My team and i are preparing for an offseason competition with our 2018 robot and bumped into some problems with the electrical system. The robot is drawing too much voltage from the battery and also heating the battery wires up.
We’ve tried trouble shooting the problem by disconnecting the compressor, because we have a minor leak in the pneumatic system, but that’s another fix for another day. This decreased the load on the battery, but we could only drive for about 30 seconds before dropping to 5 volts.
When we went to disconnect the battery, we’ve discovered the battery wire was burning hot. The wire that connects the main power switch wasn’t warm at all. Also, there were no other warm wires on the robot.
We’ve tried 7 different batteries from different years including new ones, but they started to dip below 5-6 volts about 30 seconds after driving around only with the drive train.
There were no modifications to the robot before we went on summer break. Before that, we had no problems with it.
I just wanted to ask for some ideas what the problem could be, so when we go back to fix the robot Monday, we have a place to start.
To add on to what Jamie said, there are ways to check that pretty quickly.
The roboRIO Dashboard has a current monitor per channel, and it’s also an item you can see in Shuffleboard on your Driver Station if you add it… Should help you identify if a given motor / motor controller is drawing lots of current causing those drops.
If you remove an item from that channel (physically unplug it / remove it) and it’s still abnormally high current draw, it’s possible the PDP is having issues.
Additionally, it’s possible your main breaker is problematic, as after so many trips / switches they do wear out. No quick way to check this out though, you’d want to just swap it out for another and see if that still happens.
Hot wires are also a symptom of high resistance. Since you say some of the wires are not getting hot then excessive current draw is not the problem as all of the wires would be heating up similar amounts since they are all the same gauge.
The proper way to find the problem is voltage drop testing. Put the robot up on blocks. Put your volt meter + probe on the battery + terminal and the - probe on the main breaker battery terminal. Then run the robot at full speed for a few seconds and observe the voltage on the meter. Repeat with the + probe on the PDP - terminal and the - probe on the battery - terminal and observe the voltage.
^^ This. For your battery wire to get hot in this amount of time means drawing way more current than you can pass through the compressor motor without burning it up. If you find yourself with hot wires, check around to see what else is hot, starting with motors, then motor controllers, then the PDP (may be hot near a specific circuit, or between the 6AWG wires).
A bad main breaker should not cause this sort of behavior, as it does not provide a bypass route to short out the battery. A bad PDP can do this, but it is less likely than one or more motors/controllers.
Longshot idea: perhaps you have two motors running against each other on the drivetrain or a manipulator?
A connection that is not sufficiently tight (crimped or bolted) will have high resistance and will cause heating near end of the wire with the loose connection. If you pull on the wire and it moves relative to the crimp lug, it is too loose. If you push on the crimp lug with the end of your finger and it can rotate on the bolt, it is too loose. If you have loose crimps and/or loose bolted connections, check all the rest of those connections. This is indicative of a procedural (technique) and/or equipment (crimper) fault and there often multiples of these faults.
A short circuit will cause the whole length of the wire to heat up.
Please note that it is possible to have both a short circuit and some loose connections.
Regarding this - one of the more common places we’ve found high resistance is corrosion on the Keps nuts on the main breaker leads. If you leave it for long enough the plastic of the breaker will melt (that’s how we found the issue).
Tamas,
If you are sure that there is no short on a branch circuit of the PDP then you either have stray metal in the PDP or the battery connector on the robot side is damaged. A quick check is to remove all breakers on the PDP. If the battery is still being affected then check the PDP and wiring leading to the PDP from the battery. It is not uncommon for shavings and conductive debris to get inside the PDP and short out the input.
If all appears to be normal, then add the breakers back one at a time while observing the battery voltage. If one breaker suddenly changes the battery voltage by a volt or more, leave that breaker pulled out and continue. If you get a normal response from the battery until you insert that one breaker, you will know that something in that branch is the culprit. While very rare, battery connectors have been known to fail, mostly due to mishandling.
Use a multimeter to confirm that you aren’t electrically connected to your chassis somewhere (measure resistance at multiple points, it should always be higher than 3kΩ). I have seen this happen with motors and with the compressor, although usually it just means that the chassis is grounded and won’t necessarily do anything unless an actual short develops. But if your chassis got grounded through wear and tear or damage, and later a short developed through that path, you could be shorting the battery and never seeing any sign of it in software or even the main breaker. This is why we have R48.
It might help you to run through the electrical inspection checklist and confirm that the robot passes before looking for more interesting theories.
If the wires past the main breaker aren’t becoming unusually hot, there could be a short or a bad connection in the battery connector or the main breaker. Try replacing the battery connector on the robot. If this issue was present with multiple batteries, that probably rules out an issue with the connector on the batteries, but they’re still worth a check, though this would cause the batteries to discharge very quickly even when not connected to a robot. As mentioned by others, replacing the main breaker wouldn’t be a bad idea.
The electrical path is from the flat part of the terminal (not the threaded post) on the breaker to the crimp lug. It does not go through the nut. The nut is there to hold the lug firmly against the terminal so that the contact between them is good. If you are getting enough heating to melt the plastic of the breaker, it is likely that the nuts were not tight enough, coroded or not.
While checking terminations, don’t forget to check the terminations at the battery terminals. If you checked these early in the season and haven’t since, this would be a good thing to do.
You may also want to check the battery terminals to make sure that they are free from corrosion - if you do see signs of corrosion you may need new batteries.
The OP mentioned that the batteries were hot as well. Were they uniformly hot, or was the heat concentrated in one area. As batteries age, they will eventually drop a cell. Under load that cell will be much hotter than the other cells.
Here is a thermal image of a battery that demonstrated a failed cell under testing this year.
I just reread your original post. Wre the battery wires hot just near the SB50 connector or along their whole length? One or both of the crimps for the lugs in the SB50 connector may be loose if the wires are hot mainly near the connector.
If there is a short across the 12V, you would not have any voltage to run your robot. Also, those connectors are designed to make it very, very difficult to short from one side to another.
Look inside the two SB50 connectors on the wires going to the battery and on the wires going to the robot. Are the contacts deformed?
Lastly, use a small screwdriver to depress the spring steel latch that holds the contacts in the SB50 connector and pull the wire and lug out. Look at the area on the contact where it makes contact with the the other connector. I have noticed that the silver(?) plating is physically worn off many of the contacts. This may lead to a high contact resistance and heating where the two contacts touch.
If you can get access to a thermal camera like the one phargo used, it will lead you to the problem area very quickly.
The batteries weren’t hot themselves, only the battery connector cable. It got more hot towards the end of the connector and it was normal temperature after the main breaker’s battery connector.
That sounds like bad crimps inside the battery’s SB50 connector. The resistance from the contact inside the connector to the lug at the battery should be a bit over a milliohm on each side.
If you measure the resistance of a contact with a normal DVM with a normal PAIR of probes, and you get a reading of under anything around 4~5 Ohms, it is a good connection. It requires a 4-probe technique with a higher end DVM to properly measure anything below 10 Ohms.
You can pull up driver station logs and trend voltage drops and current draws to find out what is using current and how much. You can also go back and look at earlier times when you were not having this problem and see what has changed. This feature is an often overlooked diagnostic tool. These logs stay on the driver station computer until you specifically delete them.
I bet that you have a contact/spot resistance issue in a battery connector, likely on the robot. Carefully inspect any crimps or contacts (metal pads inside the Anderson, for example) in this area. Ideally, post pictures of all of these!
Also, if you decide to probe this contact for resistance with a DMM or Ohmmeter, DO NOT PROBE THE CONTACT SURFACE DIRECTLY. You risk raising a burr on the contact, which could turn into a significant spot resistance on its own. Use a dummy connector and probe the wires off of each end of the connector (added benefit of testing the as-used Anderson connection) or probe the bottom/crimped area of the Anderson terminal.
This could still be a bad SB50 connector on the robot side if it occurs with more than one battery. Remember that temperatures on the inside of the battery will not transfer to the case immediately. That being said, that battery internal could be hot while the case seems cool.
I have seen students modify (bend) the spring steel under the SB50 contacts to make insertion easier. This will reduce the contact pressure and ultimately lead to high resistance and heat at the terminal. If you crimp your own SB50 pins and do not use a valid crimper, what method do you use? Over the years, teams have reported that they used a hammer, pliers, etc. I have a good method for crimping using a bench vise but we (WildStang) use the AndyMark crimper and then solder our connections.
A worn SB50 will allow the contacts to move quite a bit inside the connector. (side to side) If you wiggle the wires and the contacts move a lot compared to other connectors, it is time to trash that connector, shell and contacts please. A high temp terminal will show color changes to indicate the terminal is the source of the heat.
I have seen many robots that have the conditions you describe. In worse case, the SB50 shells melted and fused together.
