VEX rules for robot construction at first glance lead one to believe that one battery can be used on a robot:
The only allowable sources of electrical power for a VEX Robotics Competition Robot is any single (1) VEX 7.2V Robot Battery Pack of any type . . .
But keep reading, and the next part of the sentence says:
unless the robot is utilizing the VEX Power Expander…Robots utilizing the VEX Power Expander can use a second (2) VEX 7.2V Robot Battery of any type.
Wait. What? You can use a second battery? Yes, that's right, 2 batteries can be used on a robot if one is plugged into the $49.99 VEX Power Expander. The power expander is a small widget that's connected to the second battery, the cortex, and the motors that the team decides should run off the second battery. In designing the robot, one can think of it as a dedicated battery for a specific sub-system. In the 2015-2016 Nothing But Net game, Team 1666 used it to run the motors connected to their flywheel for shooting balls. Flywheel consistency and power were top priorities, so the team used the second battery to control just that system, and used the standard, primary battery (the one plugged into the cortex) to control everything else (chassis and ball intake).
Here is the diagram that VEX provides to explain how it works:
Whaaa??? Here's a simpler diagram:
So what's the point of the power expander, exactly, and why does it have to be set up this way? The main point is that it prevents a team from daisy-chaining batteries together, which would double the voltage available and indeed be an unfair advantage (and at the same time would risk damaging various components). The power expander provides a way to run up to 4 motors (or more, using Y-cables) off of a dedicated second battery.
Just like with the Cortex, teams should use a battery extension cable with the power expander. Team 1666 did not use one in their first year because the power expander's plug was easily accessible for attaching a battery directly to it so didn't need one. At California State Championships, one of the wires leading from the device to the plug broke off! Luckily the team at the table next to theirs lent one ASAP and they were off to the races again. But it could just as easily have been a complete disaster. Now the team uses a battery extension cable, which costs $5 to replace if it breaks, not $50 like the power expander.
For those who are interested in some technical specs on the load that can be placed on a power expander, please read this excellent VEX Forum post by the awesome jpearman. He did extensive testing on a batch of resistors similar to the ones used in the VEX power expander, and explains how much current they can draw before they trip their internal circuit breaker and turn your motors off.