Wow, 50A is gonna require a pretty serious power supply. Can you give us more info about the motor and what it's doing?
For 50A, you'll need a separate supply, with somewhat careful attention paid to connectors and such. Take a look at what the remote control people do for their high-power electric motors...
I'm actually using a remote control car as the base for my robot, see http://megabotblog.blogspot.com/.