[Al Skierkiewicz]

How about we just stick with the 24 volt requirement for all discussions. That will simplify matters for the majority of the people who read these posts.
Yes, you can stack (series wire) two 12 volt batteries to produce the 24 volt requirement. However, everyone should note that the load on any given battery will change it's terminal voltage dependent on the current supplied to the load and the internal resistance of that battery. It is not uncommon for our robot batteries to be pulled below 8 volts during normal running conditions. That is why the 8 volt trip point was selected in the IFI controller and why Eric and Russ designed the 24 volt power in the new 2008 PD to maintain output below 5 volts. The internal resistance of our batteries is typically 11 mohm and can deliver over 600 amps at full charge. CIM motor stall current is 129 amps so four or more motors in stall can draw down the terminal voltage significantly.