Light chasing BEAM robot is powered by 3.7V lipo battery which is charged by 2x 4V solar cells. Basically the robot follows light and moves faster when there is more light available in the area but when in dark it comes to complete stand still, this is done by photo resistor on the back. This may not be conventional type of BEAM robot but it gives you a lot of fun. For more information please visit my website www.andromedabots.com where you can also buy kit for 20$.