What should be added to the battery when the fluid level is low?

When the fluid level in a battery is low, adding distilled water is the correct course of action. Distilled water is free from minerals and impurities that can interfere with the battery's electrochemical reactions. Using distilled water helps maintain the proper chemical balance within the battery, ensuring it operates efficiently and prolongs its lifespan.

On the other hand, tap water may contain minerals and contaminants that can negatively impact battery performance and lead to corrosion. Mineral water is similarly unsuitable due to its mineral content. Adding battery acid is not appropriate for topping off fluid levels; instead, it is used for specific charging or maintenance purposes and should be handled with care. Therefore, using distilled water is the proper and safe method to address low fluid levels in a battery.