What should you top up your car battery with when the fluid is low?

Using distilled water to top up your car battery when the fluid level is low is essential because distilled water is free from impurities and minerals that could interfere with the battery's performance. Car batteries require a specific chemical balance to function correctly, and introducing anything other than distilled water—such as tap water or spring water, which can contain minerals—may lead to corrosion or decreased battery efficiency over time.

Additionally, topping up with battery acid is unnecessary and potentially harmful; if the battery fluid level is low, it typically means the water has evaporated, not that you need to replace with acid. Therefore, distilled water ensures that you maintain the right conditions for your battery without introducing harmful substances that could compromise its longevity or efficiency.