The Robot Browns Out or Goes Sluggish Mid-Match

Symptom: Mid-match the robot stutters, motors cut out, or the whole thing goes limp for a second and recovers. Drivers describe it as "laggy" or "it died and came back."

Root cause, almost always: a brownout. When battery voltage drops below the roboRIO threshold (6.3 V on roboRIO 1; 6.75 V default Stage-2 on roboRIO 2, which is software-settable), the firmware disables all actuators to avoid a full reboot. The trigger is a transient current spike — usually a hard acceleration, a collision, or several mechanisms moving at once — on top of a weak battery.

Diagnostic workflow:

1. Look at the evidence first. After the match, open the Driver Station Log Viewer and find the voltage chart. A brownout shows up as voltage diving toward the threshold exactly when the current trace spikes. The DS also logs explicit brownout events. Don't guess — read the chart.
2. Check the battery. Was it freshly charged and load-tested? A tired battery sags under load even if it reads 12.5 V or higher at rest. No-load voltage lies; internal resistance is what matters, so test under a load (a battery beak / load tester reports internal resistance and capacity).
3. Check for current limits. If your drive SPARK MAXs have no smartCurrentLimit, four NEOs accelerating at once can pull a transient spike far larger than what a single 12 V 18 Ah SLA can supply without its voltage collapsing.
4. Check connections. A loose main breaker, PDH/PDP lug, or Anderson SB-50 battery connector adds resistance and causes voltage to collapse under load.

The fix:

• Add or lower current limits: cfg.smartCurrentLimit(40) on drive motors is a standard starting point. CTRE users set supply/stator limits via CurrentLimitsConfigs.
• Rotate to a fresh, load-tested battery and retire any that sag.
• Re-torque the main breaker and battery lugs; you should not be able to rotate the lug by hand.
• If you're on a tippy or powerful robot, add a SlewRateLimiter so the driver can't command an instantaneous full-current spike.

Verify: Repeat the same hard-driving test and watch the voltage chart. If it now stays comfortably above the threshold under identical driving, the fix held. Brownouts are never "random" — there is always a current spike and a sagging supply, and the chart will show you both.