When using a hydrometer, make sure the top of the battery is clear of any contaminants. In low light conditions, a light source may be required. Under normal operating conditions, the hydrometer can give three readings.
Caution: Air bubbles in the magic eye may cause inaccurate readings. In this case, you need to gently tap the "magic eye." The electrolyte level in a maintenance-free battery can be determined from the outside using the maximum and minimum level marks. The exception is batteries with a black casing.
The text was borrowed from here: Nissanbook.ru