Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsBattery Diagnosis and ServiceHard
A technician is performing a specific gravity test on a conventional flooded lead-acid battery using a hydrometer. The readings for the six cells are: 1.265, 1.260, 1.270, 1.265, 1.200, 1.268. What does this set of readings indicate about the battery?
- AThe hydrometer is faulty and providing inaccurate readings.
- BThe battery has a shorted or weak cell.
- CThe battery is fully charged and in good condition.
- DThe battery is overcharged and needs to be replaced.
Show answer & explanationAnswer & explanation
Correct answer: B. The battery has a shorted or weak cell.
A difference of 0.050 or more in specific gravity between cells indicates a weak or shorted cell. In this case, cell 5 (1.200) is significantly lower than the others (which are all around 1.260-1.270), exceeding the 0.050 difference criteria (e.g., 1.265 - 1.200 = 0.065). This points to an internal fault in that cell.
Why the other options are wrong
- A. While a faulty hydrometer is possible, the consistent readings across five cells suggest it's working; the outlier (1.200) is a battery issue.
- C. The presence of one significantly lower cell reading (1.200) prevents the battery from being considered in good condition.
- D. Overcharging would typically show all cells with very high specific gravity (above 1.265-1.280) and potentially signs of boiling, not one low cell.
Specific Gravity Cell Variation
A difference of 0.050 or more in specific gravity readings between cells in a flooded lead-acid battery indicates an internal fault, such as a shorted or weak cell.
- Specific gravity measures electrolyte density.
- Healthy cells should be within 0.050 of each other.
- Low reading indicates a discharged or faulty cell.
- Requires a fully charged battery for accurate assessment.
Memory trick: Fifty Points Apart? A Cell's Heart Is Breaking Apart!