I found a new Chumby-compatible rechargeable battery yesterday, the "Lenmar Universal DVD Battery" (http://www.radioshack.com/product/index … 620&cp). It costs $50 and has a capacity of 9v 2600mAh, the Radio Shack website is wrong in saying it's 3600 mAh (I'll double check with my multimeter, but the battery and packaging both say 2600mAh).
The battery has almost the same footprint as the Chumby so I can just plop the Chumby on top with the cables hiding neatly behind. The battery, AC adapter, and car charger plugs fit the Chumby perfectly and don't appear to come loose at all. The AC adapter is rated at 12v 1amp and will power the Chumby just fine. The battery has an LED charge indicator showing when capacity is at 25%, 50%, 75%, and full. In addition, there are rubberized surfaces on the top and bottom which help keep things from slipping.
In my first test, after charging the battery to near-full capacity, I was able to pull 8+ hours with the Chumby on full brightness while streaming music. Unfortunately, I fell asleep after 8 hours so I'm not sure how much longer it went. I'm going to test it some more and will try to post pictures/video in the next few days, but I'm quite happy with it so far.
The main fault with this battery is that it can't power the Chumby while charging. I'm not sure why the battery disables itself when charging, perhaps it has something to do with eliminating surges when using the car adapter as Duane was saying above.
Edit: I tested it again today and got 8.5 hours out of a full charge streaming music/youtube half the time and rotating through widgets the other half. 8)