P.S We also had to use a voltage regulator, as we use 3 x 9v batteries, 1 for arduino and 2 for servo motors.
The voltage regulator is for servo motors, we used 1 battery and it stopped working very soon, so now we use 2 x 9v but with voltage regulator, we give them 6V from 2 sources (2 x 9v ), if that makes sense.