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Safety·Lesson 19 of 28

Troubleshooting Brownouts and Power Sag

A symptom-to-cause workflow for mid-match brownouts: read the DS log, check wiring/breakers/battery, then apply current limits.

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Symptom: the robot freezes or loses subsystems under load, the roboRIO power LED flashes amber, and the Driver Station voltage display flashes red. That is a brownout - battery voltage crossed the roboRIO threshold (6.3V on a roboRIO 1.0, 6.75V default on a 2.0) and outputs were disabled until voltage recovered above 7.5V.

Debugging workflow (work top to bottom, change one thing at a time):

  1. Confirm it's a brownout, not a disconnect. Open the Driver Station Log Viewer after the match. Brownouts show as markers and a rising '12V fault count.' A comms drop looks different (radio/CAN). Don't fix the wrong problem.
  2. Check the battery first - it's the most common cause. Test with the Battery Beak: a battery reading 12.5V on an idle robot, or internal resistance above 0.020 Ohm, will sag hard under load. Swap to a battery that reads 12.7-13.5V open-circuit and under 0.015 Ohm. Never start eliminations on an untested battery.
  3. Inspect the high-current path. A loose SB-50 connector, a corroded or under-torqued battery lug, or wire thinner than 6 AWG between battery, 120A main breaker, and PDP/PDH all add resistance and cause sag. Wiggle-test connections; redo any that move.
  4. Look for a mechanical stall. A jammed mechanism pulls stall current continuously. If one PDH/PDP channel spikes, you have a binding gearbox or an over-driven motor.
  5. Add current limits in firmware. On a CTRE TalonFX set a SupplyCurrentLimit (e.g., 70A dropping to 40A after 1.0s) and enable it; on a REV SPARK MAX use setSmartCurrentLimit. This caps draw so voltage never reaches the brownout floor.
  6. Fix gearing. If wheels can't slip until far above ~40-50A per motor, the drivetrain can stall the whole bus. Re-gear so wheels slip before motors stall.
  7. Re-test and verify. Re-run the same driving pattern, watch RobotController.isBrownedOut() / getBatteryVoltage(), and confirm the DS log is clean. Don't declare victory without re-reading the log.

The #1 mistake is blaming code when the real cause is a tired battery or a loose lug. Always test the battery and the high-current connections before touching firmware.

Key takeaways

  • Confirm brownout via the DS Log Viewer (markers, 12V fault count) before changing anything.
  • Test the battery and the SB-50/lug/6 AWG path first - these cause more brownouts than code does.
  • Cap draw with supply current limits and gear so wheels slip below motor stall.

Go deeper

Lesson quiz

Required

Answer all 3 questions correctly to complete this lesson.

1.On a roboRIO 1, at what input voltage does the controller enter its brownout protection state and disable PWM outputs?

2.What is the most common root cause of a robot browning out during a match?

3.Which measure most directly helps prevent brownouts caused by motor current spikes?

Answer every question to submit.