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Unread 10-23-2018, 11:57 AM
enderdrag64 enderdrag64 is offline
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Voltage issues, brownouts and battery drain

My team began practicing for our upcoming off season recently and we've noticed major issues with our robot's battery life.
We have to consistently change our battery at least every two runs or it browns out. This is less than the amount of time a match takes (about two minutes), and we've never had this issue before (on this robot).

There is another related issue, where driver station's listed voltage starts low and increases after a small amount of use.
For example, a new battery might read 12.6V resting in driver station, and after a small amount of driving it reads 12.8V resting.
Normally it would start high and gradually decrease.

We have encountered issues with brownouts before on a practice robot from a previous year, and the solution to that involved replacing a bad battery connector.
We replaced the connector last night and it does not appear to have solved the problem.

Any help would be appreciated, thanks.
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Unread 10-23-2018, 12:06 PM
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Re: Voltage issues, brownouts and battery drain

Have you looked at the reported current draw from the PDP? Knowing what that looks like may help point in the right direction. The total current, plus the individual channels. You may find that one or more channels are high - worn wheels can cause a drop center to not drop as much anymore, increasing scrub and power requirements on the drive train, for example. Looking at the individual channels can help lead you to the issue.

Also feel around your robot after practicing - is anything unusually warm? A set of connectors (like the battery connector) that's unusually warm would indicate high current, which may come from a bad connection. A motor that's too warm could indicate a problem with the motor, or that the motor is working harder than you expect (possibly a problem with the mechanism that introduces too much friction).

Have you noticed any performance issues while driving around, other than the battery dropping too quickly? Is the robot browning out while turning, driving too slowly or with a pronounced turn to one side or the other? Is a mechanism not responding as you would expect?

Have you tracked which batteries you were using during the tests? Is it always the same one that's a problem, or is it general across all the batteries? Have you verified the batteries are in good condition and fully charged before starting (using something like a battery beak for a quick check)? Are the leads on the battery fully tightened, such that you can't wiggle them?

It's hard to diagnose an issue like this over the internet, but hopefully looking into those areas will help identify where the issue might be.
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Unread 10-23-2018, 12:09 PM
CooperWard4499 CooperWard4499 is offline
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Re: Voltage issues, brownouts and battery drain

If all of your connections are good, there should obviously not be a problem there. However, depending on what drive train you are using, there could be some variation there.

I'm not very well versed in programming, but I know there is a thing called "brownout protection" which prevents this kind of issue. It could also be that you need to get new batteries, as they do lose their ability to hold charge over time.

if you could maybe give some more information about your robot, that would help greatly.
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Unread 10-23-2018, 12:14 PM
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Re: Voltage issues, brownouts and battery drain

Check all your connections between the battery and PDP. A loose connection there can cause significant voltage drops and battery draws.

Also check that the chassis is isolated from both the positive and negative battery terminals. A short to the chassis could (in theory) slowly drain the battery.

One more thing to check is the roboRIO power light color. When it's on, it should be green. If it's red, that means that there is a short somewhere inside the roboRIO or on one of its connections.
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Unread 10-23-2018, 03:02 PM
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Re: Voltage issues, brownouts and battery drain

Quote:
Originally Posted by CooperWard4499 View Post
I'm not very well versed in programming, but I know there is a thing called "brownout protection" which prevents this kind of issue. .
"Mitigates" is probably a better description - In a nutshell, it simply shuts off motors when system voltage gets too low. Stops the processor from resetting, but also causes some pretty abrupt behavioral changes to the robot.

The 0.2V increase in unloaded voltage after a tiny bit of driving isn't overly surprising to me. I'm sure there's some non-linearities in the behavior of lead-acid batteries that could explain this. If nothing else, a bit of temperature rise might increase the output potential of the battery? Or maybe just measurement error? I wouldn't worry too much about that.
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Unread 10-23-2018, 03:40 PM
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Re: Voltage issues, brownouts and battery drain

Concur with above about checking for heating and looking at the power logs and lights. Also check for friction in your drive train and major manipulators.

Quote:
Originally Posted by enderdrag64 View Post
There is another related issue, where driver station's listed voltage starts low and increases after a small amount of use.
For example, a new battery might read 12.6V resting in driver station, and after a small amount of driving it reads 12.8V resting.
Normally it would start high and gradually decrease.
This is strange. This must somehow be caused by the heating as current flows through the system for a bit. My best (wild) guess would be that somewhere you have a bad crimp of dissimilar metals, possibly steel around copper. Copper has about a 50% greater thermal expansion coefficient than steel*, so as the joint heats up, the copper is better forced into the steel, reducing the resistance and increasing the voltage at the RIO.

*Aluminum is even greater, but you're following R46 and R59 and using copper wires, right?
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Unread 10-25-2018, 09:16 AM
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Re: Voltage issues, brownouts and battery drain

Normally a 0.2 volt difference would not bother me as that may well be the resolution of the A to D in the PDP. However, couple that with brownout and I would start to look for a bad connection somewhere in the primary electrical circuit. I start by checking if any of the #6 connections move when you wiggle them. Check the battery, the main breaker and the PDP. Tug on the #6 wire at each of these connections to see if there is any movement of the wire in the connector. If everything seems great (be aggressive please) then I would start to look for a high friction mechanical system. It is possible that the robot frame is bent and things are not moving as well as before. Put the robot on block and move the wheels by hand with no power turned on. Does everything move as expected?
An old adage for me is to "go where the last person was" when checking out a failing system. It could be something got switched, motor polarity, speed controller etc. Make sure all lights on the speed controllers act the same way for the same motors. Since the battery connector was the last thing changed, go there first. Make sure all connections are tight. If you make up your own cables, was the wire old? Was it discolored or crusty? It is possible to have a bad connection due to the wire not being pristine. Scotchbrite works wonders cleaning the copper wire prior to assembly, crimping or soldering.
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Unread 10-25-2018, 10:17 AM
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Re: Voltage issues, brownouts and battery drain

I'll add one other easily-missed point of failure - bad crimps. I've seen teams have bad crimps in their battery lugs, or the lugs where it connects to the main breaker or PDP. Everything looks just fine because the electrical tape covers it up, and if you grab the lug when doing a tug/twist test nothing moves... but the wire has mostly pulled out of the lug. It may be worth pulling off the electrical tape to perform a visual inspection of these crimps, and then reapplying if everything looks good.
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Unread 11-07-2018, 10:03 AM
enderdrag64 enderdrag64 is offline
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Re: Voltage issues, brownouts and battery drain

Thanks for the assistance everyone. We figured out the issue, it ended up being a faulty Talon. We had to replace it because it was causing other issues (it was constantly getting too stale errors and was only reading 7 volts while all the other Talons were reading 12). When we replaced it our battery issues went away.
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