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Module 6 — Power system

Everything on the craft is a guest of the battery. Voltage sag, wire gauge and a 30-cent capacitor decide more flights than PID values do.

🟢 Foundations. A LiPo cell swings 4.2 V (full) → ~3.5 V (land) → 3.0 V (damage). Packs are named by topology: 6S2P = 6 in series (voltage adds) × 2 in parallel (capacity adds). Nominal pack voltage = 3.7 V × S (LiPo) or 3.6 V × S (Li-Ion). Capacity is in mAh; energy is what actually flies:

E [Wh]=Vnom×AhE\,[\text{Wh}] = V_{\text{nom}} \times \text{Ah}

C-rating promises max continuous current: Imax⁡=C×AhI_{\max} = C \times \text{Ah}. Li-Ion (18650/21700) stores ~1.5–2× more Wh/kg than LiPo but delivers far less current — perfect for long-range cruisers, wrong for freestyle punch-outs.

🟡 Practitioner. Batteries have internal resistance RintR_{int}; under load the voltage you get is

Vload=Voc−I RintV_{\text{load}} = V_{oc} - I\,R_{int}

A pack with 20 mΩ total at 100 A sags 2.0 V — that's the OSD voltage dip on punch-out, and the reason worn packs "feel weak" at identical charge. Flight-time estimate, continuing Module 2's 7″ example (hover ≈ 205 W): a 6S2P Molicel pack, 8.4 Ah × 21.6 V ≈ 181 Wh; using 80 % of it:

t≈0.8×181205≈0.7 h≈42 min (hover; cruise similar or better)t \approx \frac{0.8 \times 181}{205} \approx 0.7\ \text{h} \approx 42\ \text{min (hover; cruise similar or better)}

Design the power tree deliberately:

The capacitor absorbs voltage spikes from motor switching — the cheapest reliability upgrade in FPV. Budget the 5 V rail: receiver + GPS + two servos can exceed a weak BEC; brownout = mid-air reboot.

🔴 Advanced. Chemistry: energy lives in lithium intercalation; C-rate abuse, deep discharge and heat age cells by growing internal resistance. Charge at ≤1C balanced, store at 3.8 V/cell, retire puffed packs. Wiring is physics too: current density and P=I2RP = I^2R losses set copper cross-section (12 AWG mains for 100 A-class builds); ground loops between FC, camera and VTX inject the video noise you'll meet in Module 10.

⚫ Master. You spot-weld your own Li-Ion packs (with proper BMS-less balance leads and fusing knowledge), instrument a build with a power meter to map g/W across throttle, and can model the full electrical chain — pack IR, PDB losses, ESC efficiency curve — to predict endurance within 10 %.

Mastery checklist

  • Size a battery (chemistry, S, P, capacity) from a target flight time and hover-power estimate.
  • Measure a pack's internal resistance with a load and a multimeter.
  • Explain why a bigger capacitor fixes "video static under throttle".

🖼️ Image ideas: your own photo of a dissected (dead, discharged!) pouch cell next to 21700 cells; Wikimedia Commons "Lithium polymer battery" (check licenses).

📚 Free resources: Battery University (free articles); ArduPilot/Betaflight docs on battery monitoring; Oscar Liang Li-Ion pack guides.