Just a thought: one of the problems i see with reducing the weight of the robots power system is that there needs to be enough electrically energy to power and sustain the unit for x amount of time. this ussually involves one large battery or several small ones. the weight of this adds together and requires the drive motors to need more torque to move the heavier system containing all those batteries. But what if you were to use two rechargable batteries and a solar collector, a few relays and possible a comparator or time to do this: have one battery just large enough to supply the motors with enough energy for movement for x amount of time. the second battery will be charged by the soloar cell for y amount of time. When the first battery is low, a comparator, timer or relay will actuate and switch the system to the second battery which should by now be charged. The same switching then transfers the charging process to the first battery, while the robot continues on with objective. i see a few bugs in this system myself right now, but any thoughts you mighthave....

sin jimm