Distilled vs. Purified Water for batteries

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Yes, I agree there is a small amount of sodium. :P

But most soft water does not have a mineral presence which is really the problem.

Distilled would be the best though!


If you can't find distilled, don't know why you couldn't, but just in case, find something that has been purified with reverse osmosis and hasn't had minerals added back in for flavor.


From the British Water Company:

Q  Can softened water be used in car batteries and steam irons?
A   No. Only distilled or deionised waters should be used in car batteries. Many modern steam irons can use hard and/or softened water. The guidance given in the manufacturer's instructions should be followed.

From About.com:

Expert: Douglas Logan
Date: 6/1/2004
Subject: soft water

Andy, can I use the soft water that comes into the house from the water softner to fill my car battery and auto radiator? Will the sodium hurt the devices?

Hello, Sophie.
You should always follow the advice of the automobile manufacturer regarding what water is suitable for the battery and auto radiator. High sodium levels can contribute to corrosion and could effect the chemistry in the battery. Distilled water, available at many grocery stores, usually is priced at less than one dollar per gallon, making it the preferred choice for both applications.

Best Regards,

Doug Logan

Those are among just a few of the answers I got when I googled it.

There are a couple of articles that state that softened water is only good for washing, heating and filling car batteries,
however, if you think about it, the specific gravity of the electrolyte, (sulfuric acid and water) is very precise. If you have sodium mixed in with it, in any amount, you will obtain skewed readings on a hydrometer.

Just my tuppence worth.

My two cents worth?  Distilled ONLY (hee hee, then read on)

Tap, purified, drinking, etc water all has various degrees of mineral content that will kill a battery slowly or quickly depending on what that content is.

Distilled water is perfect

Deionized water is a bit too pure.  It has a tendency to leach metals because it is "hungry" so to speak.  While it's probably a better choice than purified drinking water, unless you've actually done the science I'd advise against using it in a battery.  It's one of those things that unless completely known, has a fairly decent potential to mess a battery up.
My experience with deionized water is in closed circuit laser cooling systems.  Unless you also put an oxygen removal filter in the loop, deionized water would eat the metals that the laser was constructed of with expensive damage as a result.  So knowing that, but not knowing the actual science of using it in a battery, would tend to make me stay away from it.... and distilled is so cheap and avaliable, every supermarket has it... so I can't see a reason to not use it.

Ok, the "hee hee" part... I'll admit, I use reverse-osmosis'd water in all my batteries.  R/O water is virtually as pure as distilled... the test is to put a cup or so in a clear glass vessel and allow it to evaporate... if you have a mineral deposit left don't use the water. R/O always passes the test, and functionally it's as good as distilled.  Now there is a chance that if you use home made R/O water your filters could be old, in that case you'll be back to fouling your battery.  So because I use it doesn't mean it's perfect... distilled always will be perfect...


Distilled water carries the day!  Thanks all for the very informative answers.  I shall get myself to a store and pick up a few bottles so I am no longer tempted to put in bottled water in a moment of weakness.    8)  8)  8)

Kind Regards, Phil


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