Can I extend the battery life of my portable DVD player?

[Anonymous]

Do you use an external battery to extend the life of your portable DVD player? What should I look for & where? Do universal external batteries fit all brands? Is there a big difference in quality?

I have a Sony 7 inch (DVP-FX730). It already lasts 5 hours, but we're going a LONG way before we can recharge!

TIA