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 2.1 V at the terminals with no load connected, but the voltage drops sharply when a load is applied. What is the most likely cause?
- A. The cell's electrode chemistry has changed, lowering its EMF under load.
- ✓ The cell has high internal resistance, causing a large voltage drop under load current.Correct
- C. The cell is a primary type and cannot sustain current flow.
- D. The electrolyte concentration is too high, increasing the open-circuit voltage artificially.
Why B is correct
Terminal voltage under load equals open-circuit EMF minus the internal-resistance drop (I × r). A cell with high internal resistance will show normal no-load voltage but sag significantly when current flows. This is distinct from a change in electrode chemistry, which would alter the open-circuit EMF itself. (NEETS Mod. 1 §2-1)
Cited:NEETS Mod. 1 §2-1