Aircraft batteries are one of those A&P topics that seem simple at first, but they show up in a lot of test questions. Batteries connect directly to basic electricity, Ohm’s law, series circuits, parallel circuits, charging systems, corrosion, and aircraft safety.
For an A&P student, you do not just need to know that a battery stores electricity. You need to understand what type of battery is being used, how cells combine to create voltage, how charging works, and why battery maintenance is so important.
In this post, we will look at the basics of aircraft batteries from an A&P perspective.
What Does an Aircraft Battery Do?
An aircraft battery stores chemical energy and converts it into electrical energy when needed.
In an aircraft, the battery may be used for:
- Engine starting
- Ground electrical power
- Emergency electrical power
- Stabilizing the DC electrical bus
- Helping clear electrical faults
- Powering essential equipment if the generator or alternator fails
The battery is not just a convenience item. In many aircraft, it is an important part of the electrical system and emergency backup system.
The Two Main Aircraft Battery Types
The two battery types A&P students usually study are:
- Lead-acid batteries
- Nickel-cadmium batteries, usually called Ni-Cad batteries
Both are rechargeable storage batteries, but they are not maintained the same way.
Lead-Acid Batteries
Lead-acid batteries are common in smaller general aviation aircraft.
A lead-acid battery uses lead plates and an electrolyte made from sulfuric acid and water.
Each lead-acid cell produces about 2 volts.
That means:
- A 12-volt lead-acid battery usually has 6 cells
- A 24-volt lead-acid battery usually has 12 cells
The cells are connected in series. When cells are connected in series, voltage adds together.
So for a 24-volt lead-acid battery:
12 cells × 2 volts per cell = 24 volts
This connects directly to your series circuit knowledge:
Series = voltage adds
Nickel-Cadmium Batteries
Nickel-cadmium batteries are often used in larger aircraft, commercial aircraft, and military aircraft.
A Ni-Cad battery uses nickel and cadmium plates with an alkaline electrolyte.
A Ni-Cad cell produces about 1.2 volts per cell.
That means a 24-volt Ni-Cad battery needs more cells than a lead-acid battery.
A common A&P test point is that Ni-Cad batteries require specific maintenance procedures and should not be serviced together with lead-acid batteries.
Lead-Acid vs. Ni-Cad Batteries
Here is the simple comparison:
| Feature | Lead-Acid Battery | Ni-Cad Battery |
|---|---|---|
| Common use | Small aircraft | Larger, commercial, military aircraft |
| Electrolyte | Sulfuric acid and water | Alkaline electrolyte |
| Cell voltage | About 2 volts per cell | About 1.2 volts per cell |
| Maintenance | Check electrolyte, corrosion, charge condition | Check electrolyte, capacity, temperature, charging condition |
| Main concern | Acid corrosion, sulfation, low electrolyte | Thermal runaway, electrolyte contamination, overcharging |
The biggest thing to remember is that these batteries are chemically different. You should not mix tools, service areas, or procedures between them unless the manufacturer specifically allows it.
Why Battery Cells Are Connected in Series
Aircraft batteries use multiple cells connected in series to increase voltage.
In a series circuit, the voltage of each cell adds together.
Example:
Cell 1 = 2 volts
Cell 2 = 2 volts
Cell 3 = 2 volts
Total voltage:
2V + 2V + 2V = 6V
For a 24-volt lead-acid battery:
12 cells × 2V = 24V
This is why A&P battery questions often connect back to basic series circuit theory.
Battery Charging Basics
Charging a battery reverses the chemical process that occurred during discharge.
When a battery is discharged, chemical energy is converted into electrical energy.
When a battery is charged, electrical energy is used to restore the chemical condition of the battery.
However, charging must be controlled. Too much charging voltage or current can create heat, damage the battery, boil electrolyte, or cause failure.
Lead-Acid Charging
Lead-acid batteries must be charged carefully.
If the charging voltage is too high, the battery can overheat, gas excessively, lose electrolyte, or become damaged.
A key A&P point is that the maximum charging voltage per lead-acid cell must be controlled.
For lead-acid batteries, AC 43.13-1B states that the voltage per cell must not exceed 2.35 volts.
That is important because a 12-cell lead-acid battery would have a maximum charging voltage based on the number of cells:
12 cells × 2.35V = 28.2V
That does not mean every aircraft system charges at exactly 28.2 volts, but it shows why charging voltage and cell count matter.
Ni-Cad Charging
Ni-Cad batteries are different from lead-acid batteries.
Ni-Cad batteries can accept high charging currents, but they are also more sensitive to temperature problems.
One major concern with Ni-Cad batteries is thermal runaway.
Thermal runaway is a condition where increasing battery temperature causes more current flow, which creates more heat, which causes even more current flow.
This cycle can quickly damage the battery and become dangerous.
That is why Ni-Cad battery systems often use temperature monitoring or current monitoring.
Do Not Service Lead-Acid and Ni-Cad Batteries Together
This is a major safety and test point.
Lead-acid and Ni-Cad batteries should not be serviced in the same area.
The reason is electrolyte contamination.
Lead-acid batteries use an acid electrolyte.
Ni-Cad batteries use an alkaline electrolyte.
If acid electrolyte contaminates a Ni-Cad battery, it can damage or destroy the battery. The reverse is also true.
Simple memory aid:
Lead-acid = acid
Ni-Cad = alkaline
Do not mix them
Battery Maintenance
Battery maintenance depends on the type of battery and the manufacturer’s instructions.
In general, battery maintenance may include:
- Inspecting the case
- Checking for cracks or leakage
- Checking terminals
- Cleaning corrosion
- Checking electrolyte level when applicable
- Checking state of charge
- Performing a capacity test
- Inspecting battery cables and connectors
- Checking for overheating
- Making sure the battery is properly secured
- Following the aircraft maintenance manual
The most important maintenance rule is this:
Always follow the aircraft and battery manufacturer's instructions.
FAA references are useful for general principles, but the manufacturer’s maintenance manual is the controlling source for the actual aircraft.
Corrosion Around Batteries
Lead-acid batteries can cause corrosion because of acid fumes or spilled electrolyte.
Corrosion near a battery is serious because it can damage:
- Battery terminals
- Cables
- Battery boxes
- Aircraft structure
- Electrical connections
Corrosion can also increase resistance in a circuit.
Higher resistance at a battery connection can cause voltage drop, heat, and poor starting performance.
That connects directly to Ohm’s law:
E = I × R
If resistance increases at a bad connection, voltage drop and heat can become problems.
Burn Marks on Battery Cell Straps
A common A&P-style question is:
If the cell straps in a battery show burn marks, what is this an indication of?
The best answer is usually:
The straps were not properly torqued when installed.
Why?
Because a loose connection increases resistance. When current flows through a high-resistance connection, heat is produced.
That heat can cause burn marks.
This is another example of basic electricity showing up in a maintenance question.
Battery State of Charge
Battery state of charge means how much usable electrical energy remains in the battery.
For a lead-acid battery, state of charge may be checked by measuring electrolyte specific gravity with a hydrometer, if the battery design allows it.
For sealed batteries, maintenance procedures may require voltage checks, capacity tests, or other approved methods.
For Ni-Cad batteries, state of charge is not determined the same way as a lead-acid battery. Ni-Cad batteries often require a measured discharge or capacity check according to the manufacturer’s instructions.
Capacity Testing
A battery can show voltage and still be weak.
That is an important A&P concept.
Voltage alone does not always prove that a battery can deliver the required current for the required time.
A capacity test checks whether the battery can supply a specified load for a specified period.
This matters because an aircraft battery may be required to power essential equipment during an emergency.
A weak battery may appear okay during a simple voltage check but fail under load.
Battery Safety
Aircraft batteries can be dangerous if handled incorrectly.
Battery hazards include:
- Acid burns
- Chemical exposure
- Explosive gases
- Electrical short circuits
- Burns from high current
- Corrosion
- Fire risk
- Thermal runaway in Ni-Cad batteries
Always remove jewelry before working around batteries.
A ring, watch, or bracelet can create a direct short if it touches a battery terminal and ground.
A battery can deliver very high current, and a short circuit can create intense heat very quickly.
Why a Direct Short Is Dangerous
A direct short gives current a path with very little resistance.
Using Ohm’s law:
I = E / R
If resistance becomes very small, current becomes very large.
That high current can create rapid heating, sparks, burns, fire, or battery damage.
This is why battery terminals must be protected and tools must be used carefully.
Common A&P Test Points
Here are some common A&P-style battery ideas to remember:
1. What is the electrolyte in a lead-acid battery?
Sulfuric acid and water
2. What is the electrolyte in a Ni-Cad battery?
An alkaline electrolyte
3. How many cells are in a 24-volt lead-acid battery?
12 cells
Because each lead-acid cell is about 2 volts.
4. Why should lead-acid and Ni-Cad batteries not be serviced together?
Electrolyte contamination can damage or destroy the batteries.
5. What can burn marks on battery straps indicate?
Loose or improperly torqued connections.
6. What is a major danger with Ni-Cad batteries?
Thermal runaway.
7. What happens when battery cells are connected in series?
Voltage adds.
8. Can a battery show voltage and still be weak?
Yes. A battery may show voltage but fail a capacity test or fail under load.
Simple Memory Aids
Here are a few quick memory aids.
Lead-acid = acid electrolyte
Ni-Cad = alkaline electrolyte
Do not mix service areas
Series cells = voltage adds
Loose connection = resistance
Resistance + current = heat
Heat can cause burn marks
Voltage check alone does not prove battery capacity
How This Connects to Basic Electricity
Aircraft batteries are a great example of why basic electricity matters.
You use these concepts when studying batteries:
- Series circuits
- Voltage
- Current
- Resistance
- Ohm’s law
- Power
- Heat
- Corrosion
- Conductors
- Insulators
- Charging systems
A battery is not just a box that stores electricity. It is part of the aircraft electrical system, and its condition affects safety, starting, emergency power, and system reliability.
Final Thoughts
For A&P students, aircraft batteries are worth studying carefully because they connect textbook electrical theory to real maintenance.
Remember the big ideas:
- Aircraft batteries provide starting, backup, and emergency power.
- Lead-acid and Ni-Cad batteries are chemically different.
- Lead-acid batteries use sulfuric acid and water.
- Ni-Cad batteries use an alkaline electrolyte.
- Battery cells connected in series increase voltage.
- Charging must be controlled to prevent heat and damage.
- Lead-acid and Ni-Cad batteries should not be serviced together.
- A battery can show voltage but still fail under load.
- Always follow the aircraft and battery manufacturer’s maintenance instructions.
If you understand batteries, you are also reinforcing the core electrical ideas that show up all over the A&P general section.