The VEX Power Expander is now available:
http://www.vexrobotics.com/vex-power-expander.shtml and
http://www.vexforum.com/wiki/index.php/Power_Expander. This allows builders to use two batteries for one controller, and reduces the power draw on that controller. From the product description:
* Attach a 2nd battery to your robot
o Power up to four (4) Motors or Servos
o Reduce motor load on primary battery
o Run your robot for longer!
* Protected by an Internal Circuit Breaker
* Status port provides feedback to Microcontroller
o Record when your robot trips its breaker
o Measure battery voltage
* Feedback LED shows battery and circuit breaker status
* PWM Slide Lock keeps cables from coming unplugged