Why won't my 12v 2.5A LCD driver board run when powered from a 12v 6A supply?
The converter (or batteries) can't keep up with the in rush current, dipping down too much or is too noisy for the driver. Try adding a 1000 uF capacitor to the power input of the Driver.

Other option is to put the cells in series and regulate them down to 12V instead. Or use 3 in series with a regulator. The driver likely still works at less than 12V input.