Battery Charge Indicator
Use the sight glass on the top of maintenance free batteries to gauge their (SoC) state of charge. Typically a light green dot indicates a fully charged battery. This is because 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. That’s why they 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 solutions 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.