Batteries

Function of the aircraft battery

The aircraft battery stores electrical energy chemically and delivers it as direct current (DC). It has three main jobs:

Engine starting — provides the high current for the starter motor, typically 100–300 A for a few seconds.
Backup power on alternator/generator failure — bridges power loss for 20–60 minutes, depending on load profile and battery capacity.
Voltage smoothing and buffering — stabilises the bus, absorbs voltage spikes.

Types of aircraft batteries

1. Lead-acid — widespread in PPL aircraft

Cell voltage: 2 V nominal.
6 cells in series → 12 V battery (14 V bus).
12 cells in series → 24 V battery (28 V bus, more common in airliners).
Electrolyte: dilute sulfuric acid — some models flooded (vented), modern types VRLA (Valve-Regulated Lead-Acid) or AGM (Absorbed Glass Mat) sealed.

2. Nickel-Cadmium (NiCd) — some helicopters and larger GA aircraft

Cell voltage: 1.25 V nominal.
19–20 cells for a 24 V system.
Advantages: long life, robust at extreme temperatures.
Disadvantages: memory effect, cadmium is an environmental concern.

3. Lithium-Ion — new, in modern types (e.g. Garmin G3X avionics backup)

High energy density, low weight.
Requires dedicated charging/protection electronics.
EASA STC required in certified aircraft.

Capacity

Capacity = amount of electric charge the battery can store. Unit: ampere-hours (Ah).

C172: typically a 28 Ah battery (14 V system).
Example: at a 5 A load (several small consumers), 28 Ah / 5 A = 5.6 h theoretically — practically less, as residual capacity is not fully usable.

Important: usable capacity decreases with:

Lower temperature: at −20 °C only ~50% of rated capacity.
Higher discharge current: rapid discharge reduces effective capacity (Peukert effect).
Ageing: significantly lower after 5–7 years.

Battery voltage vs charging voltage

Battery voltage (open circuit):

12.7 V on a fully charged 14 V battery (lead-acid).
24.4 V on a fully charged 28 V battery.
At 11.8 V (or 23.5 V): 50% discharged.
Below 11.5 V: deeply discharged, permanently damaged.

Charging voltage:

Supplied by the alternator/generator; higher than battery voltage to charge the battery.
14.2–14.5 V on a 14 V bus, 28.4–28.5 V on a 28 V system.
Held constant by the voltage regulator.

Cockpit ammeter shows:

Positive: battery is charging (alternator output exceeds loads).
Zero: load equals charge.
Negative: battery is discharging (alternator failure or overload).

Temperature effects

Cold: chemical reactions slow → reduced capacity, weaker starting. At −20 °C only ~50% of rated capacity.
Heat: higher self-discharge, faster ageing, risk of gassing.
Storage: in parked aircraft keep battery warm (hangar) or trickle-charge regularly.

Safety aspects

Hydrogen gassing: charging may produce explosive gas — battery compartment ventilation mandatory.
Corrosion: acid leakage attacks aluminium and steel structures; pre-flight check.
Short-circuit: very high currents; battery cables must be protected (main-bus fuse or master solenoid).