Friday, September 22, 2023


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.”

##RVT827 ##RVDT1398

Russ and Tiña De Maris
Russ and Tiña De Maris
Russ and Tiña went from childhood tent camping to RVing in the 1980s when the ground got too hard. They've been tutored in the ways of RVing (and RV repair) by a series of rigs, from truck campers, to a fifth-wheel, and several travel trailers. In addition to writing scores of articles on RVing topics, they've also taught college classes for folks new to RVing. They authored the book, RV Boondocking Basics.


  1. Just to give the Crazy Gonzo answer, you can simply put a tea kettle on your stove and distill your own with a clean cookie sheet. Hold the sheet as a “diamond” and let the steam blast against it — it condenses against the impromptu heatsink and runs off the corner of the pan. If you want to do a lot, connect fishtank line to your kettle, pass the tube through a cooling bath, and it will run distilled water out the end.

    Around the shop, I distill water by having a piezo dehumidifier (non-metallic plates) drip through a non-chemical filter (to remove any dust/dirt). At a quart or so per day, this device is not fast, but I’ve never had any issues with minerals or contamination in the final product.

      • I’d agree for most folks! I was more saying “in a pinch” if having trouble finding it in stores. For myself, I run the dehumidifier anyway so might as well get my electricity back as water…

  2. I’m retired from the power industry where we produced high purity water for use in plant steam systems. We used RO units as polishers to remove a lot of the dissolved minerals in water so we could get longer runs on our deionization units that they fed, which produced the high purity water. Our specs were in the sub part per billion range for some elements. The article is exactly right, RO significantly removes dissolved material in the water, but its not complete removal. Distilled water is the best bet that you can get off the shelf for batteries.

  3. Back in my broke college days (before dirt), a friend had a cheap car that had a leaky battery. It wouldn’t start one morning (most of the electrolyte was gone). His fix was scooping gutter water with an old big gulp cup and dumping it in the battery. I was so shocked when it actually worked. This only succeeded a few times, but it did work in a pinch and the car got us to our early morning job.

  4. When our children were wee little we needed distilled water for their formula. We always found it at the drug store. They might still sell it. I would suspect they do.

  5. I replaced two trojans 6volt batteries at 125.00 apiece, I can buy a lot of distilled water for that. .99 cents to 1.00 is petty cheap I think.

  6. I’m surprised the article doesn’t offer the option of a home water distiller, available through Amazon for $70 to $100. It makes up to 6 gallons per day. We double filter the water prior to entering RV’s then filter our drinking water through a Zero filter.
    Recommend a sealed AGM deep cycle battery and not have to find distilled water. That or buy the home water distiller.

    • A lot of ladys that ironed clothes would use water from dehumidifie rs. Because of the air passing over the coils had dirt in it and plugged up the vent holes. We recommended they quit using it.

    • AC condensers are great air scrubbers removing all sorts of crap in the air passing over it. As a hay fever sufferer I’m very grateful for how it removes pollen. Between the crap it removes from the air, and the potential for picking up metal from the condenser, I’d be hesitant to put it in a battery.

  7. 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.

  8. 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

    • Distilled has to be used in CPAP machines. I buy mine usually at Walmart, but my small local grocery store also carries it and even a few cents cheaper per gallon.

  9. 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

  10. 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.

  11. 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:
      Water softeners tend to remove some minerals, but don’t get them all. They also use an ‘ion exchange’ that adds sodium to the water output. In Washington where we lived for a number of years, when we installed a water softener, we had to go to great pains to purchase one that removed iron from the water — where we lived, the “raw” water would turn white shirts orange it was so bad. Even with a water softener specifically designed to remove iron, some still made it through. So, get all the minerals out — not safe enough to count on with the high cost of batteries.

  12. 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.

  13. 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.


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