Need help building a supplemental battery for portable DVD player.

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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