Why is my Battery Hissing and What Should I do?

One thing that is sure to get your attention is a hissing or whistling car battery when charging. Maybe you needed to hook up a battery charger after getting a jumpstart, or maybe you’re just hearing the hissing from under your hood as the car is running.

Either way a hissing battery is definitely a problem and one that needs to be corrected immediately.

A car battery will hiss when it has built up too much internal pressure due to overcharging. This can be caused by an oversized battery charger or a malfunctioning alternator. If the hissing is left unchecked, a car battery will be completely destroyed.

Let’s get into exactly what is happening when your car battery is hissing, what causes it, and what you should do about it.

Why Your Battery is Hissing

When you hear the audible sound of hissing coming from your car battery, that is the result of pressurized gases escaping at a rapid rate through a small vent. Kind of like steam escaping from a tea kettle. The water is as hot as it can get without evaporating and creating pressure with gasses. They are venting out in order to preserve the structural integrity of your battery and keep it from bulging or exploding.

What is the cause of this increased internal pressure in your battery? Overcharging.

The main culprits of overcharging are the following:

  • Using an over-sized charger
  • Leaving a charger on your battery for too long (not using a smart charger)
  • A defective cell(s) within the battery
  • A malfunctioning car alternator (or any other part of the car’s charging system)

When a battery is charging, it is converting electricity into stored chemical energy. A lead-acid battery, like those found in vehicles, can only accept so many amps at a certain voltage efficiently, and the amount changes throughout the charging process. This applies to both flooded and sealed batteries.

If the battery is really depleted, it can accept a high amount of amps and a moderate amount of volts because it has minimal internal resistance and readily converts electricity into stored chemical energy. As it approaches 85% or higher, more volts are required to overcome the internal resistance from the battery, but fewer amps are needed to complete the charge.

A good rule of thumb is to use a charger who’s amps are 10% or less than the total amp hour rating of your battery. When you exceed that amperage, you increasingly run the risk of overcharging your battery.

I personally like any of the three following smart chargers from Amazon to deliver a gentle but efficient charge that will have you back in business within a few hours (or overnight if your battery is really dead). I’m partial to the Deltran brand chargers and here’s a 4-amp and a 5-amp model. I have used the 4 amp model for over 6 years now without an issue and I use it everyday on my battery bank for emergency power and I also top off my car batteries at least once a month with this as well.

The 5-amp model is marketed towards boaters simply because it can be mounted but it is still perfectly safe for your car battery and the charge will be a little bit faster.

This other 5-amp model is an excellent choice if your battery is severely depleted. Most smart chargers require a minimum voltage of around 9 volts in order to bypass the built-in safety and start the charging process. This charger only requires 1-volt before it recognizes that it is indeed hooked up to a battery and starts pumping in current.

You can leave any of the chargers above hooked up indefinitely without fear of overcharging.

So, let’s say your battery is 100 amp hours and you are using a 10 amp smart charger and let’s assume the maximum amps that this battery can efficiently accept is 10 amps. For the most part, the battery will be able to handle what the charger is giving it and you won’t hear any hissing besides maybe a small amount of internal bubbling from time to time as the battery charger hugs the line between “maximum acceptable charge rate” and “overcharging”.

Now, let’s say you use a 50 amp charger on the 100 amp hour battery. Since the battery can only handle 10 efficiently and cannot convert the excess 40 amps into stored chemical energy, it has to do something with that electricity.

The excess electricity passes through the electrolyte, which is the acid and water mix within your battery, and essentially begins to boil it because it has nothing else to do but that. This is called electrolysis. The battery has saturated itself with the amount of charge it can accept and the excess must be released in the form of heat.

As the electrolyte boils (or undergoes electrolysis) it begins to off-gas. The molecules of the electrolyte are destroyed and evaporated out into a gas form. In the mix, you have hydrogen, water vapor, hydrogen sulfide, etc. Some of these gases are flammable, some are noxious and poisonous.

The gas will build up in pressure between the top of the battery and the level of the electrolyte and when it hits a certain threshold it will be vented out through a one-way relief check valve in order to neutralize the internal pressure.

When this highly pressurized gas vents out of a very small valve, you hear a hissing or whistling sound. The result is the same as a tea kettle coming to a boil. The water is boiling beneath in the pressurized water vapor that has evaporated out (steam), eventually reaches a certain pressure, and makes a high-pitch sound is it vents out of the relief hole of the tea kettle.

tea kettle venting gasses like a battery when hissing
Think of trying to get the water in a tea kettle to 205 degrees Fahrenheit and no higher for a “full charge” and maintaining the water level without causing it to boil and steam to come hissing out. It’s a delicate balance, just like charging and maintaining a battery. Think of the flame size like the amps, and the distance from the flame like the volts. If you change either one, you change the rate at which the water heats.

If your tea kettle was perfectly sealed with water boiling inside of it it would eventually crack or explode. The same is the case with your battery.

In fact, if you are overcharging at too high of a rate, the battery may not be able to vent out the gas is fast enough and you may see the sidewalls of your battery bulging out.

What Causes a Battery to Hiss

Now that we know why a battery will hiss when overcharging, let’s touch on the reasons that cause it.

Over-sized Chargers

As I mentioned above, using an over-sized charger for your particular battery will result and overcharging. Keep your battery charger amps at 10% or less of the total capacity of your particular car battery.

For example, If you have a 70 amp hour battery, don’t exceed anything above 6 to 8 amps to be safe. I know a lot of people have 10 amp Chargers since they are very common at big box stores. If you have one of those just be extra cautious when using it on a battery that doesn’t quite reach 100 amp-hours. If you notice any hissing, excessive bubbling, or the smell of rotten eggs, then unplug the charger and go with something smaller.

I would be remiss if I only mentioned amps in the equation when it comes to charging. Amps are the current and volts are the pressure behind the current. If your charging voltage is too high, you can also run the risk of the battery whistling or hissing. Most smart chargers take care of this problem, but if your smart charger enters into “desulfation” or “equalization” mode, then it will get up to 15 or 16 volts which can quickly lead to a hissing sound.

“Desulfation” or “equalization” mode is when a charger purposefully overcharges a battery to agitate and dislodge any sulfation that is building up on the plates inside the cells of the battery. I had a cheap charger from a big box store that automatically entered into this mode upon hooking it up to a battery and the manual said it could take 8-10 hours to complete. I unhooked it after 45 minutes when all I could smell was rotten eggs (which lead to a sore throat and sinuses for days), hear a loud rolling boil from 30 feet away, and light hissing from gasses escaping.

The Deltran chargers I linked to above do not enter into this mode automatically and that’s why I use them.

cold cranking amps on car battery sticker
As you can see, my Ford Escape battery has 590 Cold Cranking Amps, but what are the amp hours?

How Do You Find Your Car Battery’s Amp-Hours (AH) to Size Your Charger Properly?

Car batteries are commonly rated in Cold-Cranking Amps (CCA) or Reserve capacity (RC), and not an amp hours. If that’s the case with your battery, I have some easy to use calculators directly below this where you can type in the values that you see on your battery sticker to find out what your amp hour equivalency will be.

If you have cold cranking amps just type them in to the first calculator and you’ll immediately see your result. If you have Reserve capacity that is rated at 25 amps or the amps are not mentioned after the reserve capacity time then go ahead and use the second calculator. If your battery States a reserve capacity time, followed by an amperage other than 25, then use the third calculator.

These are all ballpark estimates Based on extrapolated charging data from the Schumacher battery charging company. If you want to see how I arrived at these numbers you can check out my article here. Also if you want to see how long it will take to charge your car battery with any charger for any given state of charge then you can also check out my article here with additional easy to use calculators.

Leaving a Charger Hooked up for Too Long

The next reason you might hear hissing coming from your battery is that you left a charger hooked-up for too long. If you’re using a modern, microprocessor-controlled smart charger that enters into “float” or “maintenance” mode, then this will not be an issue for you. But if you’re using an old-school charger from 30 years ago that explicitly says to remove the charger after the battery is fully charged, and you are definitely going to have overcharging occurring the longer you let the charger remain hooked up.

If your old school charger is 10 amps and the battery is at 100%, and those 10 amps that are still being pumped into the battery are going to go through the same electrolysis process that we talked about earlier and you will have increased pressure build-up and your battery will start to hiss.

If you have an old-school trickle charger that only puts out 1 or 2 amps that does not turn off when the battery reaches full charge, then you likely will not hear a hiss but your battery will certainly be overcharging and slowly venting out the electrolyte as it evaporates. If you leave your battery hooked up like this for a couple of days, there’s a good chance you will have boiled out all of the electrolyte and you’ll have a completely dead battery.

Car’s Alternator (or Charging System) is Malfunctioning

Finally, if you’re hearing the hissing occurring while your battery is hooked up under the hood, then you are going to want to have your alternator and the rest of the car’s charging system inspected by a certified mechanic. Generally your alternator we’ll have a charging voltage of about 14.1 or 14.2, but it can go up to about 14.8 or even a little higher in colder weather. Once you start getting into the 15 volt range or even higher, there’s a good chance you will be hearing a rapid boil inside your battery and hissing will soon follow.

What You Should do if your Battery is Hissing

If you notice that your battery is hissing while charging, the first thing you should do put on some safety glasses before you approach the charging source and turn it off or unplug it. It’s your car just turn off the key. If it’s a charger unplug it from the wall.

Do not remove the cables of the Charger from the battery while the charger is plugged in! Remember that when the battery is venting out these gases and hissing, many of these gases are flammable and the last thing we need is a spark to occur as you’re disconnecting the charging cables.

Most smart chargers will advertise that they will not spark on a battery terminal, but why take the chance with burning your skin, blinding yourself, or worse?

Once the charging has stopped, let the battery settle down for 30 to 60 minutes.

If you have a flooded battery, which is the type where you can pop the tops off to check the fluid inside, go ahead and carefully do that while wearing eye protection. You should see the electrolyte level about a quarter inch below the bottom of the fill tubes and the lead plates should all be under the surface of electrolyte.

The red line is the edge of the slotted fill tube. The blue is the water curve beneath it. The green is the 1/8-1/4″ 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.

If the lead plates are all covered, then go ahead and wait a few more hours if you have the time and check the specific gravity of each of the cells. You can use a hydrometer like this one seen on Amazon and you’ll be able to tell if you have any cells that are bad within your battery which could also lead to overcharging since the other cells will be compensating for the weak one.

If you have a bad cell, it’s time for a new battery.

If all of the cells check out all right and you have any electrolyte that is low in any of the cells, go ahead and carefully add distilled water to them. I use a small plastic syringe to fill them instead of pouring water directly from a gallon jug. You can be more precise this way and not accidentally add too much. Remember that if you get any electrolyte on your clothes it will eat holes in them so be careful or wear clothes that you’re not worried about.

Once the flooded battery has electrolyte levels where they should be, go ahead and hook it up to a smart charger with fewer amps and see if the result is the same. If you were using a 15 amp charger before you’ll see a world of difference when you use a 4 amp charger. The battery will charge gentler, more safely, and the charge will be much more efficient inside your battery.

Now, if you have a sealed battery and you experienced hissing, you have definitely caused irreparable damage to your battery since electrolyte cannot be replenished within the battery. Sealed batteries are designed to recombine evaporated electrolyte back into the solution, however, if you surpass that threshold of the battery’s ability to contain that pressure it will vent off the evaporated gases and they cannot be replaced.

Sealed batteries can handle light overcharging but will be damaged if you are too rough with them.

Here’s a table with the proper charging voltages for the different types of car batteries:

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.

Since you cannot check the electrolyte in a sealed battery it’s impossible to really know how badly you’ve damaged it unless you conduct a load test on it. You can use a device like this one seen on Amazon to conduct the tests right at home and here’s a video on how to do that (not mine). You can also swing by any automotive shop or parts supply store and they should be able to conduct a load test on your battery as well.

Simply reading the voltage of your battery with a multimeter is not going to give you the information you need to determine the state of health of your battery. You could have one bad cell and five good cells that are actually overcharged and are compensating for the weak one and have the overall voltage read as fully charged (12.65+ volts). Externally, the numbers look great, but internally your battery is defective.

Robert Van Nuck

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

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