Q251 was just posted to the Q&A about the legality of using the 9V backup battery to help eliminate issues with the VEXnet connection dropping out.  I have read through some old forum posts on the VEX site (many from members of this forum) and was wondering what the current thoughts were on this issue.  

We consistently have connection issues at competitions too (2 rounds this year and during practice the night before at the venue).  I believe that all connection issues were while simply driving with the 2 large motors (ports 5&6).  No motors were at stall and there should not have been overly large loads on the cortex.  

Of possible note, our hub has never replaced the power connector on the cortex/batteries.  We still use the connectors that ship from VEX.

We have not been using the 9V since it was removed from the returnables in 2015.  This year we used a 6" dongle to move the VEXnet key further away from other electronics

Would the 9V be helpful if it is deemed legal?  and if it is helpful, any ideas why is was removed?
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Make sure to bring your programming computer and programming code to regional,
maybe the regional spare parts person can swap you out with a power pole pigtail cortex, which you could then reprogram to your code.

Hypothesis 1: white battery connectors jiggle loose when bumped, causing disconnect.
Test 1: whack your robot with a hammer in all directions, to look for disconnect from that cause.
Solution if true: Spread male connector pins, shock-mount your cortex, etc

Hypothesis 2: Motor over current, (possibly in connection with weak resistor battery connection)
Test 2: Hold the wheels to make sure the motors will stall, and try to drive in all directions, to look for disconnect from that cause.
Test 2b: do many forward/backward jerks without holding wheels...
"Stalled Motor" condition is when axle isn't rotating, so there is no "back EMF" to restrict current.
Note that every time you start up from a stop, it is a stall condition.  Many drivers treat the joystick analog sticks as digital buttons, and flip 0,100%,0,100% which is many times more start from stall.
This can be fixed by reducing the slew rate of change to the motors, either by more careful driving, or by software.   See Slew rate control  in vexforum.

If the behavior of your robot is due to the above issues, adding 9v wont help.

Regarding 9v battery backup:  my understanding from talking with Corey from Vex (or someone similar), is that the 9v battery only helps if "Game Mode" is activated by plugging something into the Competition port of the Cortex, (and Game Mode keeps wifi and cpu alive, even if main battery is < 5v due to overcurrent, and Game mode also does some other stuff that may be helpful to avoid disconnect problems due to other sources).
I have made 100 Game-Mode adapters (we call them "wifi antennas" as a simplifying lie) that we tried without additional 9v at Collin County BEST on Oct 22, with harmless results.
I will use these at Texas BEST also, and I am making them available to the other regionals (or hubs that run regionals) 
We can manage the "wifi antennas" the same way we used to do with the crystal boxes for RGBY quadrants, and it will help with connection problems other than current overdraw.
We might implement 9v backup in another year.  I think this year we will treat over-draw as a condition caused by robot design and operation, which are under student control.

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Thanks for the input.  I will definitely have the students try to troubleshoot per your suggestions (although I cringe at them getting carried away with the hammer).

We already employ a basic slew rate solution through Simulink, but have wanted to refine it.  I will share this post with our programmer so he can keep in in mind as he looks into that.

I have brought the power connections to the attention of our hub director.  Hopefully I can gain some traction on having them updated.  In the meantime, I may make the power pole modification on a backup cortex/battery and see if it has an impact.

My gut (which has been known to be wrong) is that it is your hypothesis 1 is the cause.... or maybe 2b... we shall see

Of course, I don't remember any connection drops / power losses while running tethered (which we did during regionals semifinals a few years ago), but that is a much smaller sample size.
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I seem to remember a document in the file manager that detailed the conversion of the cortex and batteries to powerpole connectors.  Does it still exist and if so, where?  I have not found it yet.  I would like to ensure that any changes I make to our own equipment is in line with the approved BEST changes that the hubs can do.

OK, so I now realize that the document I am thinking of is for hub personnel to use in preparing the kits.  It is probably no longer posted for the general BEST community.  
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