Need help building a supplemental battery for portable DVD player.

Hey all! I may be in the incorrect forum so please excuse me if I'm wrong.

I have a portable DVD player (AMW m281) with a detachable battery (BT01). I would assume it's rated for at least the length of a movie (2 hrs?). I have read that with Lithium Ion batteries every deep discharge cycle decreases their capacity. I wanted to run the DVD for my 2.5 hour trip.

I wanted to build a battery pack that I could carry with the DVD that would at least double the time I could run the player. That way I would avoid a deep discharge on each battery.

There is a DC connector on the DVD and the battery. Both say DC

9volts. The battery is LI-ION 7.2 volts 3000 mAH. I would assume all I need is to supply 9 volts to the batteries dc connector to supplement it?

What is a good battery to use. *9* volt LI-ION I would assume? Is there such a thing?

I would carry the extra battery in a bag with the dvd so any consumer type battery would fit. I would probably wire two batteries in parallel and connect it to the DC in connector.

Thanks for your help.

Steve