Why is My Battery Bubbling when Charging?

If you’re new to owning a 12-volt flooded lead-acid battery,  it’s likely that you’ll get nervous the first time you hear a bubbling noise after you hook up a charger to it.

At least I was.

At times it sounded like the faint carbonation from a drink, and at other times it sounded like water at a low boil in a pot.

Is this normal?  Is my battery actually boiling?  Is it going to explode?  How long will it do this?

My mind was filled with too many destructive thoughts!  Since my battery bank is located inside my home for emergency power, I wasn’t going to risk anything without further research.

When a battery bubbles it is overcharging due to excessive amperage, voltage, or both. The excess electricity is breaking down the electrolyte in each of the cells and causing the battery to off-gas and bubble. If left unchecked, this can damage or destroy your battery.

Safe Charging Ranges for All Battery Types and Consequences of Overcharging

Battery TypeMax Range for Safe Charging (Room Temp)Tolerant of Bubbling? (Electrolysis)Consequences of Bubbling Above Safe Charging Range
Flooded Battery (electrolyte accessible)13.8 - 14.7 volts Yes, as long as plates are covered by the electrolyteEvaporated water is vented out and battery offgasses hydrogen, and hydrogen sulfide
Gel (Sealed)13.8 - 14.1 voltsLeast tolerant, but if it stays at 14.1 or below the bubbling should be fine as evaporated water is recombined into the electrolyteHisses, whistles, steams out the emergency vent if pressures exceed the batteries ability to recombine evaporated water; permanent damage occurs when this happens
AGM (Sealed)14.4 - 14.6 voltsLess tolerant, try not to exceed 14.6 volts to keep bubbling in the safe range so that the evaporated water recombines with the electrolyteHisses, whistles, steams out the emergency vent if pressures exceed the batteries ability to recombine evaporated water; permanent damage occurs when this happens
The charging ranges are temperature dependent and are rated for room temperature. Higher temps will reduce the safe charging range voltage, while colder temperatures will increase it.

To keep it simple, if the battery sounds like a carbonated beverage when charging, then I’m not at all worried (assuming it’s hooked to a quality smart charger with a float mode).

If it sounds like you have a pot of water on the stove with medium to high heat and a rolling boil, then it definitely has my attention and needs to be addressed or monitored (regardless of the charger).

With a flooded lead-acid battery the sound will usually become barely audible as battery reads 13.8 on the voltmeter (minimum voltage for charging).  As the volts on the voltmeter increase, the bubbling sound will increase in intensity. Normal charging ranges can go up to 14.8 with a flooded battery.

In the normal charging range, this bubbling is caused when an electric current from your charger is passing between the positive and negative plates in the battery’s cells and through the electrolyte solution.  This results in electrolysis which excites the electrolyte solution and releases hydrogen and oxygen gas from the “water” (evaporation).

electrolyte levels in battery to stay safe
The red line is the edge of the slotted fill tube for a Flooded Lead Acid Battery. The blue is the water curve beneath it. The green is the 1/8″ space between. You can see the lead plates at the bottom of the hole, and the slot for the fill tube at the top of the hole.

Now, sealed batteries, such as gel or AGM, certainly have the ability to make noise when charging.  However, a hissing sound (or anything indicating that pressure is squeezing out – like steam) is an indication that too much charge is being applied and irreversible damage is occurring.  The charging process must be stopped and corrective action needs to be taken to resume charging.

Never allow a sealed lead-acid battery to hiss or vent steam when charging! 

From my repeated research, a small amount of bubbling is acceptable with a sealed battery. They can recombine the evaporated water back into the electrolyte. However, if the charge amperage exceeds 33% of the total amp hours of the battery, then excess pressure will cause the battery to hiss, vent out steam, and undergo outgassing.

The water lost during such an event cannot be replaced on a sealed battery and your battery is being irreversibly damaged.

Personally, if you’re not in a rush to charge up, I would stick with a charger with an amp rating at 10% or less than the total AH rating of your battery and stick with a smart charger that has a particular charging mode based on the type of sealed battery that you have (Gel, AGM, etc.).

A TPPL AGM (common on boats) is an exception, as that type of battery actually does best if it receives an amperage that is 40% of its total capacity. For example, a 100AH TPPL AGM wants at least a 40-amp charger.

If you’re worried about your battery bubbling and don’t want to risk it, this charger on Amazon is perfectly suited for applying a gentle charge to AGM and other sealed (and flooded) lead-acid batteries and would pair well with 12-volt batteries including car, marine, golf cart, RV, and other power sports. It has a float mode and can be left hooked to your batteries indefinitely.

I’ve been using it for over 4 years and it’s the only one that I’m comfortable using on my indoor battery bank (regardless, for liability reasons I’m sure, the manufacturer doesn’t recommend charging indoors). You might hear a carbonated liquid sound every now and then as it tests the state of charge of the battery during the charging process, but I have yet to experience actual “rolling-boil, bubbling” sounds when using it.

Though it is normal for flooded lead-acid batteries to bubble to an extent, it is not always benign.  There can be dangers associated with this bubbly situation and steps you can do to keep things as safe as possible.  

Let me share with you what I’ve learned to hopefully save you some time!


Are the Gases Released when the Battery is Bubbling Dangerous?

The gasses released when a battery is bubbling due to overcharging (electrolysis) can be dangerous in a confined area without proper ventilation. Hydrogen is released which can explode with flame or spark, and hydrogen sulfide is noxious when inhaled.

Like I mentioned above, hearing a bubbling sound when your battery is charging is normal to a degree.  It might be tempting to say that your battery is “boiling” but odds are that this is not the case.  This is just a chemical reaction due to an electrical current passing through a liquid and is to be expected within a certain range (see table at the beginning of the article). 

Also keep in mind that we are speaking about flooded lead-acid batteries — not Gel, not AGM.

Assuming your battery is healthy and structurally sound, it should suffer no structural damage due to the bubbling because your battery will be able to vent out the pressure build-up from the gases.

During the normal range of charging (13.8-14.7 volts on a voltmeter), these gases, once again, are hydrogen and oxygen.  Hydrogen is what you have to worry the most about since it is explosive in concentrations as low as 4% but keep in mind that hydrogen is 16 times lighter than air and is moving at about 45 miles an hour to get out of the place that it’s in. As long as you don’t have it in a small, tightly sealed room where there is no ventilation you should be fine as long as your battery and charger are healthy and structurally sound.

Hydrogen sulfide, which smells like sulfur or rotten eggs, is released as well when the battery is at a high voltage (14.7+) — typically during overcharging, equalization,  and desulfation.

Often times during the charging process for a flooded lead-acid battery, a three-stage smart charger will creep into the 15-volt range for a while during the first 80% charge — the Bulk Phase.  This is normal as the battery can accept the charge pretty easily at this point, and the bubbling will get a bit more audible.  Just make sure to keep an eye on a charging battery as good practice.

When the charge gets into the 16+ volt range I really stop what I’m doing and pay attention.  Something is either not right or the charger is performing an equalization or desulfation process.

You can monitor your charging voltage in real-time with these two items. They are a must-have in my opinion when charging any 12-volt battery, simply for the sake of safety. 12-volt adapter with plug, and a plug-in voltmeter as seen on Amazon. I never charge a battery without these hooked up.

monitoring voltage when charging bubbling battery
Voltmeter plugged into a DC socket adapter straight to the battery (cigarette lighter plug). This is the charge reading when the battery is hooked up to a charger and charging after use. It will read around 13.15 volts when in “float mode” and if unplugged for a few hours it will work its way down to its actual charge of 12.7+.

Equalization & Desulfation on a Charger Will Cause Bubbling

To keep it basic, equalization or desulfation means is that the charger will perform a controlled overcharge on your batteries for roughly 8 to 10 hours in order to agitate the electrolyte solution in your battery cells so that the sulfites are released from the plates and also to equalize the charges within the individual cells to bring them to an even level for optimum performance.

This controlled overcharge may result in your voltmeter reading north of 16-volts and the sound from your batteries will remind you of a pot on the stove that has a medium simmer.  Because it is an overcharge it will be gassing excessively and you’ll be getting a healthy dose of hydrogen, oxygen, water, and hydrogen sulfide being released.

Ventilation is key during any charge but especially critical during the equalization or desulfation modes which are essentially pushing the battery at its upper limits and causing excessive gas build-up and release. I would never perform this inside the house!

There’s a lot that can be easily overlooked that can end in disaster when it comes to charging your batteries indoors.  I’ve got a rundown of some things you’ll definitely want to take a glance at to help keep you safe at this article that I wrote on charging batteries safely indoors.

So why don’t I want a charger with an automatic desulfation mode if this process is supposed to increase performance?

I’m not saying to have a charger without a sulfation or equalization mode, it’s just that the automatic part scares me.  When your charger does this automatically, without warning or consent, and you don’t have your battery in an environment that has the maximum chance to ventilate during an overcharge, your risk of something going wrong increases.

Not only is ventilation an issue, but any manual you look at will tell you to monitor your battery closely during desulfation or equalization modes.  If it does this automatically and without notice while plugged-in, how are you supposed to know when to monitor this?

Too many safety concerns for me when it comes to automatic desulfation.  If you charge indoors, it is critical that your charger will not conduct an overcharge on your battery and will carefully and precisely maintain it after it is brought up to 100%. 

The best smart chargers are those where you can set it and forget it.  That’s why I stick with this charger and maintainer seen here on Amazon and have trusted it for 6+ years without fail.  It’s safe, user friendly, and extremely gently on batteries (due to the way it was programmed to charge them). I do not experience bubbling sounds since using this charger, and I won’t ever go back.

Of course, it takes a longer to charge due to the lower amps (for my 215 amp hour system) but with the lower amps and gentler charge, I feel much safer using this charger indoors and know that my chances for a loud bubbling noise (due to overcharging) are slim to none with this.  

For the typical 45 amp hour car battery, this charger would bring your battery back to life from a full discharge overnight (8-9 hours) as well.

How to Select a Proper Charger to Reduce Battery Bubbling

Since we’ve established that the bubbling is normal for a flooded lead-acid battery, any 3-stage, smart charger you buy is likely going to cause the sound to happen.  If you’re looking to use your battery backup bank indoors, there are few things to look out for to help with not pushing the battery past its limits and keep things as safe as possible.

The general consensus is out there is to select a battery charger with an amp rating that is 10% of your total amp-hours.  That’s pretty solid advice for most applications but since I’m using mine indoors I prefer to have a charger that’s less than 10%.  The higher the amp rating of the charger, the higher the current and the greater the chance of overcharging (bubbling).

I want to minimize the stress on my battery as much as possible for safety reasons, so having a charger like mine which is at 4 amps is fine for me when the ideal charger size would be 20 amps.  I just understand ahead of time that my setup is going to take much longer to recharge after use, but with the frequency that that occurs, I am content with this tradeoff.

Having a good quality float charger is a must.  Float Chargers will supply your battery with just the amount of current it needs as it self-discharges.  The voltmeter should read around 13.1-13.3 volts.  The battery will start charging at anything 13.8+ volts. 

A true trickle charger, on the other hand, will constantly apply the faintest of charges to your battery whether the battery wants to accept the charge or not.  Even though you may not be able to hear it, your battery is experiencing electrolysis during a Trickle charge.  A sustained trickle charge over a long period of time is damaging to your battery and will reduce its longevity and performance.

Finally, like we previously mentioned, if you plan on using your set up indoors I highly recommend never purchasing a charger that has an automatic desulfation or equalization mode.

When Is a Bubbling Battery Dangerous

There are some key things to look out for as you’re getting to know how your battery reacts with your particular charger.

Is the battery hot or warm?  Even though the bubbling makes you think that the electrolyte solution is boiling, it’s actually only releasing hydrogen and oxygen in the excited electrolyte solution due to an electrical current.  The internal temperature is not actually high enough to be boiling in most cases.

battery bank

While your battery is charging or experiencing a controlled overcharge you want to make sure that you are still in the “most cases” category.  Touch your battery and make sure that it is not getting hot. 

If you perform the desulfation or equalization charge it may get warm to the touch especially if you let it run its course of 8 to 10 hours.  To prevent any issues with noxious and explosive gases, only perform this controlled overcharge outside of the house and in a well-ventilated garage or even outdoors!  

Are the sidewalls of the battery bulging?  You definitely want to keep an eye on the structural integrity of your battery as well.  If you see that the sidewalls are starting to bulge out from where they normally were you’ve got a serious problem.

Is the bubbling occurring in a rhythmic manner? Oftentimes you can faintly hear your charger pulsating.  Is the bubbling that you here in line with that?

If you experience any of these issues it is important to stop charging immediately.  First, make sure you always have eye protection on.  Next, unplug your charger from the wall.  Finally, after the charger has been unplugged from the wall and 15 seconds have passed, you can remove the clamps from the battery.  

Doing it in this order will keep charging cables from sparking above the battery when removing them (though most smart chargers already remedy this possibility).  With a compromised battery, all it takes is a single spark to ignite hydrogen gases that are venting directly above your battery and then you’ve got bigger problems than just a faulty battery.

Is it Okay for Gel & AGM Batteries To Bubble?

Gel and AGM batteries are definitely more finicky when it comes to charging.  They need a lower voltage supplied to safely charge them.

If you hear gurgling, or hissing from your gel or AGM battery — that is a sign that you are supplying too much current your battery and damage is being done.

Sealed Gel batteries should be charged with a voltage between 13.8 and 14.1 and sealed AGM batteries should be 14.6 to 14.6. Exceeding these ranges will surpass a Gel or AGM battery’s ability to recombine the gasses back into electrolyte and you’ll lose irreplaceable electrolyte through the pressure release vent and shorten your battery’s lifespan.

If your AGM or gel battery is making a low bubbling sound, they are capable of recombining the evaporation back into the electrolyte — so long as the pressure doesn’t get too high. Like I mentioned above, I would stick with a charger with an amp rating that is 10% or below of the total amp hours of the battery you intend to charge. My bubbling issues stopped when I started using this particular charger on Amazon.

If you have a smart charger make sure the settings indicate that they are in line with the type of battery that you’re using. The one I recommended above is automatic.  Many will have a light indicator letting the user know if it is supplying current to charge the different types of batteries.

If the settings are correct on the charger and you are still experiencing noises during charging, I recommend finding a local expert to help you troubleshoot the issue.


Final Thoughts on a Bubbling Battery

When it comes to the bubbling noises in a flooded lead-acid battery, I wouldn’t worry too much as long as it is in the normal acceptable charging range, that it doesn’t last for over 8 hours, and that the battery is not becoming hot or deformed.

Still though, make sure to always have proper ventilation no matter where you choose to charge your batteries since the charging process causes potentially harmful gases to escape.  Make sure to check the batteries every month or two to replace water loss from the electrolyte solution with distilled water.  If you’re new to battery maintenance, you can check out my article here on proper battery maintenance to avoid ruining your investment.

When it comes to the hissing noises in a sealed lead-acid battery, such as a gel or AGM, something is wrong (likely more amps than the battery can chemically accept) and you must take corrective action immediately to stop the damage that is being done to your batteries.

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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