gizzmotronics:
I'm sorry - I think you misunderstood. I want to turn left/right, not power the wheels. And I am not sure that the batteries are car batteries - They are from a razor electric scooter. They say "sealed lead acid 12v" so I assumed they were car batteries.
You will need to figure out how much torque is needed to turn the wheels of your fully loaded cart, and the speed at which the wheels need to be turned left/right. You probably need to look into a linear actuator or some type of "monster servo".