We're getting ready to take a long trip (air mostly) with the kids, and have purchased two Sony FX810's for their use. The battery's are new, and in good shape, I let them each use the players ONE day to see how long the life was, and we got about 2 1/2 moves out of them - or roughly 5 hours as stated.

So - is there a solution to the portable battery issue? I've see the Energizer battery packs, but reviews are aweful - stating they only give 2 to 3 additional hours of time - I could almost buy a second FX810 for what one of those cost and just keep the battery.

For a solution to be worth while, it would have to be under $50 and provide up to 4 additional hours of play time. I have the settings set very conservativly on the players for brightness and warmth... so that should help.

Any other suggestions?

A battery used for alarm systems could work. It is 12 Volt, 7Ah(Amp/hour). Physical dimensions are about 3.5" tall, 3" deep, and about 6" wide. May have to use voltage adapter from 12 to ? They also make a little charging board, and transformer to recharge.
What is the voltage and Amp/Hour of current battery? This could give an idea how long this will last, It is slightly heavy for travel.

I believe the Sony runs at 9.4v and the current battery is a 4500ah Lithium rechargeable - there aren't any real specifics on the battery itself, that's just what I've found from other message boards.

4500 Ah is 4.5 Ah(Amp/hours)They do make a smaller battery if the excess capacity is not needed.