On-board electrical systems

Why this matters

A survey drone has dozens of electrical devices needing different voltages: the motors take 22 V straight from the battery, the flight controller uses 5 V and some cameras 12 V. If a converter cannot supply enough current, or the supply dips momentarily as the motors accelerate, the flight controller may reset in mid-air (a brown-out), which means the drone falls. Power distribution design is therefore directly a safety matter.

The whole course uses one hypothetical case: designing the electrical system of a 6S survey quadcopter with a 3.2 kg take-off mass and a camera on a gimbal. The Python code of every module can be downloaded from /downloads/uat-203/.

Power distribution architecture

• The power distribution board (PDB) splits battery power to the ESCs and other devices, and must carry the maximum total current
• The power module gives the flight controller a stable supply and senses voltage and current for the battery failsafe. Per the ArduPilot documentation, a separate stable supply reduces the risk of brown-outs resetting the controller
• A BEC (battery eliminator circuit) is a switching converter stepping battery voltage down to 5 V or 12 V for other devices

The power budget

A power budget tabulates the power of every device by voltage rail, to choose converters with enough capacity and know the system’s total draw. Converters do not give power for free: output power divided by efficiency is the power drawn from the battery.

loads = {  # device: (rail voltage V, maximum power W)
    "flight controller": (5, 2.5), "GNSS + compass": (5, 1.0), "RC receiver": (5, 0.5),
    "telemetry radio": (5, 2.0), "LEDs": (5, 2.0), "camera": (12, 8.0), "gimbal": (12, 6.0),
}
eta = 0.85                       # assumed converter efficiency
v_batt = 22.2
for rail in (5, 12):
    watts = sum(p for v, p in loads.values() if v == rail)
    print(f"{rail:>2} V rail: {watts:.1f} W = {watts / rail:.2f} A out; "
          f"draws {watts / eta / v_batt:.2f} A from the battery")

avionics_w = sum(p for _, p in loads.values()) / eta
hover_w, max_motor_a = 4 * 95, 4 * 25
print(f"avionics {avionics_w:.1f} W from the battery; hover propulsion {hover_w} W")
print(f"peak battery current ≈ {max_motor_a + avionics_w / v_batt:.1f} A (all motors at maximum)")

 5 V rail: 8.0 W = 1.60 A out; draws 0.42 A from the battery
12 V rail: 14.0 W = 1.17 A out; draws 0.74 A from the battery
avionics 25.9 W from the battery; hover propulsion 380 W
peak battery current ≈ 101.2 A (all motors at maximum)

Choosing converters

• Output current must exceed the maximum load with margin, such as 1.5–2×
• Input voltage range must accept a fully charged battery (25.2 V for 6S)
• Noise: switching converters create interference; keep them away from the compass and GNSS antenna
• Measurement: the power module’s voltage and current multipliers must be calibrated (see UAT 321)

Module lab

Common mistakes

Summary

• A drone’s power system has a battery, PDB, ESCs, a power module and BECs at several voltages
• The power budget is split by rail, uses maximum power and divides by converter efficiency
• The electronics draw little power but matter most, and need a stable supply with margin
• Follow the power requirements of the flight controller model used

Check your understanding

1. 5 V devices totalling 10 W need how much output current?
2. An 80%-efficient converter delivers 10 W from a 20 V battery. What input current does it draw?
3. A 5 V BEC’s maximum load is 1.6 A. With 1.5× margin, what rating is needed at least?
4. What is the voltage of a fully charged 6S pack, and how does it affect converter choice?
5. Why is a momentary supply dip at the flight controller dangerous?