After nationals, we put power expanders on the two robots that didnt have them (A and S), brand new power expanders…
We are experiencing some weird behavior from the S robots one, it seems to be back powering the cortex - so that even if the cortex has no battery, it gets it from the power expander and powers the cortex, all the cortex motors and sensors… and the power expander…
Has this happened to anyone else?
EDIT: there is also no backup battery 
Yep yep, I have definitely seen that happen before! The motors don’t get very much power though, so they perform very badly. This can be a major downer, too, when during a match your robot looks connected (because it is), but it isn’t switched on, therefore breaking the circuit for your main battery, and your robot has just enough motor power to drive off the alliance starting tile, so you can’t touch it to switch it on. The tell tale sign for this, is if you have an LCD screen with the backlight on, the backlight will flicker if it’s not getting it’s power from the main battery. Beware the flickering LCD screen.
I wish that would happen to us! Sometimes our brain battery comes loose. It cost us a match at our last tournament…
Im not sure if this is good or not, im more worried about damage to the cortex from the backpowering, because it appears that all 8 motors are being powered from the 4 motor wires from the power expander, it cant be good to have that much power going INTO the motor ports 
This should not be possible. There are some technical details on the power expander in this thread, the power from the cortex should in no way be connected to power from the second battery. Are you sure it is wired correctly?
apparently it is possible!!!
I should think that an experienced forum member like yourself would have taken up the attitude by now that any crazy thing is possible.
There could be a wiring problem or a problem with the power expander. I don’t have too much knowledge about the power expander, but I’d suggest checking your wires.
I’m a little confused by this comment.
What I said was “This should not be possible” rather than “this is impossible”.
My attitude would more accurately be described as “there is a logical explanation for this unexpected behavior” rather than “any crazy thing is possible”. As engineers our job is to troubleshoot the problem and find out what is wrong, it may be cockpit error or a manufacturing defect, but a methodical approach will always lead to a satisfactory answer.
I agree in that there are a number of theoretical possibilities including my suggestion that perhaps something was connected incorrectly to the cortex. If this is not the case then what must be suspected next is a short from the row of center pins on the input port to the row of center pins on the output port, this is where the cortex battery and power expander battery voltage exists. When I look at the schematic I created there are few other possibilities, perhaps a short between pins 1 & 2 of the pic, this would feed 5V via a 1K resistor the the cortex but the 5V regulator probably only has 150mA capability, not enough to power the cortex (not to mention the 1k series resistor).
So the troubleshooting would be, simplify the system, one motor and one connection to the cortex, is the problem still present. Are the white wires (pwm signal) next to each other, perhaps one is reversed, problem still present? If so, disconnect the motor, is the problem still present? At some point the problem should disappear, let us all know when.
For the record, I believe our Cortex was being powered by the 9v backup battery, and only the motors were being backdriven by the Power Expander (the cortex itself was powered from the 9v I believe). Is it possible voltage could be sent back through the PWM signal wires from the Power expander? Perhaps the ground is held at a different level?
Or maybe only the motors plugged into the power expander moved on our robot when this happened, I can’t remember.
This reminds me of a Guy I know, who is “basically” an EE and an Embedded Programmer for the last 36 years, working on a lot of various types of Control Systems, in various Industries…
He Amazed by the Number of People that Believe in Magic, or the Failure of it… Because they Can Not believe there is a Reasonable Explanation for Systems that Misbehave…
Yes, Crazy Things happen, but in the Mundane, there is an Rational Explanation behind it…
Im fairly sure we have it wired correctly, we have done them multiple times before
Just to clarify, nothing, not even a backup battery can be plugged in, so the power is definitely coming from the power expander.
Could it be a fault in a diode or something? Allowing the power to go both ways out of the power expander?
There are no diodes in the power expander, there is a FET that is used to enable power to the motors when the power expander senses that the cortex is turned on.
The only place the cortex power and expander power come close is for voltage sensing at the embedded micro-controller (a small pic), however, this is through resistor networks to drop voltage down to approximately 2.5V so it is within the range of the A-D converter in the pic. I would talk with VEX tech support as if the 7.2V from the power expander battery is making it’s way back to the cortex and into the motor ports then that will be bypassing the PTC breakers in the cortex.
Does the LED on the power expander operate correctly? Does the power expander only turn on when you first turn on the cortex? Does it shut down after 3 or 4 minutes when the cortex is turned off?
You said these were new, perhaps the design has changed. I’ve had one on back order for several weeks and understand they will be back in stock mid April, I’ll take a look then as the one I have works correctly but is old.
Pairs of Diodes are sometimes used on Devices that can be Powered from Two Different Sources, to prevent a Back-feed to the Adjacent Devices powered from one Power Feed or the Other…
One example from where I work has a Common Ground, but Two different Banks of Batteries, that Power a Monitor, through Two Different Switches.
In this case, the Power Expander has a Common Ground, but there is No Common on the Positive Side of the Batteries. The Battery voltage should be Totally Separate… Unless there is a Short between two sections of the Circuit Board, or the PIC16F616 has Turned into a BIG Conductive Lump of Silicone…
See jpearman’s BatteryExtendBlockDiagram1.jpg and power_expander_schematic.pdf.
Even the Inputs and Outputs, the PWR for Both is in the Center of the Three Pins, Unless you have a Jumper going from the Inputs to the Outputs, there is no connection… YOU might check for that, since the PWR is the Only NON COMMON pin in the Input/Output Banks.
jpearman: My meaning was that so many strange things have been mentioned on this forum before, so this shouldn’t surprise you.
jacko: To find out whether the problem is in the power expander, hook up a working power expander from one of your other robots to the cortex of the robot with the problem. Use the exact same wiring and port arrangement and see if it does the same thing. If the working power expander doesn’t backpower the cortex, then your original expander is faulty. If the working expander does the same thing as the one you have now, then the problem is either in your wires or your cortex.
I think I’ve seen reports before that the power expander can power up the cortex.
According to jpearlman’s pdf schematic, the cortex can’t be getting much power from the power pins of the expander, because of the 1k series resistor.
Here is another hypothesis:
The cortex is being powered up through the PWM signal wire, which is passed through the power expander and is sourced by the chip in the motors that are plugged into the power expander.
It is common to (poorly) power up a chip from an IO pin (cortex side), if the Vdd is not connected, as there is usually an ESD diode that prevents the IO pin from being driven higher than the VDD source. When VDD is undriven, a high IO pin can power up the whole chip at a voltage 1 diode drop down from the IO.
I’m not sure how the chip on the motor side is sourcing the current though.
I noticed a year or so ago that hand-driving a motor would wake up the cortex when a backup battery was plugged in.