When an FTC robot abruptly loses Wi-Fi connection, freezes, or reboots during an intense pushing match or an aggressive directional change, the problem is almost never code instability or faulty radio signals. It is an electrical brownout.

Under heavy mechanical load, your drive motors draw their maximum stall current. This massive current draw pulls the overall system voltage below the threshold required to keep the REV Control Hub’s internal Android processor running, triggering a hard reboot.

What Actually Happens During a Brownout

The REV Control Hub runs an internal regulator that powers the Android operating system and Wi-Fi radio. It expects a nominal 12V supply from your standard TETRIX or REV 12V 3000mAh NiMH battery pack. While the hub is designed to tolerate dips down to around 8V–9V for short fractions of a second, any drop below roughly 7V–8V causes the internal buck converters to collapse.

When that rail drops, the processor reboots. The robot drops off the Driver Station Wi-Fi network, the indicator light blinks through its boot sequence, and your alliance loses drive capability for 30 to 45 seconds—effectively ending your match.

The Physics of Stall Current and Voltage Sag

The arithmetic behind a brownout is straightforward. A standard FTC drivetrain runs four heavy-duty planetary gearmotors (such as REV HD Hex or goBILDA Yellowjackets). At free-run speed, each motor draws between 0.3A and 0.5A. Under heavy defence or when reversing direction at full throttle, those motors instantaneously enter a stall condition, drawing between 9A and 11A each.

If all four drivetrain motors stall at the same moment while an intake or linear slide motor is also engaged, total instantaneous current draw can exceed 40A. Every battery has an internal resistance (R_int). According to Ohm's Law (V_sag = I × R_int), pulling 40A through a battery with an internal resistance of even 0.1Ω causes a direct 4.0V drop across the battery terminals alone. A pack sitting at 12.0V suddenly delivers only 8.0V to the system before accounting for wiring losses.

1. Battery Hygiene: Why Resting Voltage Lies

A battery that reads 12.6V on a digital multimeter can easily drop to 6V under a 30A load if it is damaged, poorly conditioned, or old. Multimeters measure open-circuit voltage, drawing practically zero current and concealing high internal resistance.

• Retire worn packs: NiMH competition packs typically degrade after one to two seasons of aggressive charging and high-discharge competition cycles.
• Load test before matches: Test batteries under an actual load (using a commercial automotive battery tester or a dedicated battery analyser) rather than relying on the Driver Station voltage widget at idle.
• Temperature matters: A battery fresh off the charger is hot, which temporarily raises internal resistance and shortens run duration. Allow packs to cool to ambient temperature before putting them on the field.

2. Electrical Losses in Wiring and Connectors

High current amplifies every milliohm of resistance in your wiring loom. A small resistance that causes no visible issue when driving around the pit table can drop vital volts during a match.

• Tamiya connectors: The standard white plastic Tamiya connectors found on older batteries are notorious for loose pin retention and high contact resistance. Standardise on Anderson Powerpoles or genuine XT30 connectors across your robot and battery fleet.
• Switch resistance: The mandatory 20A main power switch can accumulate carbon fouling or mechanical looseness after heavy use, introducing significant resistance. If your robot reboots when lightly tapped near the main switch, replace it immediately.
• Gauge and length: Keep high-current runs between the battery, switch, and Control Hub as short as practically possible using 14 AWG or 16 AWG silicone wire. Avoid unnecessary adapter cables.

3. Software Fixes: Slew Rates and Current Management

Mechanical and electrical fixes ensure the hardware can withstand peak current, but good software prevents unnecessary current spikes altogether.

The most common trigger for a brownout is an instantaneous shift from full forward (+1.0) to full reverse (-1.0) on the gamepad joystick. When a spinning motor is forced to reverse instantly, back-EMF combines with the supply voltage, creating current spikes that far exceed normal stall ratings.

• Implement a Slew-Rate Limiter: Rate-limit your joystick inputs in code. Instead of applying 100% motor power on the first control loop, ramp the target power over 100 to 200 milliseconds. Drivers will barely notice the latency, but it eliminates the peak current spike.
• Use RUN_USING_ENCODER carefully: The FTC SDK's built-in velocity PID controller will command 100% duty cycle if an obstacle prevents the motor from reaching its target velocity. If your drivetrain gets pinned against the arena perimeter, the PID loop will maintain maximum stall current until the hub browns out.
• Monitor real-time bus voltage: Write safety logic into your teleop loop. If controlHub.getBatteryVoltage() drops below 9.5V, scale back non-essential mechanisms (such as intake rollers or active outriggers) until voltage recovers.

Summary Checklist

Eliminating brownouts requires treating the power budget as a core design parameter rather than an afterthought. If you are setting up or upgrading your team's hardware for the season, you can find competition-grade power harnesses, sensors, and structural components via the Sheen Robotics store, or explore our mentoring resources at Sheen Robotics FTC support.