When we start our automobiles the starter motor takes energy from the car battery. After the vehicle’s engine is running the alternator replaces the energy removed by the starting process. The alternator also supplies the energy to keep the battery charged while electrical items such as the lights, spark system, and much more are being used.

While the alternator is operating electrical current is passing through the water/sulfuric acid solution called the “electrolyte”. As the current passes through the electrolyte it causes the water to be broken down into its two elements, hydrogen and oxygen. Those two gasses enter the atmosphere and thus some of the water is lost.

Sulfuric acid does not readily evaporate but water does so water is also lost by evaporation.

The electrolyte in fully charged automobile battery should be a solution of water (63%) and sulfuric acid (37%). During the discharging process the sulfur in the sulfuric acid is combined with electrodes, the lead and lead oxide plates in the battery. During the charging process the sulfur is removed from the electrodes and returned to the solution to reform the sulfuric acid.

As the battery loses water the percentage of sulfuric acid compared to the water in the electrolyte will increase above the most efficient operating percentage, 37%. The level of the electrolyte will drop below the tops of the electrodes, if the water is not replaced, which will reduce the surface area of the electrodes that is submerged in the electrolyte this will reduce the charging capacity of the battery and increase the resistance of the battery making it more difficult for the battery to supply the energy to start the engine.

Even the best tap water contains impurities which, if used, will react with the sulfuric acid changing its properties which will reduce the percentage of sulfuric acid in the solution and leave contaminants in the solution that will reduce the batteries efficiency and life.

Distilled water is pure water which means it contains no impurities and should be used to replace the water which was lost. The water that was lost was pure water and the water that replaces it should be pure.

If tap water is used the difference may not be noticed immediately but the contaminants along with the decreased sulfuric acid will increase the resistance of the electrolyte. This increased resistance, just as with levels of the electrolyte dropping below the electrode tops, will make it more difficult for the battery to start the engine. It will also increase the operating temperature of the battery which will increase the rate of evaporation. The time it takes for the alternator to replace the energy used to start the engine will take longer

Mechanics and service station attendants do not always, or more correctly rarely use distilled water. Sometimes they will buy a bottle of distilled water but when it is empty they will fill it from the tap so people will think they are using distilled water.

A good practice, if you do not replace the water in your own battery, is to carry a bottle of distilled water in your vehicle and don’t let anyone replace the water in your battery from any other source. A gallon of distilled water will cost you a little over a dollar to purchase but in the extended life it will give your battery it will pay for itself several times over.