I've been trying to put together a "lazy man's" ps2 portable using off the shelf parts and as little hackage as possible.
I bought a Joytech 8" PS2 screen and it worked great (except for two bad pixels - of course it's only replaceable under warranty if three are bad).
I bought the same universal rechargeable battery that Benjamin Heckendorn did, and it worked great too, up to a point. The default tip on the battery's output power cable fit nicely into the Joytech's power socket, and the PS2 and screen powered up nicely.
The problem came when I attempted to use a disk. Apparently the Joytech screen uses more power than the PSOne screen Mr. Heckendorn used, because as soon as I tried to start a game the system died and reset. I found that I could start a game with the screen off, or run the system with no disk, but that the combination of DVD-ROM activity and the screen drew more power than the battery could deliver.
At this point my plan is to purchase a second battery, and splice the power output cables together in parallel so I get the same voltage but double the amperage. That should provide enough current flow to power the system. Unfortunately that means two wall adapters for the batteries. I'd splice those in parallel as well, but suspect that charging both batteries from a single wall wart would double charging time.
Once I have a working setup, I'll probably velcro the batteries to the underside of the ps2, though I have concerns about heat dissipation. It may end up being safer to leave them detached from the ps2, in which case I'll velcro them to each other.
The other drawback I noticed was that the universal battery won't output power while it's being charged. This means you have to drag along the ps2's brick if you want to play while recharching the batteries.