Problem with External Battery
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Author:  SSI [ Sun Apr 03, 2011 9:48 pm ]
Post subject:  Problem with External Battery

We have been seeing a problem with our external battery monitoring. For reasons we have not been able to determine, sometimes the displayed value for the external battery drops to 0. While the value is 0, we can put a meter on the last HiTechnic controller in the chain and the voltage is good. When this happens we cannot drive the robot and if it is already moving, we will loose control (software no longer responds to joystick commands). We have set the internal system parameter (muxLowBatteryShutdown from this thread viewtopic.php?p=3396) to 0 to ignore the external battery. Apparently this is not ignoring the external battery voltage reading. Is there any way to ignore the external battery voltage and keep running the robot? We are obviously getting a bad reading of the external voltage because we have seen the voltage drop to 0 while the robot is sitting waiting for Teleop to be enabled and the robot was not moving. Sometimes the voltage will bounce from 12 to 0 and back with no hands on the robot. We have spent a lot of time pushing and pulling on wires in an attempt to find a loose wire that could be causing this with no luck so far.

We have also swapped out the first motor and the servo controller in an attempt to narrow down the problem.

Our current setup is motor-servo-motor-motor on Sensor port 1.

Can anybody explain how the voltage monitoring works on the HiTechnic controllers?


Author:  MHTS [ Sun Apr 03, 2011 10:35 pm ]
Post subject:  Re: Problem with External Battery

Do you have a different motor controller to try? It may be the motor controller, not the wiring.

Author:  SSI [ Mon Apr 04, 2011 12:42 pm ]
Post subject:  Re: Problem with External Battery

As I stated in the original post, we already swapped in a new motor controller for the first motor controller in the chain.

My main questions are:

Is it possible to tell RobotC not to shut down control of the robot if RobotC thinks the battery is low?

How does RobotC determine if the battery is low? Does it get a value from the fist motor controller in the chain, the last, an average?

Author:  MHTS [ Mon Apr 04, 2011 1:56 pm ]
Post subject:  Re: Problem with External Battery

Sorry, I am not very familiar with how that's done in the NXT firmware. You may want to send a support request to the developers (

Author:  SSI [ Tue Apr 05, 2011 8:48 pm ]
Post subject:  Re: Problem with External Battery

Well, we opened a ticket with support yesterday. We are still waiting to hear from them.

We have continued testing...

We have determined that if we don't hook up our Sensor Multiplexer, then the problem goes away. Unfortunately, we would really like to use it in St. Louis.

Author:  mightor [ Wed Apr 06, 2011 1:06 am ]
Post subject:  Re: Problem with External Battery


I saw your mail, unfortunately, I do not know how this system works. Is the sensor mux daisy chained into the motor mux or something?

I don't have one of those motor muxes so it would be hard for me to test this.

- Xander

Author:  SSI [ Wed Apr 06, 2011 7:57 am ]
Post subject:  Re: Problem with External Battery


No, the sensor mux is not daisy chained with the motor controllers. I was more wondering if there is any battery voltage monitoring for the sensor mux battery that may possibly be interfering with the external battery monitor for the 12V battery for the HiTechnic motor and servo controllers.


Author:  mightor [ Wed Apr 06, 2011 8:03 am ]
Post subject:  Re: Problem with External Battery


None that I can think of. I really wish I had one of these MUXes so I could see what the issue was :(

- Xander

Author:  SSI [ Sun Apr 10, 2011 8:36 pm ]
Post subject:  Re: Problem with External Battery

Well, we thought the problem had gone away. Unfortunately it came back. We found a way through robotC to monitor the voltage from each motor controller and it looks like the values are sometimes steady and sometimes jumping around (sometimes up to 20 volts). If anybody has any suggestions, we are all ears. We even have run just the basic competition template only modified for our motor setup and we had the false battery off indication. During one of our tests today, we received the false battery off indication while the robot was moving and we could only stop it by turning off the power. As I mentioned earlier, we opened up a support ticket but we have had no reponse from the robotC people.

Author:  team3717 [ Tue May 10, 2011 3:38 pm ]
Post subject:  Re: Problem with External Battery

We experienced the same problem and boiled it down to poor electrical connection between the battery and the motor controller...the female end of the cable plug stretched out over time and became slightly loose. Simply reclamping the female connectors helped with this problem.

Author:  kfilar [ Wed May 18, 2011 5:41 pm ]
Post subject:  Re: Problem with External Battery

We also had the same problem, but under a different set of circumstances:

In 2 different matches at the FTC World Championships in St. Louis, we ran into another robot during the autonomous period. We were driving at a slow speed (20), but it was PID speed controlled. The PID speed control tried to compensate for the slower/stopped speed by ramping up the motor power to something extreme, which quickly ran down the 12V batteries, and created a nasty smell. When the batteries reached somewhere around 13.3V (they typically get charged to the high 13V's or low 14V's) the brick decided the external battery was too low and stopped communicating with the motor controllers (or at least that's how I interpret it), including not giving the motor controllers the motor stop command that was in our program. The program ran fine and terminated, but there was still more than 13V in the external battery and the motors continued to run until the referee flipped the 12V power switch.

We have a heavy robot (36 pounds), so the wheels get lots of traction and that complicated the problem. With a lighter robot I am guessing the wheels would simply have spun at nearly the desired speed, and consequently not nearly as much power would have been fed to the motors by the PID speed control system, and the batteries would likely not have drained to the point of being ignored by the brick.

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