Question 51 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is C **Why Option C is Correct:** Option C, "comparing the specific gravity of the electrolyte in each cell," is the most definitive and reliable method for detecting a weak or dead cell in a lead-acid battery. The specific gravity (SG) of the electrolyte (sulfuric acid and water) is directly proportional to the state of charge of the cell. During discharge, the sulfuric acid is consumed and replaced by water, causing the SG to drop. * A fully charged cell will have a high, uniform SG (typically around 1.265–1.280, depending on the battery type and region). * A weak or dead cell (one that has internally shorted or sulfated severely) will refuse to hold a charge, resulting in a significantly lower SG reading compared to the other healthy cells in the battery (e.g., 1.100 or lower). * By comparing the SG readings across all cells using a hydrometer, technicians can immediately pinpoint the specific cell that is failing. **Why the Other Options are Incorrect:** * **A) visually inspecting the electrolyte levels of each cell:** While low electrolyte levels can indicate poor maintenance and potential damage (which might lead to a weak cell), inspecting the level only checks the volume of liquid, not the chemical concentration (state of charge) or health of the plates. A cell can be full of water and still be completely dead. * **B) taking each cell's temperature with a calibrated mercury thermometer:** Temperature measurements are useful for detecting significant internal issues like thermal runaway or excessive resistance during charging or discharge. However, a slight difference in temperature is not a primary or precise indicator of a *weak state of charge* or a *dead cell*. Specific gravity provides a direct measure of the cell's chemical health, which temperature does not. * **D) taking an open circuit voltage test of individual cells:** While the open circuit voltage (OCV) of a healthy, fully charged cell should be approximately 2.1 to 2.2 volts, testing the individual cell voltage is often impractical (requires specialized equipment and access to internal cell connectors) and less sensitive than SG testing. Furthermore, a faulty cell that is still holding *some* charge may show an OCV that is deceptively close to the other cells, especially if the battery was recently charged. The SG test provides a much clearer distinction between a healthy cell and one that cannot maintain its acid concentration.