I have a vextra 0419644 dvd player. How do I get it to play w/battery?

I purchased this DVD player for my children and I can't for the life of me get it to work with the battery power. I have charged it as it states and it still does not play off of the battery. Any ideas of what I am doing wrong or what can I do to make it work? It works off of the both plugs, (1) car lighter and (2) the regular house one but won't run off of the battery.