Red

22 cm has worked for me so far, I'm using cardboard boxes as stairs, so they might be compressing with the weight of the robot, I'll do some more tests with stronger boxes, and send a video. I have the EV3 brick on it right now, in order to do testing with the sensors, but after my science fair project is completed, I will be taking it off, and work on making it lighter. I've also found that having the 2 larger battery boxes increases the maximum torque of the motors, since they give out 11.4 V of power, rather than 7.4 V, which makes it easier to climb up the stairs. I removed the EV3 sensors, and weighed the robot. It has a weight between 1.7 and 1.72 KG. It appears that the weight of the large battery boxes is offset by the use of the SBricks, instead of IR control. 

This is what I've made so far. I'm testing different kinds of stair climbing robots for my science fair project, it is able to climb stairs up to 22cm tall.