Troubleshooting Brownouts and Power Sag

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.