Please Don’t Water Your AGM Batteries

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.

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.

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.

 

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.

 

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!

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.

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.

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.

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