What is the main idea of the passage? A car battery that is low on water will not deliver enough current to start your car. The average life expectancy of a vehicle battery is three to four years. Being low on water also shortens the life of the battery. The water level should be checked regularly—more often in the summer than in the winter. Always check the water in your battery before a long trip. When the battery is low, always fill it with distilled water, which is water without impurities such as salt. Tap water contains salts that will corrode the battery. You should also be careful not to overfill the battery. When the water mixes with the acid in the battery, it can damage your clothing or other parts of your car.