Our team is reassembling an old KOP chassis to help teach programming, and we’ve gotten to setting up the electronics. We wired everything up and connected to the driver station, but the driver station displays that the battery is at 11.8 volts even though we’re using a new battery that measures over 12 volts when we check it with a battery beak. We’ve tried multiple batteries, and checked all the electrical connections all the way up to the RIO. The problem seems to be somewhere between the RIO and the driver station. Has anyone experienced this before or have any advice on how to fix it?
Are you using a Rev PDH or Vex PDP?
What voltage does the Battery Beak give you exactly? Also could you include a picture of the robot as it currently is?
To add to this, which voltage are you looking at?
What is all connected to your robot? If there is a coprocessor and many LEDs its possible that the current draw is not near the no load range.
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Connected to the robot are 4 CIMs, 4 Talon SRXs, a Vex PDP, VRM, RIO, and Radio. We’ve been looking at voltage with no load, but all of the voltages are always within .1 of each other.
Depending on the battery, a battery fresh iff the charger will give bewteen 13.6 to 13.3 volts. The voltage usually drops down to around 12.8 or 12.7 after being on the robot for a couple minutes, but doesnt signifigantly drop past that volatge.
I can’t get a picture of the robot right now, but I will get one as soon as I can.
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Sorry I guess I realized that the roborio is likely what is doing the voltage measuring for the DS. How is the roborio wired?
I’m thinking that either the roborio is bad or it’s wired to the VRM causing the voltage to hover at 11.8V which is just what the “12V” output of the DC regulation is providing.
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The rio is wired to the vrm. We checked the 12V output of the vrm and it was at 12.2 volts, but we figured this was normal since the output was supposed to be around 12 volts. We also replaced the rio with a different one and still had the problem.
Don’t wire the Rio to the VRM. It should be wired directly to the PDP/PDH. The voltage reported at the DS is the Rio’s input voltage, the Rio uses its input power to determine when to start browning out motors based on low voltage, and the rules require it be wired this way for competition.
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Please reference Introduction to FRC Robot Wiring — FIRST Robotics Competition documentation
But yeah that’s the expected behavior. The regulated output will prevent the rio from reading the correct voltage. Per the rules this is not allowed.
I’m not an expert on Boost Buck Regulators but the measured voltage difference is likely the load on the regulator vs being unloaded. Or just accuracy in the different measuring tools.
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Battery beak reports voltage at a different loads than when the battery is plugged into the robot. Therefore a different voltage. The driver station reports the voltage measured by the roborio.
The VRM output is boosted by the VRM. You should measure the voltage at the battery or PDB and compare that to the voltage on the driver station. When the Rio is connected directly to the PDB per rules…
Battery voltage for a new, charged, unloaded battery should be 12.6+. If not charge the battery
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On the somewhat related topic of Battery Beaks, has anyone noticed different battery beaks giving different readings? This past year we discovered that one of our battery beaks was reading Rint (internal resistance) values about 0.004 - 0.005 ohms higher than the other (throwing off other values as well). Couldn’t think of a good explanation for it, but it’s enough of a difference that we’d consider it significant (it doesn’t sound like a lot, but 0.015 ohms and 0.020 ohms is basically the difference between a great competition battery and a pit battery, IMO).
Did you try cleaning the battery contacts on the battery beaks? That should be the only place you could introduce a difference.
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I would assume the other values changed as well? Because you can’t measure the internal resistance directly it’s more of a calculation
We have between our V1 and V2.
Thanks! That fixed it.
I suppose that’s worth trying. Neither seemed particularly more dirty/worn than the other but that could certainly contribute.
I mostly pointed out the resistance because that was the most noticeable (and consistent) difference, voltage values can vary even between multiple checks with the same battery beak so it was harder to get a good idea of the change, Rint tends to be pretty consistent between checks of charged batteries in my experience (TBH, we basically ignore voltage values beyond checking if it’s over a certain point, mostly we compare Rint between charged batteries).
I didn’t realize there had been a second revision of the Battery Beak released at some point. That could very well explain the differences too.
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