Battery Charge Indicator
Use the sight glass on the top of a maintenance-free battery to gauge the (SoC) state of charge. Typically a light green dot indicates a fully charged battery. The electrolyte solution is close to 1.265, a solution that is heavier than water (1.0).
Maintenance-free batteries are equipped with relief valves that prevent pressure buildup. Manufacturers refer to them as VRLA or valve-regulated lead-acid batteries.
A dark green/black indicator on a maintenance-free battery typically indicates that the battery needs a charge. The electrolyte has gone through a chemical reaction; it's now closer to water. Charging a battery with a dark indicator restores the solution's specific gravity.
A light yellow or clear sight glass indicates that the electrolyte solution has dropped below the level of the built-in hydrometer. The battery may have a dead cell. Most battery manufacturers recommend replacing a maintenance-free battery that has low electrolyte levels.