Checking Electrolyte Level

WARNING:

Never allow battery fluid to come in contact with skin, eyes, fabrics, or painted surfaces. After touching a battery, never touch or rub your eyes until you have thoroughly washed your hands. If acid contacts eyes, skin or clothing, immediately flush with water for 15 minutes and seek medical attention.

Failure to do this may cause personal injury or damage to clothing or the painted surfaces.

Remove the cell plug using a suitable tool.

Fig 1: Removing Cell Plug
Courtesy of NISSAN NORTH AMERICA, INC.
Add distilled water up to the MAX level.

sulfation

Fig 2: Battery Charging Graph

Courtesy of NISSAN NORTH AMERICA, INC.

A battery will be completely discharged if it is left unattended for a long time and the specific gravity will become less than 1.100. This may result in sulfation on the cell plates.
To determine if a battery has been "sulfated", note its voltage and current when charging it. As shown in the figure, less current and higher voltage are observed in the initial stage of charging sulfated batteries.
A sulfated battery may sometimes be brought back into service by means of a long, slow charge, 12 hours or more, followed by a battery capacity test.