USCG Exam PracticeDC circuits and electronic principles
A lead-acid storage cell has a nominal open-circuit voltage of 2.1 V. A technician measures the same cell under a heavy load and reads only 1.7 V. What is the most likely cause of this voltage drop?
- A. The cell chemistry has permanently changed, reducing its rated EMF.
- ✓ The internal resistance of the cell is causing a voltage drop equal to the load current times the internal resistance.Correct
- C. The cell has been connected in series with another cell, dividing the voltage.
- D. The electrolyte has increased in temperature, raising the cell voltage above 2.1 V and triggering a protection circuit.
Why B is correct
Terminal voltage under load is always less than open-circuit EMF by the amount I × R_internal. A cell with significant internal resistance will show a normal no-load voltage but a depressed terminal voltage under load. The rated EMF is set by chemistry, not by load conditions. (NEETS Mod. 1 §2-1)
Cited:NEETS Mod. 1 §2-1