[RyanN]

The voltage stays the same, but the amperage increases by 100% for every extra power source that you use as long as they are all the same. I would use some diodes to keep every battery separate. In this case, the voltage will be the (voltage - (I think) .4V) of the most charged battery, but the amperage will fluctuate as you use the batteries. For example, if you are using a 9V battery, 4AA batteries, and 2D batteries, the amperage at 9V will be much lower than at 6V, and then the amperage at 6V will be much lower than at 3V. Hopefully this makes sense.

Without some sort of power separator device, your voltage should be the average of all the power sources, and the amperage should be steady.