Aircraft Electrical Batteries and Thermal Runaway Protection

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.