this topic is indeed a hot issue, when using the arduino stand-alone with battery-supply. i´ll try sleep_mode next days, but also throw up the question which battery supply do you prefer?
for my last project, i was dealing with atmega8L working on 3,3V and 8Mhz supplied by cell-phone battery (3,7V) from an old nokia-phone. good thing about these batteries that they are really cheap NimH or even Lithium Ionen solutions and thousand times rechargeable, plus as an extra also very "handy"
Nice work on getting the sleep code up and running.
For my project I want the microcontroller to wake up every 5 minutes or so and update one variable. Seems like a waste to not put it to sleep during the in-between times, but I don't want to have to manually wake it up, I was hoping it would wake up itself automatically.
Is there any way to use an internal interupt to wake it up, or alternatively, is there another (better?) way to do it? I think some of the PIC range have built in real time clocks, but of course who wants to use one of them...