Which of the following should never be added to the battery?

Adding battery acid to a battery is not a regular maintenance task and is typically only needed in specific circumstances, such as when refilling a battery that has low acid levels. However, even in those situations, it’s important to ensure that the correct concentration and type of battery acid are used, as improper handling can lead to dangerous chemical reactions, damage to the battery, or reduced performance.

On the other hand, distilled water is often added to maintain the proper electrolyte level in batteries, especially in lead-acid batteries. Mineral water and tap water contain various impurities and minerals that can harm the battery's functionality over time by causing corrosion or scaling. Therefore, the correct answer emphasizes that battery acid should not be arbitrarily added and highlights the importance of using the right materials for battery maintenance.