UAV Power Systems: Chemistry & Physics

CRITICAL The battery is not just a fuel tank; it is a voltage source with internal resistance. Understanding this resistance is key to preventing mid-air failures (voltage sag) and fires.

1. Battery Chemistry: The Electron Exchange

1.1 LiPo (Lithium Polymer)

The standard for high-performance flight.

Structure: Uses a solid polymer electrolyte.
Voltage: 3.7V Nominal / 4.20V Max.
Advantages: Extremely low Internal Resistance (IR). Can dump massive current (100A+) instantly.
The "Puff": When a LiPo is abused (over-discharged or overheated), the electrolyte decomposes into gas (mostly Oxygen and Hydrogen). This gas is trapped in the foil pouch, causing it to "puff." A puffed LiPo is a fire hazard. The internal structure is compromised, and the layers can short-circuit.

1.2 LiHV (High Voltage LiPo)

Voltage: 3.8V Nominal / 4.35V Max.
The Trade-off: By charging to a higher voltage, you get ~10% more energy density. However, the chemical stress on the cathode is significantly higher.
Lifespan: LiHV cells degrade faster than standard LiPo cells if consistently charged to 4.35V. They are for racing/performance, not longevity.

1.3 Li-Ion (Cylindrical Cells)

Used for Long Range / Endurance. Common cells: Sony VTC6 (18650), Molicel P42A (21700).

Density: Superior Energy Density (Wh/kg). They hold more fuel for the weight.
Power: Inferior Power Density (W/kg). They have higher Internal Resistance.
The Sag: Because of high IR, they sag heavily under load.
- LiPo: 100% throttle -> Voltage drops 1V.
- Li-Ion: 100% throttle -> Voltage drops 4V.

2. The C-Rating Lie vs. Internal Resistance (IR)

2.1 The Marketing Lie

Manufacturers label packs with "100C" or "150C."

The Math: A 1300mAh (1.3Ah) battery at 100C claims to deliver $1.3 \times 100 = 130$ Amps continuous.
The Reality: The XT60 connector melts at 60A. The 12AWG wire melts at 100A. The "C-Rating" is effectively a marketing number with no standardized physical basis.

2.2 The Real Metric: Internal Resistance (IR)

IR is the impedance to electron flow inside the cell. It is measured in milliohms ($m\Omega$).

Ohm's Law: $V_{drop} = I \times R$.
Example: If you pull 50 Amps from a pack with 10$m\Omega$ (0.01 $\Omega$) total resistance:
$$ V_{drop} = 50 \times 0.01 = 0.5 \text{ Volts lost as heat inside the pack.} $$
Heat: $P = I^2 R$. $50^2 \times 0.01 = 25$ Watts of heat generated inside the battery.
Rule of Thumb (per cell):
- < 2 $m\Omega$: Race Grade.
- 2 - 5 $m\Omega$: Good / Freestyle.
- > 10 $m\Omega$: Old / Degraded. Relegate to goggle battery or disposal.
Temperature Factor: IR decreases as heat increases. This is why batteries perform better in summer than winter. Pre-heating LiPos to 30°C before a race increases performance.

3. Power Distribution Architecture

3.1 Connectors: The Bottleneck

The connector is often the point of highest resistance in the circuit.

XT30: Rated 30A continuous. Use for micro drones (< 250g).
XT60: Rated 60A continuous. The industry standard. Realistically handles 100A bursts (3s).
XT90: Rated 90A continuous. Use for X-Class or Cinelifters (10").
AS150: Anti-Spark. Mandatory for high-voltage (12S+) setups to prevent the "pop" that erodes contacts.

3.2 Capacitors: The Electronic Shock Absorber

Electronic Speed Controllers (ESCs) chop voltage thousands of times per second. This creates massive inductive voltage spikes ("Noise").

The Physics: A capacitor resists changes in voltage. It acts as a reservoir, filling up during spikes and draining during dips.
Low ESR: You must use Low ESR (Equivalent Series Resistance) capacitors (e.g., Panasonic FM/FR series). Standard capacitors cannot react fast enough to filter 96kHz switching noise.
Placement: Must be soldered at the source of the noise—directly onto the ESC battery pads. The longer the wire between the ESC and the Cap, the less effective it is (due to wire inductance).

3.3 Wire Gauge

Voltage drop over wire length is significant at low voltages (DC).

Battery Leads: 12AWG or 14AWG.
Motor Wires: 18AWG or 20AWG.
Signal Wires: 28AWG (silicone).