Automotive Electronics Practice Test

A charge indicator (eye) operates by showing green or red when the battery is charged and dark if the battery is discharged. This charge indicator detects _____.

1. Battery voltage

2. Specific gravity

3. Electrolyte water pH

4. Internal resistance of the cells

The correct answer is: Specific gravity

The charge indicator, often referred to as a "battery eye," operates primarily based on the specific gravity of the electrolyte solution within the battery. Specific gravity measures the density of the electrolyte in relation to pure water and is an important indicator of the state of charge of a lead-acid battery. When the battery is charged, the specific gravity of the electrolyte increases due to the higher concentration of sulfuric acid in the solution. Conversely, as the battery discharges, the specific gravity decreases, indicating a lower concentration of sulfuric acid. The charge indicator is designed to provide a visual representation of this change: it may show green when the battery is adequately charged, red when it is low on charge, and remain dark when the battery is fully discharged. Other options like battery voltage, electrolyte water pH, or internal resistance are not directly indicated by the charge eye. While battery voltage is a critical parameter for battery performance, the charge indicator specifically reflects the state of charge through specific gravity rather than voltage levels. Similarly, pH and internal resistance do not provide a direct indication of the battery's charge status in a practical, user-friendly manner as the specific gravity does.