Internal resistance of batteries?
You wouldn't know it by purchasing from a department store, but battery manufacturers (like Energizer, Duracell, etc.) publish datasheets.  Look for their industrial/OEM sections. 

A standard alkaline Energizer AA battery has an internal series resistance of 150 to 300 milliohms, from the datasheet here.  The asterisk by that number will refer you to Energizer's whitepaper on battery internal resistance. 

Take some time to look around data.energizer.com, it's really a very good resource.