I've built a nice little project (a MIDI controller for the Behringer DEQ2496 - I'll write it up somewhere when I have time). The project is designed to be portable. At the moment I have the arduino in a handheld case along with a 9v PP3 battery, connected with a wire similar to http://www.sparkfun.com/products/10512
To turn the arduino off, I unplug the wire. To turn the arduino on, I plug in the wire. This is not ideal.
Ideally (and I know this might be a bit ambitious), I'd like to be able to have a rechargeable battery, that the project runs off when it's not plugged into the mains. When it is plugged into the mains, I'd like it to run off the mains, and recharge the battery. Additionally I'd like an on/off button/switch, so that I can save the battery when I'm not using it. Finally (I said it might be a bit ambitious), I'd like a battery indicator. I'm thinking of attaching a graphic display to the project, so if this data could possibly be sent to the arduino, I could display the battery status on the screen.
Any idea which parts of this are feasible, and which parts aren't?
What I know about batteries could be fitted onto the back of a postage stamp, so if anybody has ever come across a nice tutorial on them, I'd appreciate it.