Is it Safe to Charge a Car Battery Indoors?

I’m assuming you’ve found yourself needing to bring in your car battery indoors for a proper charge and wondered if it was safe to do so. Perhaps you don’t have a garage and are worried your charger will be stolen outside. Perhaps it’s winter and the battery is too cold to take a charge.

Either way, what could possibly go wrong? Right?

Internet forums are rife with “end of the world” scenarios about keeping or charging one of these batteries indoors, and the level of extreme talk is enough to cause anyone to pause and tremble in fear.  

A car battery can be safely charged indoors if a properly-sized 3-stage smart charger is used that will switch to “float” mode when charging is complete. The car battery should also be kept from open flame when charging and not be confined to a sealed-off room or confined space.

That’s the quick and easy answer, but there are still plenty of mistakes that could be very costly if you aren’t paying attention. I’ll cover the following 8 topics that I believe are most critical to take note of to keep things as safe as possible when charging your car battery indoors.

  1. Proper charger
  2. The Order of Terminal Connections
  3. Immediate Surrounding Environment
  4. Ventilation
  5. Insulated Tools
  6. Charge Time of Day
  7. Temperature of the Battery

While there is a very real potential danger in storing and charging a 12-volt battery indoors, I do believe that almost every aspect of the equation can be mitigated and risk can be reduced to a bare minimum with some common sense steps and considerations.

Let’s go over what you’ll need to do to stay as safe as possible and some hazardous mistakes that you will want to avoid or at least be aware of.

***If you are planning on charging your car battery in a garage or shed, I would still take note of the steps in this article, but for all intents and purposes, you shouldn’t have much to worry about if you select a proper charger. This article will deal primarily with charging a car battery in your home.***


Proper Charger

proper and safe battery charger for indoor use
Having the correct charger for the application (charging indoors) is critical to make things as safe as possible.

Over the years, my criteria for a proper charger for indoor use are as follows:

  • The charger does NOT have an automatic desulfation/equalization mode
  • The charger is a 3-stage / smart charger (microprocessor controlled)
  • The charger will enter into “float” or “maintenance” mode when finished charging
  • The charger has an inline fuse
  • The charger has reverse polarity protection
  • The charger’s amps do not exceed 10% of the total amp-hour capacity of your battery

Stay Away from Automatic “desulfation” or “equalization” Modes

The last thing I want when charging indoors is a charger that will automatically enter into “desulfation mode”.

Desulfation is when the charger will enter into a phase (generally upon plugging the charger in and every month or so thereafter) that will conduct a controlled overcharge of your battery in order to break up any sulfate crystals that may have built up on your lead plates, to equalize the charge of each of cells to be more uniform, and to stir the electrolyte around to prevent stratification.

Sulfate crystals build up if your battery is left in a state of partial charge (anything below 100% for any length of time). It’s a naturally occurring chemical process in the battery and can be reversed to a point. Your battery’s electrolyte will stratify, or separate the water and sulfuric acid based on their densities if they sit undisturbed long-term.

Why wouldn’t we want a charger to help us out to correct these problems automatically?

It would be all good and well if these batteries were in the garage or in the shed, but we’re talking about a place where we live and spend 1/3 of the time unconscious when sleeping.

When a battery conducts this “controlled overcharge” that can take up to 8-10 hours to complete, the battery is releasing explosive and poisonous properties into the air and you will be breathing them in and taking all of the risks that come with that.

I won’t get into it all in this article, but if you want to nerd out a bit, I have a full article on what happens when a battery bubbles (overcharges) here that you can check out and why it’s important to pay attention to.

Essentially, the reason the automatic feature bothers me is that it raises the temperature of your batteries and causes a higher rate of outgassing than even a normal charge, which releases hydrogen, oxygen, and hydrogen sulfide into your house.  

This can take up to 10 hours or more to complete, but maybe less as well. Manuals will tell you to closely monitor your battery during this time. How are we supposed to increase ventilation for these sessions and monitor them if they are automatic?  

It’s fine to have a charger that has the “desulfation” mode, just make sure it has to be manually toggled.

How to prevent sulfation and stratification if you’re storing your car battery indoors?

  • To prevent your battery from sulfating, keep it hooked to a float charger at all times
  • To keep your battery’s electrolyte from stratifying, gently shift and move your battery around a bit to splash the contents around inside every 2 weeks or so

If you’re looking for a charger that is gentle on your batteries and will keep your batteries maintained indefinitely, I cannot recommend this 5-amp charger highly enough. It meets all of the requirements that I am covering in this article. It is also perfect for flooded, AGM, and Gel batteries. I’ve used it for 6+ years. It’s simple and user-friendly and is a true “set it and forget it” charger.

It’s the only charger that I trust to be hooked up to my battery bank 24/7, 365 while indoors (the manufacturer doesn’t advocate for the use of their product indoors for obvious liability reasons). The 5-amp model that I linked to above would be perfect for anything 100AH or below, in my opinion — which is nearly every car battery.

3-Stage Charger / Smart Charger

This shouldn’t be much of an issue if you’re purchasing a new charger, as nearly all of them are 3-stage. You’d be hard-pressed to find a charger that isn’t “smart”.

You do not want an “old school” charger that will just keep emitting a constant charger regardless of your battery’s ability to absorb it or not. True “trickle chargers” are NOT smart chargers as they will continue to emit a small current even after the battery is charged (slow overcharging).

“Float charge”

When I say “float charge”, I do not mean “trickle charge”.  A charger that has a float charge feature has microprocessors that give the battery only the exact amount of charge that the battery loses as it naturally self-discharges.  

A trickle charger also supplies a very small current, but it never stops feeding the battery the small current whether the battery needs it or not, so it will reduce the life of your battery in the long run due to overcharging.

To put it plainly, self-discharge is what happens when a battery sits idle but naturally loses minute amounts of power due to the chemical reaction reversing internally (5-20% per week with a flooded lead-acid battery, depending on condition).  

For example, your Duracell AA alkaline batteries in the package will have 90% of their original charge after 10 years of sitting in the package.  That 10% loss is self-discharge.

A normal “float” range will be 13.00-13.3 for a 12-volt battery.

If a 12-volt battery hits 13.8, it is technically “charging” and no longer in float mode.

Inline Fuse

Not much needs to be said here, but I prefer an inline fuse so that if something happens the fuse will blow and not destroy your charger or cause a fire.

Reverse Polarity Protection

This is common on many newer battery chargers. It is a safety feature that ensures that the charger will not emit a charge if you hook up the cables to the wrong battery terminals. We all make mistakes, so having this feature is a must in my opinion if you’re going to be charging inside.

Amp Rating of Your Charger

Typically, you’ll hear the rule of thumb that the amp rating of your charger should be more than 10% of the total AH (Amp hours) of your battery.  Having one that is very high relative to your total amp hours can be catastophic for your battery’s health and yours (for instance, a 50 amp charger on a 45 amp hour car battery).  Plates can warp, acid can leak, and dangerous gasses are released.

Batteries can only accept so much electrical current at a time while efficiently converting it to stored chemical energy.

If you exceed the amount that it can convert, the battery turns the excess electrical amperage into heat and causing electrolysis (breaking down of water molecules into hydrogen and oxygen) in the electrolyte (battery acid).  Simply put, your battery will sound like it’s boiling and will be venting off dangerous gases including hydrogen and hydrogen sulfide.

By charging at a lower rate (4 to 5 amps for a car battery) you’re removing most of the possibility for the charger to overcharge and damage your car battery. 

The Order of Terminal Connections

I’ll assume that you’ll be using a 3-stage smart charger like I mentioned above. If that’s the case, then this point this step can likely be skipped, but I still adhere to it just out of habit.

When hooking up your charger to your battery, I recommend connecting the cables to the corresponding battery terminals and then plugging the charger in. When you’re finished, unplug the charger before you disconnect the terminal connections.

Hook up before you plug in, and unplug before you unhook.

Doing this would make sure that no sparks would be created when using an old-school charger (like a spark when using jumper cables) that didn’t have all of the safety features that the newer models have.

The reason we don’t want to have a spark occur directly over our battery is due to the possibility of igniting the hydrogen or oxygen gasses that can be vented out. In reality, you shouldn’t have that problem at all unless the battery is currently overcharging or just came off of an overcharge.

Regardless, I stick to this habit just to hedge my bets even though my current charger will not create a spark if I plug it in before I connect the cables.

Immediate Surrounding Environment

Before you hook up your charger to your car battery indoors, take a moment to look around the immediate area. A few things can easily happen if you’re not careful, and for all completely different reasons.

Make sure that your battery will be inaccessible from all unauthorized persons and pets. The last thing you want is for your kids to start messing with your battery, or for your dog to knock it off a platform.

Next, look at what is directly above the battery. When charging (and especially short periods of overcharging which will happen briefly at times even with a great charger), the car battery will be releasing lots of different gasses that can be corrosive to metals and damaging to fabrics. Give your battery space to “breath and exhale”, so to speak.

Finally, make sure that you’re not in a room with an open flame, such as a pilot light, or a gas burner. We’ll get to proper ventilation next, and it’s not as scary as it’s made out to be, but we’re indoors here and we want to mitigate all possible hazards.

Ventilation

This is where you will read horror stories online about charging batteries indoors. There is definitely a healthy dose of respect that needs to be given to your battery as it vents the various explosive and poisonous gasses during an overcharge.

Now, ideally, you will have selected a 3-stage smart charger whose charging amperage is not greater than 10% of the total amp-hour (AH) rating of the battery to prevent any significant off-gassing to begin with and keep overcharging at a bare minimum.

Most car batteries will fall between 40-80AH, so a 4-8 amp charger would be about as high as I would recommend going. 10 amps at the most. A 4-8 amp charger is plenty enough to fully charge your car battery overnight.

If you’re curious what the AH equivalency rating of your car battery is, simply take the RC or CCA rating on your battery’s sticker and plug it into the table below. Car batteries rarely show an AH rating since they are a starting battery (not a deep-cycle) and aren’t rated that way by the manufacturer.

Amp Hour Estimates Based On Cold Cranking Amps or Reserve Capacity

If Your Sticker Reads: 200-315 CCA, or 40-60 RCf Your Sticker Reads: 315-550 CCA, or 60-80 RCf Your Sticker Reads: 550-1,000 CCA, or 80-190 RC
36 - 46.2 AH46.2 - 58.8 AH58.8 - 111
Check the sticker on the top of your car battery and find where it matches up on the table. The approximate amp hour range times are estimated below.

If you are planning on using a charger that is a bit over-sized for the job or one that isn’t a smart charger, then by all means open some windows for any anticipated out-gassing.

I trust my charger and haven’t found it necessary to ventilate the house more than it already is since the charging voltage is gentle enough to not cause excessive out-gassing.

With that being said, let’s talk about some of the gasses to be aware of when it comes to ventilation.

Hydrogen

According to batteryuniversity.com the risk of hydrogen buildup in your house due to a single battery (or even a small battery bank) is very minimal.  

While a 4% concentration in the air can be explosive, a small setup of a few batteries (let alone a single one) just doesn’t produce enough hydrogen (even when outgassing) to do anything unless you have them perfectly sealed and not vented in any way within a room or box.  You can check out the batteryuniversity.com article here.  

Hydrogen has no smell but is 16 times lighter than air, and seeks to rise up and escape the confines of your house at an extremely rapid speed — faster than you can run.  Even faster than Usain Bolt can run.  In fact, hydrogen molecules move about 45 mph through the air to escape  (https://www.arhab.org/static/h2_safety_fsheet.pdf).  It does not linger unless perfectly sealed.  

So, I am not worried about hydrogen building up to levels that will explode in my house. That being said, I still make sure to follow to the step in the section above about keeping my charging battery away from any open flame or pilot light. There’s no sense in testing Murphy’s Law.

Hydrogen Sulfide

Hydrogen sulfide is what you would smell during overcharging — and that includes controlled overcharging like in the “desulfation” and “equalization” modes (usually around 15 volts and above for a flooded lead-acid battery).  

It is denser than air, so it sinks to the lowest point and smells like rotten eggs.  This gas is harmful to people and lethal if in high enough concentrations and the exposure is long enough.  After prolonged exposure and when the ppm (parts per million) is high enough, you will lose your ability to continue smelling this gas and it can be fatal in heavy enough concentrations.

Best case scenario when encountering hydrogen sulfide would simply be to smell it and address the problem immediately by stopping the charge if it’s out of control. You might also experience a sore throat or sinuses.

I had a charger that performed an automatic desulfation one time without my knowledge and the hydrogen sulfide produced caught my attention but it still caused my throat to feel sore for about 2-3 days. It’s not something to mess around with is all the more reason to make sure you have a solid charger to work with.

Using a proper smart charger that has a float mode feature when charging is complete, that does NOT have an automatic desulfation feature, and that is sized correctly is critical to minimize any issues with ventilation.

Anytime the voltmeter reads around 13.8 or higher, you know that your battery is charging. As you approach 14.75+ volts (flooded battery), you start to get into territory where off-gassing can occur. The higher the number on the voltmeter goes, the more it’ll vent out its gasses. Sealed batteries (AGM, GEL) are more susceptible to off-gassing at lower voltages.

A battery off-gasses because it can no longer convert electrical energy to stored chemical energy fast enough from what the charger is supplying (simply put, it’s overcharging). This causes the battery to overheat and the excess electricity causes electrolysis in the electrolyte. Electrolysis is electricity passing through a liquid that destroys the molecules within the liquid and releases them as gasses.

digital voltmeter when charging
Voltmeter plugged into a DC socket adapter straight to the battery (cigarette lighter plug). This is the charge when the battery is 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+.

The following is a table from another article of mine that will show you the safe ranges for charging a Flooded, Gel, or AGM battery and what will happen if you exceed those ranges. This is why I always use a digital voltmeter that is hooked up with a DC adapter (link to my recommended choices in the following sections).

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.

Insulated Tools

This really shouldn’t be an issue with a single car battery, since we’re not dealing with a bank of batteries where hooking up cables between them is necessary. Still, if for whatever reason you need to use a tool around your battery, try to use one that is insulated (has a rubber handle covering the metal) so that you do not accidentally bridge the gap between the positive and negative terminal of your car battery.

use insulated tools when working on battery
Along with creating unnecessary sparks on the terminals, this is your other nemesis when it comes to indoor safety (or outdoor, for that matter)! If you use any tool — insulated or not — never set it on your battery in between uses. Use it with care, and move it away from the battery immediately when not in use.

Shorting out your car battery with a metal tool (or anything metal that might fall on the battery) will cause the tool to become red hot in an instant. Sparks will fly, and the tool will basically weld itself to the terminals and melt into the battery. Shortly thereafter, it will ignite the gasses inside the cells as it melts low enough and you will have a dangerous explosion.

Exercise great caution when using anything metal above or around your car battery and do not place a tool on top of the battery when not using it.

Charge Time of Day

This is especially important if you are not using a charger that meets the criteria in the first section of this article. I’m not saying that the charging process if different at 3am vs. 3pm, but you are likely awake during one and asleep during the other.

Let’s say you have a 25 amp charger and your car battery is roughly 50AH. This is way above 10% of the total capacity of the battery!

You hook it up in the other room and go right to bed. You are now unconscious and your battery is going to be neglected as is inevitably overcharges your battery and releases explosive gasses and poisonous hydrogen sulfide into your house.

Best case scenario would be that you wake up with a sore throat from something like that.

It is best to use a charger that is properly sized for your battery, and again, I wouldn’t go any higher than 10amps if you are charging inside the house. I recommend 4-5 to be as gentle as possible on the battery.

If you’re using a battery charger that you’re not sure of when it comes to performance and reliability, it is best to use it when you’re awake and to keep a digital voltmeter connected to the battery at all times when charging. I prefer this voltmeter (and the direct-current adapter) seen on Amazon.

Temperature of the Battery

This is worth mentioning if you’re bringing your car battery inside from freezing weather because it’s dead and won’t take a charge outside.

The worse thing you can do is begin charging your battery if it is dead and was just in freezing weather, as the electrolyte is almost guaranteed to be frozen solid. Connecting it to a charger can cause a fire or explosion and is very unsafe to do indoors (or outdoors, for that matter).

Bring your car battery inside and let it completely thaw before doing anything with it.

Keep in mind that when the water inside your electrolyte froze, it also expanded. It’s very likely that your car battery’s cells ruptured internally and your battery is no longer safe to use. If you see the side-walls bulging, it’s definitely time to recycle it.

For more on freezing car batteries and how easily they can freeze, I wrote an article here that you can check out.

Recap

From my car battery to my battery bank in my basement, I treat all of them with the respect that I would give to any dangerous weapon.  I don’t let the kids have access to it, I don’t play around with it, and I don’t try to force it to do things it doesn’t want to do.  

I keep it in a safe place, and I have a digital voltmeter on it at all times for verification of the state of charge and to alert me if the charger is malfunctioning.

Make sure your charger is a smart charger that will enter into “float mode” when the charging is complete and make sure that is appropriately sized for the battery at hand. I recommend 4-5amps when charging a car battery. I’ve been charging batteries indoors for over 5 years with a charger this size without issue.

Good luck and stay safe!

Robert Van Nuck

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

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