Another thread that nobody can respond to 
So if one were to put regular alkaline AA batteries into the 6 battery holder that comes with the Vex system, you’d get 9v. Of course, those batteries would not last very long and you’d have to replace them all the time. So that basically means you should be safe running anything up to that.
I personally have switched to an 8.4v 4000mah battery, 7 cells instead of 6. I wouldn’t take it one further because 9.6v batteries can output 10-12v at a fresh charge, but I have seen videos of people hooking transmitter batteries up to their robot.
Of course, all that is out of the intended spec of 7.2v.
That all said, the much smarter solution to your problem is to reduce the load on your servers. Make the device lighter, add some springs or rubber bands or surgical tubing to reduce the load. That solution will make your robot much happier.
Another thing that I noticed about your arm was the fact that you are trying to get it to go 1:1, Just as a heads-up gearing it for torque is advisable since if that arm was even liftable, it would move far too fast to be controllable
I didn’t actually look at the pictures before, but now that I have… I think you should be using a geared down motor. You could gear down the servo, but you won’t get as much range of motion.
Is there a reason you are using servos here instead of motors? In my humble opinion, there are very few places where the use of a servo is better than a motor, which is rather unfortunate.
Also, you have an intake roller where a tank tread drive gear should be 
You might consider adding some counter-weight to the arm so that it is closer to being balanced. You could do this by extending the arm back and adding some weight, or you could use springs or elastic to help overcome the weight of the arm.
I agree with Lmui that the servos are going to go try to go turn faster than the arm can move. You might want to program the controls to ramp the servos so they don’t start or stop too quickly.
A better option would be to switch to using geared-down motors to run the arm, and add a potentiometer to read the arm position. Again, you’ll have to do some interesting programming, but you would have a great deal more power.
Cheers,
- Dean
Here are some more thoughts. First, put the tread back on your wheels. A wide robot with a short wheelbase like yours will work fine with the tread on, and you can use the grip. If you don’t know what people are talking about when they say to gear up the arm, take a look at this picture: https://vexforum.com/gallery/showimage.php?i=2646&catid=searchresults&searchid=279806 That’s a 12-tooth gear driving a 60-tooth gear. If your arm is not really lightweight, I’d go with a 12-tooth driving an 84-tooth. Also, try putting your tracks closer together. It will work better than counting on the wire ties.
For a first robot, this is very nice. Keep working at it! Don’t forget to look through the gallery on this site for more ideas.