Power Expander Burning Controllers

We have attempted to connect power expanders to be able to use 2 batteries, but every time we made the attempt it has burned a hole thru the motor controllers. It has to be some kind of short but we can not seem to put a finger on the root of the problem. The last time it caught on fire. Safety is a issue now. Any ideas?

Just to confirm, you are plugging it in the motor controller in the 3 wide section ( so it is possible to get 4 next to each other.) I have seen parts smoke if plugged in the opposite way (90 degree turn). Post a picture of your setup (WITOUT A BATTERY)

As 536C suggests, it sounds like a wiring problem. Can you conform that you have followed the wiring scheme show on this wiki page (which is dated and shows a PIC but that’s not important).