MAVLINKHUD

Power Forensics

Executive Summary

Power failure is the most common cause of non-pilot-error crashes. Analyzing power logs allows you to distinguish between Battery Exhaustion (ran out of capacity), Voltage Sag (over-current/old battery), and Brownouts (flight controller power failure).

Theory & Concepts

1. Voltage Sag vs. Capacity

  • Capacity (mAh): The "Fuel" in the tank. Consumed over time.
  • Sag (Volts): Instantaneous voltage drop due to load ($V_{drop} = I \times R_{internal}$).
  • Danger: High current causes deep sag. If voltage drops below the ESC cutoff (e.g., 3.0V/cell), the ESCs will reboot mid-air even if 50% capacity remains.

2. Brownouts

A brownout occurs when the 5V rail powering the Flight Controller drops below ~4.5V.

  • Effect: The MCU resets. Log ends abruptly.
  • Cause: Overloaded BEC (too many servos/LEDs) or short circuit.

Codebase Investigation

1. Battery Monitor: AP_BattMonitor::read()

Located in libraries/AP_BattMonitor/AP_BattMonitor.cpp.

  • It reads raw ADC values for Voltage and Current.
  • Integration: 
    _consumed_mah += (current_amps * dt) / 3600.0f;
  • Logging: Writes BAT (Basic info) and BCL (Cell levels if available).

2. System Power: log_POWR

  • Vcc: The voltage of the 5V rail powering the board.
  • VServo: The voltage of the Servo rail.

Source Code Reference

Practical Guide: Forensics

1. The "V" Shape

  • Look at BAT.Volt vs BAT.Curr.
  • If Voltage dips sharply exactly when Current spikes (Throttle Punch), your battery has high internal resistance (old/weak/cold).
  • Fix: Use higher C-rated batteries or warm them up.

2. The abrupt End

  • If the log ends mid-flight, check POWR.Vcc in the last few seconds.
  • If Vcc drifts below 4.8V or has noise, your power module is failing.

3. Total Consumption

  • Compare BAT.CurrTot (mAh used) against your battery's rated capacity.
  • Rule of Thumb: You should land before using 80% of rated capacity.