Charon
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The electrolyte in the battery is a mixture of distilled water and sulphuric acid. The sulphuric acid takes part in the reaction in the battery in such a way that, as the battery discharges, the acid becomes more dilute (the sulphate ends up on both the positive and the negative plates). A fully-charged battery has a more concentrated solution of acid. As the battery reaches full charge, part of the water is dissociated into hydrogen and oxygen. This mixture is explosive. It bubbles up through the acid, giving the appearance of boiling. The acid becomes more concentrated as the water departs, and of course the electrolyte level drops. You need to replenish the water, not the acid (unless there has been a spill), so add water, not acid. The reason for using distilled water instead of tap water is because of the minerals usually dissolved in tap water, which can contaminate the battery.
By the way, those battery testers do not measure pH. They measure specific gravity. Sulphuric acid is heavier than water. The acid solution becomes more concentrated as the battery charges, so the specific gravity of the mixture increases. In short, the more fully charged the battery, the heavier the electrolyte and the more balls float.
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