I have a robot with 8 motors so far. It has 4 drivetrain, 2 mogo, and 2 scissor lift (one side). I am adding the other side of the scissor lift, but I’ve used all ports 2-9. I have a power expander. I have a serial Y-cable (like the one for LCD). I don’t know how to add any more motors. Is there a way to do this with the materials I’ve listed?
For the first side of the scissor lift, use a y cable connected to a power expander and connect the motors to that. That frees up one more port on the cortex where you can attach a second y-cable. As far as i know, this is the easiest way.
As @avspark said, use y-cables. I would recommend 3-wire y cables, but you don’t seem to have any. I cannot think of a way to use a serial y-cable for this purpose off the top of my head, but if you can come up with a way, it would be entirely legal provided you do not physically modify the cable. You are able to use ports 1 and 10 of your motors are close by (or you have 2-wire extension cables), and they will actually give much better performance than ports 2-9, as the motor controllers are integrated into the cortex, so if you can use them I would actually recommend it (for more info, see this). However, you must make sure not to mix motors from 2-9 and 1/10, (e.g. the left mogo lifter is in 1 and the right in 2) because they will respond differently. I would recommend buying y-cables though; you’ll probably need them in the future anyway and they’re pretty cheap.
So what does the power expander do?
Allows you to plug in another battery and direct that battery’s power to specific ports.