Hi Frank,
If you dont mind a thin battery stuck to the rear of the unit, you can use any lithium device from a cellphone. Just solder leads from the battery to the spots on the nano. The nano already has a lithium charging chip. I did that on my first unit. Standalone cellphone batteries already have protection circuitry built-in. This has already been discussed back in June.
If you would rather have a small lithium so you can install it inside, go to your local discount dollar store and buy a $5 mp3 player, open it up and use the battery.
Regards
Larry
