How do I determine the maximum amp output of a battery pack?
Batteries, like any source, can be modeled as a Thevenin-equivalent circuit. They have some equivalent impedance.



As you pull more current out of the battery, it's terminal voltage will drop. The datasheet for a particularly nice Energizer AA battery lists an internal resistance of 90-160 mOhm. You'll get some voltage drop just from the instantaneous current draw, which will go away once the load is gone. The sheet also gives other useful data, like discharge curves for different loads. I got that particular datasheet from Digikey, where they list several other similar batteries. I'd look for the manufacturer of your battery (or alternately, get a different battery for which you can find a datasheet) and see if they provide similar information.