battery to spin your platter - the speed of which is already electronically regulated very accurately.
Now, I'm not going to get into a big Audiogon argument about the nature of momentum, speed regulation, and platter mass - many audiophiles have vivid imaginations, but IMO there is absolutely no way that powering properly designed belt drive TT (which will have a heavy platter and isolated motor) by battery can possibly have any discernible effect.