1.5 Aircraft Electrical Power Systems: Batteries, Generation, Buses, and Circuit Protection
Key Takeaways
AC 43.13-1B says most small private aircraft use lead-acid batteries and most commercial and military aircraft use NiCad batteries, and they must never be serviced in the same area.
A U.S. lead-acid battery is fully charged at a specific gravity of 1.275 to 1.300, and a difference of 0.050 or more between cells signals the end of its useful life.
Circuit breakers protect the wire, not the equipment, and must open before the wire's rating or the load's rating is exceeded, whichever is lower.
AC 43.13-1B Table 11-3 pairs 18 AWG copper wire with a 10 A breaker, 16 AWG with 15 A, and 14 AWG with 20 A for its stated bundle conditions.
If continuous load can exceed 80% of generator output and no placards or monitoring are fitted, the load must be reduced or charging capacity increased (a rule of thumb, para 11-35).
1.5 Aircraft Electrical Power Systems: Batteries, Generation, Buses, and Circuit Protection
Quick Answer: The CAET's "Basic Electrical Theory & Systems" area expects you to name and explain the parts that make, store, distribute, and protect aircraft electrical power: the battery, the alternator or generator and its voltage regulator, the bus bars and master/avionics switching, and the circuit breakers and fuses that protect each wire. FAA Advisory Circular AC 43.13-1B, Chapter 11 supplies most of the testable numbers: a breaker must open before its wire's rating is exceeded (Table 11-3), breakers protect the wire, not the black box, all resettable breakers must be trip-free, and the total continuous load should stay near 80 percent of generator output when no load placards or monitoring exist.
Why This Topic Matters on the CAET
AEA's own CAET prep course lists "identify the function of basic aircraft electrical components like batteries, alternators, bus bars, and circuit breakers" as a course objective. Every avionics installation draws from this system, so an installer must know where power comes from, how it reaches the radio, and what protects the wire in between. Most avionics squawks that look like "bad box" problems are actually low bus voltage, a tired battery, a corroded bus bar, or a nuisance-tripping breaker.
Storage Batteries
AC 43.13-1B paragraph 11-15 notes that aircraft batteries provide ground power, emergency power, DC bus stability, and fault clearing. Most small private aircraft use lead-acid batteries; most commercial and military aircraft use nickel-cadmium (NiCad) batteries. The AC also warns that any change of battery type may be considered a major alteration.
| Fact | Lead-acid | NiCad |
|---|---|---|
| Typical users | Most small private aircraft | Most commercial and military aircraft |
| Electrolyte | Sulfuric acid | Potassium hydroxide (alkaline) |
| Corrosion neutralizer (para 11-5) | 10% sodium bicarbonate solution | 3% acetic acid solution |
| Freezing risk | Discharged batteries freeze easily; keep specific gravity at 1.275 | Electrolyte freezes near -75 °F; little change with state of charge |
| Charge check | Hydrometer (vented) or open-circuit voltage (sealed) | Manufacturer's procedure |
Warning
AC 43.13-1B says it is extremely dangerous to store or service lead-acid and NiCad batteries in the same area. Acid electrolyte destroys a NiCad battery, and alkaline electrolyte destroys a lead-acid battery.
Lead-acid state of charge
- A U.S. lead-acid battery is fully charged at a specific gravity of 1.275 to 1.300. About 1.240 means one-third discharged and about 1.200 means two-thirds discharged (readings at 80 °F; correct readings at other temperatures with Table 11-2).
- A specific-gravity difference of 0.050 or more between cells means the battery is nearing the end of its useful life.
- For sealed lead-acid batteries, Table 11-1 uses open-circuit voltage at 70 °F: a 24-volt battery reads 25.8 V at 100%, 25.4 V at 75%, 24.8 V at 50%, and 24.0 V at 25% (12.9, 12.7, 12.4, and 12.0 V for a 12-volt battery).
- During charging, lead-acid cell voltage must not exceed 2.35 volts per cell.
- Only a load check determines overall battery condition, and a normal charging system restores a battery during a flight of about one hour to ninety minutes.
Power Generation and Regulation
A 14-volt system uses a 12-volt battery and a 28-volt system uses a 24-volt battery. The charging source runs slightly above battery voltage so that the battery charges in flight.
- Alternators generate AC internally and convert it to DC with built-in rectifier diodes. Paragraph 11-33 warns that these diodes are easily damaged by rough handling, overheating, or reversing the battery connections.
- DC generators are found on older aircraft and need a reverse-current relay so the battery cannot motor the generator. An alternator's diodes already block reverse current.
- The voltage regulator varies field current to hold bus voltage steady as load and engine speed change. Paragraph 11-6 notes that a replacement regulator usually needs adjustment, because the manufacturer's test equipment and the aircraft wiring never present exactly the same impedance.
- Overvoltage protection removes the field or takes the alternator offline if regulation fails, protecting avionics from a runaway bus.
Distribution: Bus Bars and Switching
- Bus bars are the common connection points that feed branch circuits. Paragraph 11-8 says to check them annually; a bus bar showing even limited corrosion should be disassembled, cleaned and brightened, and reinstalled.
- The master switch usually closes a battery contactor rather than carrying battery current itself.
- Many aircraft add an avionics master switch or relay so the avionics bus can stay off during engine start, when voltage transients are most likely.
- Larger aircraft add essential or emergency buses that keep critical equipment powered after a generator failure.
Load management (paragraph 11-35)
Check the generator or alternator rating against the loads the installed equipment can impose. If the total continuous load can exceed 80 percent of the generator's output limit and no special placards or monitoring devices are installed, reduce the load or increase charging capacity. The AC calls this a rule of thumb, not a substitute for a full electrical load analysis. It also describes low-voltage warning lights that trigger below 13 volts on a 14-volt system or 26 volts on a 28-volt system. Paragraph 11-36 adds that new electrical devices should not be installed until the status of the electrical system has been determined accurately.
Circuit Protection: Breakers and Fuses
Paragraph 11-47 says every electrical wire must have circuit protection, placed as close as possible to the power source bus. Paragraph 11-48 sets the sizing rule: a breaker must open before the current rating of its wire or the cumulative rating of its loads is exceeded, whichever is lower, and before anything downstream can overheat and smoke.
| Copper wire (AWG) | Circuit breaker (A) | Fuse (A) |
|---|---|---|
| 22 | 5 | 5 |
| 20 | 7.5 | 5 |
| 18 | 10 | 10 |
| 16 | 15 | 10 |
| 14 | 20 | 15 |
| 12 | 30 | 20 |
| 10 | 40 | 30 |
| 8 | 50 | 50 |
Excerpt from AC 43.13-1B Table 11-3 (bundles of 15 or more wires in a 135 °F ambient up to 30,000 ft). The equipment manufacturer's specified breaker always governs.
Key protection rules:
- Breakers protect the wire, not the black box (paragraph 11-51). Do not "upsize" a breaker to stop nuisance trips; find the cause.
- All resettable breakers must be trip-free, opening on a fault even if the handle is held in. Automatic-reset breakers are not recommended (paragraph 11-50).
- Do not use a breaker as a switch; it shortens breaker life.
- Periodically cycle breakers with no load to clean the contacts (paragraph 11-52).
Switch derating
A switch's stamped rating is its continuous rating with the contacts closed. Table 11-4 derates it for the kind of load:
| 28 VDC load | Derating factor | 12 VDC load | Derating factor |
|---|---|---|---|
| Lamp | 8 | Lamp | 5 |
| Inductive (relay or solenoid) | 4 | Inductive | 2 |
| Resistive (heater) | 2 | Resistive | 1 |
| Motor | 3 | Motor | 2 |
To size a switch, multiply the device's continuous current by the factor. Incandescent lamps can draw an initial current up to 15 times their continuous current, which is why lamp loads carry the largest factor.
Exam Traps
- The 80 percent load figure is a rule of thumb, not a regulation.
- Table 11-3 is based on a stated set of conditions. The manufacturer's specified breaker, or a properly derated choice, governs a real installation.
- Never store or service NiCad and lead-acid batteries together, even briefly.
- An alternator that "won't charge" after a battery was installed backwards probably has damaged rectifier diodes.
Using AC 43.13-1B Table 11-3 as a guide, which circuit breaker rating is listed for an 18 AWG copper wire run in a bundle under the table's stated conditions?
15 A
5 A
10 A
20 A
A radio's circuit breaker keeps tripping. A technician proposes installing the next larger breaker so the radio stays on. Why does AC 43.13-1B reject that fix?
A larger breaker increases voltage drop on the avionics bus beyond Table 11-6 limits
Breakers protect the wire, so a larger one may let the wire overheat before it trips
Larger breakers are not trip-free and cannot be reset in flight
Breakers are sized only by the radio manufacturer's weight limits
A 28 VDC incandescent lamp load draws 2 A continuously. Using the AC 43.13-1B Table 11-4 derating factor, what minimum nominal switch rating is needed?
16 A
8 A
6 A
4 A
A sealed 24-volt lead-acid battery reads 24.8 V open-circuit at 70 °F. According to AC 43.13-1B Table 11-1, what is its approximate state of charge?
25%
50%
75%
100%
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