I second the nomination of the double life battery from www.newmp3technology.com
I routinely get 8 days of service from my Archos 7 before the unit shuts down due to lack of voltage.
I strongly recomend that you do not recharge your battery while it is half way discharged.
The battery will develop a memory and will rapidly discharge to the halfway point and then run normally for extended periods.
The price is (US) $60.00 from www.newmp3technology.com
I also picked up the Archos 5 extended pattery plug on and while it does work on my Archos 7 it doesn't offer any significent run time since the 7 uses more current and dies in short order. I consider this $40.00 badly spent. Go with the Double Life battery instead.