Electrical Power Consumption and Endurance Modeling

Status: Theoretical Analysis & Component Validation
Methodology: Thrust-stand datasheet extrapolation, component TDP (Thermal Design Power) analysis, and C-rating feasibility modeling.

1. Introduction

Endurance in a multirotor system is fundamentally an optimization of the $Energy / Power$ ratio. Unlike freestyle FPV drones, which prioritize peak burst current for acrobatic maneuvers, an autonomous research drone must minimize continuous hover current to maximize flight time.

Because physical hover testing has not yet been conducted, this document presents the rigorous theoretical models used to design the drone's power distribution network, size the voltage regulators, and select the optimal Li-ion battery chemistry.

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2. Power Distribution Architecture

A critical design requirement was safely powering a Raspberry Pi 4 and dual high-power Wi-Fi adapters without browning out the Flight Controller. Relying on the Flight Controller's onboard 5V BEC (typically rated for 2A–3A) to power a Raspberry Pi is a primary cause of in-flight failure in DIY autonomous drones.

To prevent this, the power architecture physically decouples the avionics load from the flight control load via a dedicated step-down (buck) converter.

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flowchart TD
    BAT[6S / 4S Li-ion Battery] --> ESC[SkyStars 60A 4-in-1 ESC]
    BAT --> BUCK[XL4016 Buck Converter 5V/5A]

    ESC --> M1((Motor 1: 2807))
    ESC --> M2((Motor 2: 2807))
    ESC --> M3((Motor 3: 2807))
    ESC --> M4((Motor 4: 2807))
    ESC -.->|VBAT| FC[SkyStars H7 Flight Controller]

    BUCK --> FILTER[RushFPV Blade LC Filter]
    FILTER --> PI5V[5V Avionics Rail]

    PI5V --> PI[Raspberry Pi 4]
    PI5V --> WIFI[RTL8812EU Wi-Fi Module]

    FC --> GPS[M10 GPS + Compass]
    FC --> ELRS[Radiomaster RP4TD-M Receiver]

    classDef power fill:#f9a873,stroke:#ffbc7a,stroke-width:2px,color:#000000;
    classDef avionics fill:#4a7fc4,stroke:#7cb9ff,stroke-width:2px,color:#ffffff;
    classDef propulsion fill:#4ac485,stroke:#7cffb3,stroke-width:2px,color:#000000;

    class BAT,BUCK,FILTER power;
    class PI,WIFI,GPS,ELRS avionics;
    class ESC,M1,M2,M3,M4 propulsion;

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3. Avionics Power Budget

The continuous electrical load of the onboard compute and communications hardware acts as a parasitic drain on the battery, effectively reducing available flight time.

Estimated Current Draw (at 5V)

Component	Nominal Draw (A)	Peak Draw (A)	Max Power (W)
Raspberry Pi 4 (4GB)	1.2 A	3.0 A	15.0 W
RTL8812EU Wi-Fi (Tx Mode)	0.8 A	1.8 A	9.0 W
Pi Camera V2	0.2 A	0.25 A	1.25 W
Flight Controller + GPS + ELRS	0.3 A	0.5 A	2.5 W
Total Avionics Load @ 5V	~2.5 A	~5.55 A	~27.75 W

Analysis: The XL4016 Buck Converter is rated for 8A continuous. During normal operation (OpenHD encoding + tracking), the system will draw ~2.5A to 3.0A, remaining safely within the regulator's thermal and electrical limits.

%%{init: {'theme': 'dark', 'themeVariables': {'fontSize': '14px', 'primaryColor': '#1e1e1e', 'primaryTextColor': '#ffffff'}, 'pie': {'useMaxWidth': true}}}%%
pie title "Avionics Power Distribution (Peak Load)"
    "Raspberry Pi 4 CPU/GPU" : 54
    "RTL8812EU Wi-Fi Tx" : 32
    "Flight Controller & Sensors" : 9
    "Camera Payload" : 5

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4. Propulsion Power and Flight Time Modeling

The drone's All-Up Weight (AUW) dictates the required thrust per motor to maintain a hover. The relationship between thrust and power is non-linear; as disk loading increases, aerodynamic efficiency decreases.

Hover Thrust Assumptions

• Target Hover Thrust: Drone AUW
• Thrust per Motor: $AUW / 4$
• Motors: Emax ECO II 2807 1500KV
• Propellers: Gemfan LR 6026-2 (Bi-blade, highly efficient for low RPM).

Battery Options Evaluated

The physical airframe bay and center-of-gravity (CG) was constrained to fit a 4S1P pack. However, a 4S2P pack was modeled mathematically to represent the platform's theoretical maximum endurance.

1. 4S1P Li-ion (Molicel P45B): 4500mAh | 300g | 45A Discharge | AUW: ~1.2 kg
2. 4S2P Li-ion (Samsung 50S): 10000mAh | 600g | 70A Discharge | AUW: ~1.5 kg

Current Draw & Endurance Math

$Hover_Time = \left( \frac{Battery_Capacity_Ah \times Usable_Capacity_Factor}{Total_Hover_Current_A} \right) \times 60$ (Assuming a safe 80% discharge rule for Li-ion cells to prevent voltage sag).

Metric	4S1P Configuration (Built)	4S2P Configuration (Modeled)
Total AUW	~1.20 kg	~1.50 kg
Required Thrust per Motor	300 g	375 g
Propulsion Current (Total)	~14.0 A	~19.0 A
Avionics Current (at 14.8V)	~1.2 A	~1.2 A
Total System Current Draw	15.2 A	20.2 A
Usable Battery Capacity	3.6 Ah (80% of 4.5)	8.0 Ah (80% of 10)
Theoretical Flight Time	~14.2 Minutes	~23.7 Minutes

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5. Propeller Efficiency Comparison

Using 2807 motors, selecting the correct propeller is the single largest variable in total endurance. High-pitch tri-blades (freestyle props) require significantly more torque, drawing excess current. The model below visualizes the theoretical flight time penalties of aggressive propellers on the 4S2P configuration.

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xychart-beta
    title "Theoretical Endurance vs. Propeller Geometry (4S2P 1.5kg AUW)"
    x-axis ["Gemfan 5129 Tri", "Gemfan 6030 Tri", "Gemfan 6030 Bi", "Gemfan 6026 Bi"]
    y-axis "Estimated Flight Time (Minutes)" 5 --> 30
    bar [15, 18, 22, 24]

Conclusion: The Gemfan LR 6026-2 bi-blade propeller was chosen for the build. Moving from a standard 5-inch freestyle prop (5129) to a 6-inch low-pitch bi-blade yields a theoretical efficiency gain of nearly 60% in sustained hover time.

6. C-Rating Safety Margin

Lithium-ion batteries possess immense energy density but low discharge rates compared to LiPo batteries. It is vital to ensure the drone's peak current does not exceed the battery's maximum Continuous Discharge Rate (CDR).

• 4S1P (Molicel P45B): CDR = 45A.
• Drone Peak Current Draw (Full Throttle): Limited via ArduPilot MOT_THST_MAX to prevent exceeding 40A total system draw.
• Verdict: The battery operates at ~35% capacity during hover (15.2A / 45A), keeping cell temperatures low and ensuring safe autonomous operation.