FAA Aviation Mechanic General (AMG)Fundamentals of Electricity and ElectronicsMedium
A technician is testing a 12-volt lead-acid aircraft battery composed of six cells connected in series, each with a fully charged open-circuit voltage of 2.1 volts. What is the expected total open-circuit voltage of the battery?
- A12.6 volts
- B6.3 volts
- C7.2 volts
- D12.0 volts
Show answer & explanationAnswer & explanation
Correct answer: A. 12.6 volts
In a series-connected battery, the total voltage equals the sum of the individual cell voltages. With six cells at 2.1 volts each, 6 × 2.1V = 12.6 volts open-circuit, which is normal for a fully charged 12-volt lead-acid battery.
Why the other options are wrong
- B. This value assumes only three cells, not six.
- C. This value does not correspond to lead-acid cell voltage of 2.1V per cell.
- D. 12.0 volts is the nominal rating, not the actual fully charged open-circuit voltage.
Lead-Acid Battery Cell Voltage
Each fully charged lead-acid cell produces approximately 2.1 volts open-circuit; cells are connected in series to reach nominal battery voltage.
- 6 cells in series = 12.6V nominal 12V battery
- 12 cells in series = 25.2V nominal 24V battery
- Nominal rating is lower than actual fully charged voltage
Memory trick: Each lead-acid cell adds 2.1V — stack six for a '12-volt' battery that's really 12.6V.