Even maintenance free batteries need some attention every now and again. The balance of electrolytes and water in the cells of the battery will eventually become too low, and you will need to top off the cell with distilled water. While some batteries come with a hydrometer built in, that only reads the state of one cell. You have to use a battery hydrometer and test each cell to really know the condition of your battery.
Disconnect the negative lead on your battery, then disconnect the positive one.
Put on safety goggles and rubber gloves. Using a flat screwdriver, pry open the cover on your first cell. The covers are round pieces of plastic inserted into the top of your battery. If you have a maintenance free battery, the covers to the cells are hidden beneath the paper labelling on the top of your battery.
Insert the tube or syringe part of your battery hydrometer into the liquid in the cell. Depending on what type of hydrometer you have, you will either squeeze and release the bulb at the end to draw liquid into the body of the hydrometer, or you will operate it like a syringe and pull the plunger up to draw liquid. Draw liquid into the hydrometer until you see the float in the body (usually a coloured plastic ball) begin to rise. Note the reading on the gauge or scale printed on the side of the hydrometer, then release the liquid back into the cell.
Repeat step 3 for all of the cells in your battery, drawing in liquid until the ball floats, and noting the measurement on the side. A good battery will only have a difference of 0.5ml between the measurements from all the cells. A bad cell will exceed that difference.
Add a small amount of distilled water(no more than a cap full at a time) to the bad cell, and test it with the battery hydrometer again. Keep adding water and testing until the float measurement is within the acceptable range.
Follow the instructions on how to read the gauge on your specific battery hydrometer before testing the cells. There are several brands and styles of hydrometers on the market, but many have slight design differences in how they present the readings.
Wear safety goggles (not glasses), because goggles are designed to prevent liquid from getting into your eyes in the event you get splashed with the electrolyte/battery mix; this will happen if you are not extremely cautious when adding water to the battery.