Keep in mind, most 9v rechargeables (at least the ones I've used) have significantly less capacity than alkalines. I tried some in a wireless guitar body pack. 1-2 hours, tops. And the charger snapped so tight to the terminals, it took the top off of one. Not worth the effort at all.
Quotethe OP is probably talking about a standalone atmega328 or similar arrangement,Ha, you read a lot more into his post than I did, LOL. He better not be drawing more than about 25 mA total load, else the 9V battery won't last very long, cf 500 mA-hr of energy,http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=198791&langId=-1&storeId=10001&productId=198791&search_type=jamecoall&catalogId=10001&ddkey=http:StoreCatalogDrillDownView
the OP is probably talking about a standalone atmega328 or similar arrangement,