So after seeing many people mod the Sega Nomads with the Lithium-ion packs and then doing a couple myself, I decided to give the Turbo Express a shot. After fitting and testing, I finally came up with a battery pack and wanted to see if there was any interest in this mod (no guarantees yet that i will do these after just a couple).
Side notes: the internal battery tray needs some lite dremel work to ensure the batteries have a stock fit (so the tray will close properly). Basically two grooves are made to hold the batteries better. From the outside you can't tell anything has been modded.
I did a benchmark test on the batteries, and with a stock screen got just over 5 hours gameplay. With a modded LCD screen (a member on pcefx.com performs this) you could probably get 7-8 hours gameplay. This is a huge increase from what you would get on normal AA's.
And here it is installed, fits like stock door.
You can see the internal charging port on the first photo, and there is a quick disconnect that can allow it to be removed from the Turbo Express.
Also another benefit of the mod is weight reduction, just weighed 6 AA batteries - came in at 5.2 ounces, the new lithium ion pack weighs 3.5. Unit doesn't feel weightless now, but you can tell the difference. These also charge pretty fast, in about 3 hours you get a full charge. The mod would require use of your battery cover, some dremel work inside the battery tray and would also include a charge for the new battery.