LiPo batteries

Summary¶

Lithium polymer (LiPo) batteries power drone propulsion because they combine high energy density, low internal resistance, and a favourable voltage range. Each cell has a nominal voltage of 3.7V, a fully charged voltage of 4.2V, and a minimum safe discharge voltage of 3.5V. Cells connect in series for higher bus voltages. The same electrochemistry that makes LiPo batteries powerful makes them dangerous if mishandled: overcharging, over-discharging, or physical damage can trigger thermal runaway — a self-accelerating reaction producing fire that cannot be extinguished by conventional means.

Concept¶

Cell chemistry and voltage¶

Inside each LiPo cell, lithium ions move between a graphite anode and a lithium cobalt oxide cathode through a polymer gel electrolyte. During discharge, ions migrate from graphite to cathode; electrons travel through the external circuit doing work. Charging reverses this migration.

The cell voltage reflects the electrochemical potential between anode and cathode materials. Fully lithiated cathode = 4.2V; fully delithiated = 3.5V. Below 3.0V, the cathode changes phase irreversibly — permanent structural damage. This is why the minimum voltage cutoff is enforced in firmware.

Series cells and the "S" designation¶

Cells in series add their voltages. "6S" means six cells in series:

Fully charged: 6 × 4.20V = 25.2V Nominal: 6 × 3.70V = 22.2V Storage voltage: 6 × 3.80V = 22.8V Minimum safe: 6 × 3.50V = 21.0V Hard cutoff: 6 × 3.30V = 19.8V

6S was chosen for libdrone because the 2507 1750kV motor at 22.2V nominal spins at approximately 38,850 RPM unloaded — settling under prop load at 28,000–34,000 RPM. This is the optimal range for 6-inch propellers. At 4S (14.8V), the motor spins too slowly for efficient thrust. At 8S (29.6V), RPM exceeds safe bearing limits for this stator size.

C rating¶

The C rating is maximum continuous discharge as a multiple of capacity. For the Tattu R-Line 1800 mAh 150C:

Theoretical max continuous: 150 × 1.8A = 270A

This is a marketing ceiling. In flight, libdrone draws 20–60A total depending on throttle. The XT60 connector is rated 60A continuous — the practical limit is the connector, not the C rating.

Thermal runaway¶

At approximately 130°C the polymer separator melts, directly shorting anode and cathode. The short generates more heat. Above ~200°C the cathode decomposes, releasing oxygen. Flammable electrolyte plus oxygen produces fire. This is self-reinforcing: heat → decomposition → more heat. Once started it proceeds to completion regardless of external action.

In a multi-cell pack, runaway in one cell triggers adjacent cells within seconds. Water reacts with lithium. Smothering removes external oxygen but not the internal source from cathode decomposition. The only effective response is prevention.

Triggers: overcharge (above 4.2V/cell), over-discharge (below 3.0V/cell), physical puncture, internal short from crash damage.

Reference¶

libdrone battery specification¶

OSD voltage thresholds¶

Procedure¶

Safe storage¶

1. After each flight, check pack voltage. If above 23.5V (>3.9V/cell), run a storage charge to 22.8V.
2. If not flying within 3 days, storage-charge or discharge to 3.8V/cell.
3. Store at room temperature in a fireproof LiPo bag or steel ammunition box.
4. Never store in a vehicle or in direct sunlight.

Post-crash inspection¶

1. Inspect for swelling, deformation, or puncture.
2. Check individual cell voltages — any cell above 4.25V or below 3.0V: retire the pack.
3. If pack is warm after a crash without heavy flight: wait 30 minutes in an open area away from flammables before handling.
4. Any swollen pack must be fully discharged in a fireproof container and disposed of properly. Do not continue using a swollen pack.

Winter operations¶

Do not fly below 0°C in standard configuration. Transport batteries inside a pocket or insulated bag; bring to site in the last 5 minutes. A battery at +5°C may behave as if 30–40% of rated capacity is unavailable. Land at higher remaining voltage and monitor OSD more frequently.

Rationale¶

Why 6S and not 4S¶

4S nominal (14.8V) would put the 2507 1750kV at ~25,900 RPM under load — below the efficient range for 6-inch props. Additionally, higher voltage for the same power means lower current: halving the current reduces I²R losses in wires and connectors by 75%, meaningfully improving efficiency and reducing heat across the entire power distribution chain.

Why storage voltage matters¶

A LiPo stored fully charged slowly degrades through electrolyte oxidation. Stored fully discharged, it risks falling below 3.0V/cell through self-discharge, causing irreversible cathode damage. Storage voltage (3.8V/cell) minimises both degradation mechanisms simultaneously.

Connections¶

requires: [] related: - power-rail-architecture - power-sequencing - li-ion-batteries leads_to: - power-rail-architecture - power-sequencing - li-ion-batteries