Deionised water has had its dissolved mineral ions removed, leaving water with very low conductivity and virtually no scale-forming content.

That matters in several everyday applications. Lead-acid batteries must be topped up with deionised or distilled water, because the calcium, magnesium and chloride in tap water attack the plates and shorten battery life. Coolant concentrate should be diluted with de-ionised water for the same reason, since minerals form scale on hot surfaces and reduce heat transfer while chlorides accelerate corrosion in aluminium engines. It is also used in steam irons, humidifiers, laboratory work and as a final rinse in cleaning and detailing to avoid water spots.

Store it in a clean sealed container, as deionised water picks up contamination readily from dirty vessels.