Hybrid robots? What's the hybrid part of it? Electical and Electrical?
First of all, I do not think you will actually be able to charge your battery by any significant amount, if at all.
Motor D: Drive Motor
Motor G: Generator Effect
Motor D has 2 places where it's torque is being used. Motor G and the wheel(s) it is powering. Each provides a different load on the motor. The motor can only provide x output, whereas power to G + Wheels = D output. Also, for D to power G, D must have an input power source (from the battery).
Code:
|BATT|--RES--[ M ]--***--[Wheel]
| |
------RES-------[ G ]
The circuit would look something like this. RES being electrical resistances, *** is Resistance from friction within the gearbox (although relativly low). The loads upon M are outward from that. Anywho.. the turning of G is a load on M, so any electrical force created from G will be less than what is given to M. So the Net electricity used will be lower. BUT... to have equal output to the wheel, (as if G was not attached) more current must be supplied to M.
All in all.. we don't have the tools for perpetual motion.
In the hybrid cars, inertial force is used to generate electricity as well as the gas motor, which is not driven by the electrical motor.
Take a look at Toyata's Flash model of how the Hybrid Prius works:
http://www.toyota.com/vehicles/2004/...syn_drive.html