If you add too much water to a car battery what happens is the excess water can cause the water to overflow out of the battery and damage the battery and also having too much water in the battery can throw off the proper electrolyte dilution balance in the battery and negatively affect the batteries performance and even lead to a dead battery.
Distilled water is commonly used for batteries and is basically the same as battery water which is water that is pure and free of impurities and minerals.
Distilled water is purified through distillation, which involves heating water to evaporate it and then condensing the vapor back into liquid form, which leaves behind any impurities that were in the water.
You cannot use tap water even after boiling the water contains minerals as well as other impurities that can damage the battery and cause internal shorts and reduces the batteries lifespan.
You want to use distilled water or deionized water or battery top up water.
Distilled water, deionized water and battery top up water are free of the minerals and impurities which makes it the best water choice for lead acid batteries.
Tap water contain minerals and impurities which can harm the battery's components and lead to corrosion and reduced battery life.
You should use deionized water or distilled water or battery top up water made for topping of the water level in batteries.
Battery top up water is a type of water that is deionized or distilled to be used in replenishing the water and electrolyte solution in lead acid batteries.
Lead acid batteries are a mixture of water and sulfuric acid and when the water level drops in the lead acid battery due to evaporation or gas conversion during charging it can prevent the battery from holding a charge or become depleted quickly.
The electrolytes in a battery are a mixture of sulfuric acid and water and the water in the battery protects the batteries active material while the battery generates and produces power.
Without water in the battery, the active material within the battery will oxidize and the battery will then lose power.
To check if your battery needs water you can open the filler caps and ensure the electrolyte "fluid in the battery" covers the plates or the rectangular metal parts inside it fully.
If the car battery has filler caps that are removable then it's a type of battery that requires water as it's a maintenance based battery.
If the battery has no removable filler caps then it's a maintenance free battery that doesn't need water top up.
A marine or golf cart battery that is only used on the weekends may only require watering once a month.
A forklift that is used every day, may need to have its battery watered once a week.