Aircraft Electrical Batteries and Thermal Runaway Protection

021-09-02Pablo Asensio Martínez2026-03-242 min

Battery Basics

Batteries convert chemical energy into electrical energy.

  • Capacity: Measured in Ampere-hours (Ah). Represents the amount of charge a fully charged battery can supply.
    • Example: A 5 Ah battery can supply 5A for 1 hour, or 2.5A for 2 hours.
    • Capacity depends on the physical size of the battery plates (not voltage).
    • Series Connection: Voltage doubles, capacity remains the same.
    • Parallel Connection: Capacity doubles, voltage remains the same.

Condition Check

  • On-Load Check: A test applied to give a better indication of battery condition using the aircraft's voltmeter.
    • Requires applying a load (e.g., lights, pitot heat) for a specific time (10-20 seconds).
    • Voltage must remain steady and not fall below a specific value.
    • Involves comparing on-load and off-load voltages.

Battery Types

Lead-Acid Batteries

  • Composition: Anode (Lead Peroxide), Cathode (Spongy Lead), Electrolyte (Water and Sulphuric Acid).
  • Voltage: 2V per cell on load, 2.2V off load.
  • Characteristics:
    • Good energy storage but heavy.
    • Lower energy density.
    • Discharge rate decreases with lower temperature (internal resistance increases).
  • Hazards: Overcharging boils electrolyte, damaging plates.

Nickel-Cadmium (NiCd) Batteries

  • Composition: Plates of Nickel Oxide and Cadmium, Electrolyte (Potassium Hydroxide).
  • Voltage: ~1.2V per cell (remains relatively constant during discharge).
  • Characteristics:
    • Low internal resistance.
    • Wide operating temperature range.
    • Thermal Runaway Risk: High.
    • Venting: Required.

Lithium-Ion (Li-ion) / Lithium-Polymer (LiPo)

  • Characteristics: High energy density.
  • Hazards: Extremely susceptible to Thermal Runaway.
  • Wear: Performance degrades over time; internal resistance increases, causing worse performance under load.

Thermal Runaway

A rapid, unstoppable chain reaction where an increase in temperature changes internal resistance, causing more heat generation, which further increases temperature (positive feedback loop).

  • Causes:
    • Internal Short Circuit: Dendrite formation, compressive shock/impact (physical damage), deformation.
    • External Short Circuit.
    • Overcharging: Beyond maximum voltage.
    • Overheating: During charging or due to high currents.
  • Process: Electrolyte decomposition (exothermic reaction) -> Rapid temp rise -> Release of stored energy -> Fire/Explosion.
  • Risk: Li-ion fires burn at thousands of degrees and are very difficult to extinguish. Fire can spread to neighboring cells.
  • Protection/Containment:
    • Metal Boxes: Li-ion batteries are often housed in vented boxes made of galvanized/stainless steel with fire insulation to contain thermal runaway.
    • Venting: Allows dissipation of heat and release of flammable gases.

Operations and Charging

  • Charging Method: Most aircraft use Constant Voltage Charging.
    • Generator voltage exceeds battery voltage (e.g., 28V generator for a 24V battery).
  • Ammeter: Connected in series. A positive reading (e.g., +24A) indicates the battery is charging.
  • Loss of Generated Power: If all generators fail, the remaining electrical power from the battery is time-limited (typically 30 minutes for essential systems).
  • Dangerous Goods: Spare lithium batteries are restricted/forbidden in cargo due to fire risk.