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On 5 Jun 2001, at 17:50, Steven Ayres wrote: > I've heard that liquid-cell batteries outgas more or less according to > temperature, furnishing a pretty steady flow of acidic vapor around > the battery, which accounts for the tendency to rust even in dry cars. My impression is that the only part that leaks out is water vapor. The vapor pressure of the sufuric acid component is, I believe, much lower. I don't recall this subject actually being covered in any of the material I have on batteries, but it's consistent with the fact that all my sources agree that it should only be topped up with pure water, not acid. This implies that only water is lost in measurable amounts. The rate of water evaporation certainly has lots to do with the temperature, as well as the charging voltage. In the distant past, most water was lost when it was dissociated into hydorgen and oxygen during the charging cycle. Minor changes in battery chemistry have somehow managed to push the dissociation voltage up slightly so that this almost never happens. This allowed "maintenance free" batteries to be introduced about 25 years ago. I've heard that this had something to do with adding a trace of calcium to the alloy in the plates, but I don't know the details. - Jim Adney jadney@vwtype3.org Madison, WI 53711-3054 USA ------------------------------------------------------------------- Search old messages on the Web! Visit http://www.vwtype3.org/list/