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Water from dehumidifiers

In the January Waterways World you have an article on dehumidifiers. It states that the water from them is suitable for topping up batteries - but I was always taught that you shouldn't use water that has been in contact with metal, as this will have been. Can you clarify please?

Asked by: Dave Cleaver  | 3.08pm, Wednesday 5 January

WW says:

Sorry for the delay in replying.
The water from dehumidifiers is quite close to that of pure distilled water. You are correct that there is contact with metal- the usually stainless steel or aluminium (less commonly) evaporator.
However, the limited contact time, plus the low temperature, means that very little metal ions dissolve in the water. The water from the dehumidifier will have quite a low pH
(around 5.5 to 6.5) due to absorption of carbon dioxide from the air, which is common to all purified water.
Most battery top up fluids are deionised water, rather than pure distilled, and so might actually have a greater conductivity (and hence higher ionic concentration) than the distilled water, or that from the dehumidifier.
So, in practice, the water from a dehumidifier will be perfectly suitable for topping up batteries, provided the long-term storage is not in a metallic container.
Incidentally, most commercial distilled water is done in stainless steel pressure plants, and the level of dissolved solids is very low, though some are then polished by passing through deionising resin.
There is a much greater chance of contamination (from iron III) from drips of condensation falling onto the battery caps from the steel shell, or dirt entering the cells.
There is one proviso about using water from dehumidifiers, and that is the particle pre-filter must be kept clean, otherwise small dust particles might contaminate the water, though this is low-risk.

Mark Langley  | 5.15PM, Sunday 9 January

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