FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsHard
A 24-volt nickel-cadmium battery is rated at 30 ampere-hours. If a continuous electrical load of 5 amps is applied after the aircraft's generator fails, approximately how long can the battery be expected to supply this load before full discharge?
- A10 hours
- B150 hours
- C3 hours
- D6 hours
Show answer & explanationAnswer & explanation
Correct answer: D. 6 hours
Battery capacity in ampere-hours divided by the discharge current in amps gives the approximate time to discharge: 30 amp-hours ÷ 5 amps = 6 hours. This calculation assumes a constant discharge rate and ideal conditions, though actual endurance may be less due to voltage sag and temperature effects.
Why the other options are wrong
- A. This overestimates the time by using an incorrect divisor.
- B. This results from multiplying instead of dividing (30 × 5), which is not the correct operation.
- C. This would result from dividing incorrectly or using a wrong capacity value.
Battery Ampere-Hour Capacity
A rating expressing how much current a battery can supply over time, calculated as capacity (Ah) divided by discharge current (A) equals hours of use.
- Ampere-hour = amps × hours battery can sustain
- Formula: time (hr) = capacity (Ah) / load current (A)
- Actual endurance is reduced by higher discharge rates and cold temperatures
Memory trick: Amp-hours divided by amps drawn tells you how long the juice will run