Is reverse osmosis water safe for your RV batteries?


By Russ and Tiña De Maris

David Guo’s Master on

Not long ago we got a message from a reader who was traveling across the United States. Before he left home, he’d hit the local shopping centers looking for distilled water for use in his RV batteries and, much to his dismay, couldn’t find any. Could he use RO (reverse osmosis) water to keep his battery cells topped off?

What kind of water to use in RV batteries isn’t an issue for folks who use the more expensive “gel” batteries or other types of sealed units that don’t require regular maintenance. But for those of us who still use flooded acid batteries, just what you put down the “throats” of those batteries is a critical issue

Battery manufacturer Trojan reports that, “Water usage needs to be viewed as a priority for maximum performance.” When you think about the considerable cost of replacing a set of RV batteries, “maximum performance” and longevity is nothing to write off as unimportant.

Just how “pure” should your water be? Trojan has put out a “white paper” that lists water contaminants that can have an adverse effect on flooded acid batteries, as well as “what ifs” if those contaminants get into battery electrolytes.

Here are a few examples:

Magnesium and calcium, known as major constituents in “hard water,” is said by some to be great for the human heart. Batteries don’t quite see it the same way: As few as 40 parts per million of magnesium reduces battery life. The same amount of calcium increases the “shedding” of material from the battery.

Nickel, often found in ground water because of its presence in rock, can really create havoc. ANY amount of nickel in water fed to batteries can result in “intense lowering of on-charge voltage.”

Copper, in amounts more than 5 parts per million, will cause in increase in battery self-discharge. It’s true: All batteries, left alone, will eventually lose their charge – but who wants to speed the process up?

Those are just a few of the contaminants that can cause your batteries grief. For a complete list, check out Trojan’s white paper here

So what’s to be done? What water can you count on that ensures you’re doing your best to keep your batteries happy and healthy? The “best” choice is deionized water. Water is passed through special resin beds that exchange dissolved minerals (that you don’t want) for hydrogen and hydroxide ions, then recombine them to form PURE water. Sounds great? Sure, until you figure out the cost. We found several sources on the internet, but figuring about $25 per gallon (including shipping costs) made the really pure stuff rather off-putting.

What about reverse osmosis (RO) water? It’s generally recognized for seriously knocking down the amount of impurities in water. Trouble is, not all RO systems are created equal – varying amounts of some minerals batteries shouldn’t have can be left in the “finished product.” For example, RO “will remove 97 – 98% of the nickel from drinking water,” says one prominent internet source. But since ANY amount of nickel will lower a battery’s charge voltage, if there was some in the water before it was processed with RO, then there could be enough after processing to create serious issues.

If you’ll pardon a pun, from a “battery health and longevity” and financial standpoint, distilled water is your best bet. Says Trojan’s white paper of the distillation process, “This process creates a finished product free of minerals, having left all the impurities in the original water sample.” The reader who contacted us about not being able to find distilled water in his home town did report that as he traveled down the road he was able to find it for sale in a Walmart store. In our area, it’s about $1 a gallon, which certainly compares favorably with deionized water.

When shopping for distilled water, read the label. We once found the regular distilled water shelves cleared out, but nearby bottles of Nursery Purified Water promised to be “processed by steam distillation for purity.” Distilled, indeed – Trouble was, that same “glorious purified water [had] minerals and fluoride added for taste.”


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Tom Bender

While the FAA inspector may be right about adding water after charging. If the acid is below the plates, I would say at least cover the plates.

Charles Howard

Wal-Mart in Montgomery AL and at least one major Grocery chain there NO longer carries Distilled water.
Wal-Mart Distilled Water i found in Donna TX came from Fort Worth..
I suspect cost/customer demand (more Gel or Sealed Batteries?) will eventually reduce Distilled Water availability. – as it already apparently has in one market


hi, i’m john and i am an faa maintenance inspector, use only distilled water and nothing else. charge the batteries to full charge then add the water, not the other way. only fill the battery after charging and only to the vent poles inside the battery. read your paper work or forget the paper work and buy more batteries. ????j

Gene Bjerke

My first reaction (before reading) is don’t use RO water. We have spent a lot of time in North Carolina’s Outer Banks, which uses RO seawater. The taste is not very good (though one can get used to it). I assume that means that there are some things that get through the membrane. I don’t want ANYTHING in my battery water.


I don’t know where you’re paying $25 a gallon for deionized, but that’s absurd. We were in the bottled water business for many years. We sold reverse osmosis, deionized, and distilled gallons and 5 gallon bottles. All of our products were within a few cents of each other as were the products of our competitors.

Mike Logan

How about water that is run thru a water softener using salt to remove minerals?

Smyrna, TN

Tommy Molnar

I can’t imagine not being able to find distilled water. 7-11 may not carry it (though I would check if I had to), but almost every supermarket has it, as does Walmart, many truck stops, and almost anywhere that sells food.

John Snell

For less than the cost of 4 gallons of deionized water , I can buy a new deep cycle battery. Regular distilled water is just fine.