Maintenance Free Batteries
We do not typically add water to a maintenance-free battery. It's sealed and contains either a gel or glass fiber mat instead of the liquid solution found in standard batteries.
Instead of vents, they're equipped with relief valves that prevent pressure buildup. That's why the manufacturers refer to them as VRLA or valve-regulated lead-acid batteries.
These batteries have a small hydrometer with a sight glass located on the top of the case. A bright green dot indicates a charged battery. The electrolyte solution is close to (1.265), a solution that's heavier than water (1.0).
A dark green or black sight glass indicates that it 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 to a weight closer to (1.265).
A light yellow or clear sight glass means the electrolyte solution has dropped below the built-in hydrometer. Most battery manufacturer's recommend replacing the battery when the level drops below this threshold.