It's important to regularly check the electrolyte (it really isn't just water) level in your car's battery, for two reasons: first, because it naturally evaporates and second, because a small amount electrolyzes into hydrogen and oxygen every time you charge the battery. Learning how to safely check and replace the water in your car battery is an important aspect of car maintenance. Start with Step 1 below for detailed instructions on how to check car battery water levels, while protecting both you and your car.
Turn the car engine off. Open the hood of the car and locate the battery. Remove the vent caps from the battery. Clean the battery surface top before opening the inspection caps to avoid getting dirt and debris into the car battery.
Check the water level in each cell of the battery. Use only distilled water when filling a car battery. Using anything other then distilled water will cause permanent failure of your battery.
Add water to the battery to bring the level to just below the bottom of the cell inspection hole. Do not over fill the water level. This can cause the acid water mixture to leak. Flush any spilled acid water mixture with fresh water. Put the vent caps back onto the battery.
Battery terminals should be cleaned periodically with a mixture of baking soda and water and wire brush if necessary. The terminals should be rinsed with clean water and then can be coated with a commercially available sealant or high temperature grease.
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