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The electrolyte is too low if any part of the plates is exposed to air. If the plates are not fully covered with electrolyte, then the battery cannot operate at full capacity. Exposing the plates to air ruins the area exposed to air in a matter of days. If the electrolyte is only about 1/2 inch (1 cm) below the top of the plates, adding enough water to the battery to just cover the plates may return it to serviceable condition at a slightly reduced capacity. (Instructions for adding water follow in Part 3 of this wiki article.) Otherwise, you will need to consider replacing the battery. A low electrolyte level could be caused by overcharging, so if this is the case, you should consider having your alternator checked. The normal fluid level is about 1/2 inch (1 cm) above the tops of the plates or about 1/8 inch (3 mm) below the bottoms of the filler tubes that extend down from the port openings. If this is the case it may not be worth the effort to fill the battery at this time. Just replace the port covers and inspect again in three months. The maximum safe fluid level is just touching the bottom of the filler tubes. Most filler tubes have a pair of slots in the side somewhere near the bottom of the tube. This causes the meniscus (the little bit of liquid that bends up near the edge of the tube) to have a distinctive eye-shape if the fluid is touching the filler tube, whereas there is no meniscus if the fluid is below the bottom of the filler tube. The eye-shaped meniscus is designed as a signal to stop filling. You may need to use a flashlight to see the fluid level and the presence or absence of the meniscus clearly. You should always follow the advice of your battery dealer or the battery manufacturer if it conflicts with the information provided in this article. Also be aware that batteries for golf carts, floor cleaning machines, and nickel cadmium batteries especially may require other electrolyte levels.
Recognize when electrolyte levels are low. Recognize when electrolyte levels are normal. Recognize when the electrolyte is at its maximum level. Keep in mind that these levels are for lead-acid automotive batteries only.