Batteries - ATPL 021 — Airframe, Systems and Powerplant

Everything electrical so far has assumed a source, and before the generators run, the source is chemical: the battery. This lesson covers what the aircraft battery is for, the four rechargeable chemistries and how they differ, the arithmetic of cells, voltage and capacity, and the hazard that has earned its own paragraph in every modern operations manual: lithium batteries and thermal runaway.

What the battery is for

The aircraft battery is the electrical system's stored reserve, energy held chemically and released as direct current on demand. Its functions follow from that: it powers the aircraft before any generator turns (lighting up the flight deck, running fuel pumps for the start, and supplying the starter itself for the APU or engines); it buffers the running system, absorbing brief dips and spikes while generators and loads switch; and above all it is the final reserve, the source that keeps the essential instruments, radios and computers alive if every generator fails. The battery is sized, checked and kept charged for that last role.

The four chemistries

Four rechargeable battery types serve aviation:

Lead-acid — the traditional aircraft battery: heavy for its energy, tolerant of simple charging, and modest in cost.
Nickel-cadmium (NiCd) — the transport-aviation workhorse: lighter, robust through many cycles, and able to deliver very large currents for engine starting.
Lithium-ion — far more energy for the same weight, used in newer aircraft designs and in nearly every portable device carried aboard.
Lithium-polymer — the same lithium chemistry in a flexible flat pouch instead of a rigid can: lighter and shapeable, but with a soft casing that damage reaches more easily.

The comparison the syllabus asks for runs on three axes:

Load behaviour — how the voltage holds up as current is drawn: lead-acid sags noticeably under heavy load and as it empties; NiCd holds its voltage almost flat until nearly exhausted (excellent for starting, but it gives little voltage warning of a low battery); lithium types also hold up well while delivering the most energy per kilogram.
Charging characteristics — lead-acid accepts simple constant-voltage charging; NiCd charges well but can overheat late in the charge, so aircraft NiCd installations carry temperature monitoring; lithium types demand precisely controlled charging, supervised cell by cell by electronics, because overcharging is one of the routes to runaway.
Risk of thermal runaway — the danger that a hot cell gets hotter by itself: low for lead-acid; real for NiCd (an overheating aircraft NiCd is a known emergency, which the temperature monitoring exists to catch); highest for the lithium types, which pack the most energy into the least material and burn fiercely once started.

Cells, voltage and capacity

Cell voltage is the voltage one electrochemical cell produces, and it is fixed by the chemistry:

Lead-acid — about 2 volts per cell.
NiCd — 1.2 volts per cell.
Lithium-ion — around 3.6 volts per cell.

No single cell suits an aircraft's system, so a battery is built from cells combined: connected in series, their voltages add until the battery reaches the desired voltage (twelve lead-acid cells or twenty NiCd cells make the standard 24-volt aircraft battery), and using larger cells, or groups in parallel, provides the desired capacity.

Battery voltage and charging voltage are different numbers by design. Current flows from higher voltage to lower, so to push charge back into a battery, the charging source must sit above the battery's own voltage. That is the quiet logic of the aircraft's standard numbers: the battery is nominally 24 volts, the generators regulate the bus (the common supply bar the consumers draw from) to 28 volts, and the 4-volt difference drives charge into the battery whenever the generators run. A '28-volt system' and its '24-volt battery' are not a contradiction: one is the charging voltage, the other the battery voltage.

Capacity is the charge a battery can deliver, measured in ampere-hours (Ah): a 40 Ah battery can nominally supply 40 amperes for one hour, or 20 amperes for two hours. The trade is not perfectly linear (heavy currents extract a little less than the label promises), but ampere-hours are the currency in which battery endurance is calculated.

Temperature, and how long the battery lasts

Temperature moves a battery's capacity and performance: the battery is a chemical machine, and cold slows chemistry. A cold-soaked battery delivers less current and noticeably less capacity than its rating, which is why winter starts are the hardest thing most batteries ever do. Heat works the other way at first, but sustained heat ages the chemistry and shortens the battery's life, and for some chemistries heat is the doorway to runaway.

With all generated power lost, the remaining electrical power is time-limited: the battery's ampere-hours are all there is, and every switched-on consumer spends them. This is why the total-generation-failure drill sheds load down to the essentials, and why the manual states a battery endurance (commonly on the order of half an hour) that the crew plans a diversion around. The battery does not fail at that moment; it simply runs out, like a fuel tank.

The lithium hazard

Lithium batteries carry the most energy in the least material, and that is precisely their danger: a failing lithium cell can release all of it as heat and fire. The threat to aircraft safety is shaped by four factors:

Numbers on board — the aircraft's own lithium batteries are few and engineered; the hundreds of phones, laptops and power banks the passengers carry are not, and every one is a potential source. The fleet's lithium exposure walks aboard with the passengers.
Temperature — of the battery and of its environment: a lithium cell already warm (from use, charging or a hot cabin) sits closer to the temperature at which its chemistry feeds on itself.
Physical condition — a crushed, punctured, swollen or dropped battery may be internally short-circuited: the classic cabin case is a phone crushed in a seat mechanism.
Charging — charging pushes energy in, and faulty or uncontrolled charging (damaged cables, counterfeit chargers, overcharge) is a standard ignition route; a device that will not stop getting hot on charge is announcing itself.

Containing a thermal runaway

Thermal runaway is a chain reaction of heat: a failing cell's chemistry releases heat, the heat accelerates the chemistry, and the cell climbs to destruction, venting flammable gas and often flame. In a battery, no cell is alone: the runaway cell cooks its neighbours, and each neighbour that lets go adds its own energy, cell after cell, until the whole battery is consumed. Containment therefore has one goal: take the heat out fast enough that the next cell never reaches its trigger temperature.

In flight, the challenges multiply. The burning item cannot be thrown away; the cabin is full of people and the hold is unreachable. An extinguisher knocks down the flames but does not stop the chemistry: the cell keeps making its own heat (and its own oxygen-rich gases), so the fire relights. The working answer for a burning device is cooling: douse it and keep it drowned in water or other non-flammable liquid, soaking the heat out so neighbouring cells never trigger, then isolate it (fire-containment bags exist for exactly this) and treat the event as a reason to land. For the aircraft's own lithium batteries, the same logic is built in: containment boxes, venting overboard, and monitoring.

What the exam asks

The chemistry comparison and the cell/voltage/capacity arithmetic carry the definitional marks; the lithium-hazard factors and the runaway containment are asked as operational reasoning. The overview, following the learning objectives:

State the battery's function

Name the four rechargeable types and compare them on load behaviour, charging characteristics and thermal-runaway risk

Explain cell voltage, building a battery from cells, battery vs charging voltage, and capacity with its unit

State temperature's effect on capacity and performance, and that battery-only power is time-limited

Explain the lithium threat and its risk factors (numbers aboard, temperature, condition, charging)

Describe containing a thermal runaway: cell-to-neighbour propagation and the in-flight challenges