I'm oversimplifying it, but...
Batteries in SERIES:
- You add the voltages of the batteries
- Higher voltage = higher RPMs from the motor
Batteries in Parallel:
- You add the capacity (maH) of the batteries
- Higher capacity = longer runtimes
Also, there's been a lot of experimentation with using Lithium Polymer (aka Lipo and LiPoly) batteries. Lipo offers close to the same capacity, but 3x the voltage.
I've got these batteries on order to test how well they'll work in my Bits and ZipZaps:
http://www.indoorflyer.co.uk/index.a...&productid=116