A charge indicator operates by showing green or red when the battery is charged and dark if the battery is discharged. What does this charge indicator detect?

The charge indicator in question primarily detects the specific gravity of the electrolyte within the battery. Specific gravity is a measure of the density of the battery electrolyte compared to water, which changes based on the state of charge. When a battery is charged, the specific gravity of the electrolyte increases due to the higher concentration of sulfuric acid (in lead-acid batteries, for example). This higher density allows the charge indicator to show a green light. Conversely, when the battery is discharged, the specific gravity decreases, causing the indicator to show red or to turn dark, indicating a low charge state.

By utilizing specific gravity to provide a visual indication of battery status, the charge indicator is a simple yet effective tool for users to assess battery health and operational readiness. Other measurements such as battery voltage, water pH, or internal resistance do not provide the same direct correlation to battery charge state as specific gravity does in this context.