Skip to Content

Please Don’t Water Your AGM Batteries

| Updated Aug 20, 2014

Class B owners are a diligent bunch. They want to do maintenance to head off problems. This is very commendable, but there are occasions when you can maintain your unit to death. One such area is AGM batteries.

Your basic AGM battery. Those are the vent caps on top.

For the past four years, Roadtrek has provided AGM (absorbed glass mat) batteries as standard equipment. These differ from the old flooded lead-acid batteries, which were mounted on slide-out trays and needed to be checked for water every month or two. AGM batteries don't lose water, thanks to an ingenious catalytic cap that recombines the hydrogen with oxygen to make water, which trickles back down into the cell and never gets lost. No more battery maintenance – you don't have to do anything except replace them when they wear out.

Right there in black and white – DO NOT ATTEMPT TO OPEN VENTS.

But the old ways die hard. My grandaddy watered his batteries, you say. My daddy watered his batteries. And by gum, I'm gonna water my batteries. All batteries need water. Well, OK, if you can't stop yourself, go ahead, but remember, you're damaging your batteries by doing this.


DO NOT OPEN in three languages. I think they're trying to tell you something.
DO NOT OPEN in three languages. I think they're trying to tell you something.

So haul your batteries out of the permanent, non-sliding tray. It's a pain, because the manufacturer is trying to keep you from messing around with this. But ignore this warning sign. Also, ignore all the labels on the top of the battery saying “DO NOT ATTEMPT TO OPEN VENTS” and the lettering on the cap saying “DO NOT OPEN”. Those are for the regular owners, not experts like you.


Vent plug for an AGM battery.
Vent plug for an AGM battery.

Blowing through these two stop signs, pry the cap off, and figure out a way to unscrew the vent plug. It's hard, because you're not supposed to be doing this. The vent plug is an ingenious device which opens at a preset pressure, like a radiator cap, and contains the catalyst which recombines the hydrogen. Now look inside. Aha! you say. It's low! I don't see a liquid surface over the plates!

Those are happy glass mats, not thirsty plates.

Of course you don't, Einstein – those are the glass mats which absorb the electrolyte and keep these batteries from spilling. They aren't the plates you would see in a flooded cell battery.  What looks like a flooded cell battery that needs water is actually a perfectly fine AGM, which doesn't need anything. It just wants to be left alone. But NOOOO, you have to mess with it.

Battery compartment for a 190. See that nice shiny tray? Keep it that way.

OK, go ahead.  Slosh copious quantities of water into each cell, close them back up, and now charge your batteries. Watch the excess electrolyte bubble out of the caps like a volcano, spilling concentrated sulfuric acid all over your battery tray. Rinse this off, and it will continue to reappear every time you charge your batteries, because you overfilled them. Buy an new battery tray within a year or two because your old one looks like Swiss cheese. Replace your batteries, because you diluted the electrolyte down to the point where they won't hold a charge. They sure cost a lot more than regular batteries, don't they?  THAT is why you aren't supposed to water your AGM batteries. It's expensive.

Yowsa! A giant battery bank for an Etrek package. Don't fool around with these - they outnumber you.
Yowsa! A giant battery bank for an Etrek package. Don't fool around with these – they outnumber you.

Seriously, do not monkey around with these batteries. In addition to ruining the batteries, the caps hold a fair amount of pressure sometimes, and you risk getting splattered with sulfuric acid, as well as risking battery explosions. These are no fun, even with eye protection, because that sulfuric acid really, really stings when it gets all over your skin.  If you want an exfoliant scrub, try the beauty salon, not the garage.  Your batteries will provide you with many happy years of service if you resist the impulse to mess around with them.

RV Lifestyle

Published on 2014-08-20

7 Responses to “Please Don’t Water Your AGM Batteries”

January 17, 2018at12:17 am, Jon Smythe said:

And domagm batteries need any kind of maintenance?…..these were batteries inside car jump,starters i didnt use for 7 years new in bos..left uncharged… i tried to,revive them by adding waterbsince they were empty….but now i know agm should be empty… do u know if,a battery is agm or water filled,wet battery?……i have other batteries that just say sealed lead acid non spillable… this an agm battery ?

January 17, 2018at12:12 am, Jon Smythe said:

I filled my agm battery with water…..not good…what do i do now? ….is battery ruined now?….

June 28, 2015at4:19 pm, Barry Barron said:

Years ago, I too succumbed to opening a gel battery.. Fortunately I was just curious and didn’t add water! thank you for the great article Campskunk. Recently I have had my inverter repeatedly cutting out on me. I have checked the settings, reset it following the procedures outlined in the manual, but it continues to trip – with nothing plugged in. I’m wondering if perhaps it is time to replace my batteries. I have a 2010 190, with wet cells. I’m wondering if I can replace them with AGM gel-type batteries. The inverter has a selection dip switch which allows gel type batteries. Have you ever done this?

Another questions. In my 2010 190 I have two batteries, which have been located in front of the right rear tire. The configuration does not look like the picture in the manual and I cannot slide out the battery tray without damaging an aluminum stop that is located near the toggle bolt. How do I open the tray? Here is a picture.

September 09, 2014at11:14 am, Steven | PSE Battery warehouse said:

Good article! Watering you agm would be very stupid indeed. But even agm batteries need to be looked after from time to time. Proper maintenance keeps your batteries in a good condition and can even extend their lifespan.

August 22, 2014at6:02 pm, campskunk said:

yes, I still languish in captivity, as you can tell by my topics. hopefully I’ll be making a break for it soon;-)

August 22, 2014at1:00 pm, Dave Speakman said:

Love the article and the satire. But no matter what you tell them, some idiot, sorry RV’r will still fill the AGM batteries. The more AGM batteries that are sold just lowers the cost for us. Are you still held captive in Ontario?

August 20, 2014at11:08 am, Karsten Askeland said:

So … if I understand you correctly, what you are saying is NOT to add water to AGM batteries. Right!!

You know that some folks will anyway … and then they’ll whine and complain about getting defective batteries from RT.

But … like always, thanks for the good information. I have never ever looked at my batteries in 2 1/2 years. I sure hope Winnebago Industries used the AGM on my coach battery. 🙂

Comments are closed.

Back to top