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I sleep in the same room with 2 car batteries. I'm not a battery person so I don't know what type they are. I tried looking for voltage but didn't find it printed on the battery. They're standard acid batteries, I use them to power LED lights and charge a phone/laptop.

I've been told they emit dangerous chemicals and it's not safe to sleep in the same room, is this true?

I only found one other thread similar to this, but it was about deep cycle batteries, I don't think those are the same as car batteries.

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    \$\begingroup\$ Do you charge them? \$\endgroup\$ – John Birckhead Jul 6 '17 at 21:13
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    \$\begingroup\$ Doesn’t it make a huge difference if it’s a sealed lead battery or an unsealed one? Sealed should be safe, as long as you don’t over/undercharge. \$\endgroup\$ – Michael Jul 7 '17 at 6:22
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    \$\begingroup\$ The usual recommendation is to have car batteries in a well-ventilated room because (as discussed in the answers) they may produce H2 which will rise and then collect below enclosures, forming explosive mixtures with air. Since H2 does not smell this will only be discovered by the little spark your light switch produces when you flick it in the morning. The Fukushima explosions give you an idea of the energy produced by H2 combustion. This is true for all lead acid types, although sealed and deep cycle/marine types are more robust to this type of failure. \$\endgroup\$ – Peter A. Schneider Jul 7 '17 at 8:18
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    \$\begingroup\$ @pipe I live in Syria, the power goes out daily, so everyone has car batteries in their houses for lights or other devices. \$\endgroup\$ – MichaelX Jul 7 '17 at 8:20
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    \$\begingroup\$ @MichaelX: I'm going to assume you-re not a native speaker - "pipe" was making a lewd joke. \$\endgroup\$ – MSalters Jul 7 '17 at 9:59

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With any larger battery with significant potential energy, exceeding the discharge rate, over charging, charging when at end of life, or physical abuse can lead to explosions, unexpected venting of acid, or overheating that can cause a fire. The battery should be in a properly vented, protective case that safeguards the terminals and there should be an appropriately sized fuse on one or both terminals of the battery where the load and charger connects.

If the battery is simply being stored (not charged or discharged) in the room and it is in a proper protective case, then it is perfectly safe.

If the battery is the type with removable caps, then care must be taken during charging due to the release of hydrogen gas during the charging process. Of course the acid in the battery is quite corrosive should the battery be tipped or if the case becomes damaged.

If the battery is an AGM (absorbtive glass mat) or gel cell type then there is no danger of off-gassing during normal operation. These styles of batteries will not leak if tipped - many are rated to work even when placed on their side or inverted.

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  • \$\begingroup\$ Sealed batteries are safer during normal operation, true. But they run on the same chemical process as unsealed batteries and produce hydrogen under the same conditions, especially during (over-) charging. It is released through valves (thank goodness, lest they explode). So while they should not leak acid when tilted, they can still produce explosive H2 while being charged, which the OP does. \$\endgroup\$ – Peter A. Schneider Jul 7 '17 at 8:15
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Batteries also produce hydrogen and oxygen if overcharged, in just the right ratio for an explosion, especially with sparks that can also be present when connecting and disconnecting. These can build up inside the battery and rupture the case when they blow (see @Eugene Sh. comments about H2SO4).

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    \$\begingroup\$ sleeping + carpets = static discharge... I don't know anything about the particular topic but sparks seem pretty much guaranteed indoors. \$\endgroup\$ – Mehrdad Jul 7 '17 at 23:23
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Gassing:

With a sealed battery and a modern charger it may often not be a problem, but the possibility of danger exists.

Modern sealed batteries are generally not rated to withstand substantial pressure. If "gassing" occurs due to overcharging they will almost certainly "vent".
This raises both the Hydrogen gas and acid hazards mentioned by others.

It happens! - ask me how I know:

I can confirm from painful & health affecting (long long ago) personal experience that sleeping in a room where a lead acid battery was charging caused severe inflammation of my mouth and throat - and probably to some extent my lung "input" area. This was extremely painful and took weeks to heal.

In my case the battery probably "gassed" badly during charging. This should not happen with a properly designed and correctly functioning charger. The result was that sulphuric acid was transferred from battery to air in a manner that allowed it to cause me harm. The room was about 3m x 3m and may have been relatively unventilated.

So, while it MAY not cause problems on many occasions, I can attest that the possibility definitely exists.

As Dmitry notes, you'd not expect sulphuric acid vapour alone to cause problems. But, by experience shows that it can. The mechanism by is uncertain. eg generated Hydrogen may have assisted vapourisation of liquid droplets in excess of the vapour pressure. Regardless of HOW it happens, it can.

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Explosion:

If a charger is faulty and keeps charging when the battery is at full capacity - or if the battery has a shorted or heavily sulphated cell so that the overall battery will not reach full voltage, then charging can continue indefinitely. This will generate copious Hydrogen (as mentioned elsewhere).

The "used top be sealed" battery shown below exploded extremely violently under these circumstances due to internal Hydrogen gas buildup.

I saw this after the event but was not present at the time. I was told that the explosion was "very impressive".

enter image description here

Probably didn't look quite like this photo of me (really). Hydrogen flames tend to be clear. Battery acid mixed with the Hydrogen could be expected to add some colouration.
This photo shows acetylene burning "oxygen rich".
DO NOT try this at home.

enter image description here

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  • \$\begingroup\$ Overcharged batteries can release hydrogen sulphide. This can combine with water in the air or on your mucus membranes to form sulphuric acid. That's probably what got you, not the direct transport of the sulphuric acid from the battery. \$\endgroup\$ – Olin Lathrop Jul 12 '17 at 14:24
  • \$\begingroup\$ @OlinLathrop Maybe. H2S has an extremely characteristic odo[u]r which I am very familiar with. We have a geothermal area with a town with the greatest number of people in the world exposed to high levels of H2S. Rotorua I have visited Rotorua on holiday many times over decades and the whole town has an H2S smell. [At higher concentrations it is odorless as it paralyses the nasal sensors as the first stage in killing you! \$\endgroup\$ – Russell McMahon Jul 14 '17 at 15:28
  • \$\begingroup\$ People in Rotorua very occasionally die from H2S in unventilated rooms or eg manholes. \$\endgroup\$ – Russell McMahon Jul 14 '17 at 15:28
  • \$\begingroup\$ +1 for best ESE photo of 2018. More please. \$\endgroup\$ – Henry Crun Sep 8 '18 at 0:46
  • \$\begingroup\$ @HenryCrun Non-electrical, alas, but this may keep you going Boom boom - from a Facebook post of mine. My text says: "Photo: Rev Royce Luck. Muppets Swedish Chef aka yours truly. "Tenderised Licorice Cricket Flambe" - Pot lid is the flat shape at right above pot and heading right and up. Add carbide to water in pot. Set lid slightly offset to leave small gap. Carbide in water in bottle lit from candle stage left off. Wave bottle variously with squeezes for flame gouts with Swedish Chef patter. ... \$\endgroup\$ – Russell McMahon Sep 8 '18 at 10:36
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Note that if your batteries are sealed (which is common for modern car batteries), their ability to emit dangerous chemicals is practically nil, unless of course you lick the lead electrodes or keep charging them until they bulge, rip open and spew the acid all around. The latter is only practically possible if you use a self-made or defective charger.

Since car batteries are designed to deliver high current (300-500A, more in case of a short circuit), I would be more concerned with the fire hazard they represent.

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  • \$\begingroup\$ please see my answer. It CAN happen,. It usually doesnt. \$\endgroup\$ – Russell McMahon Jul 12 '17 at 12:11
  • \$\begingroup\$ @RussellMcMahon I'm talking about sealed batteries only, I'm pretty sure there's no way such batteries release anything unless their case cracks. I'm inclined to think the incident you've had involved a "serviceable" battery with a hole in each cell. Those are hard to find nowadays, at least where I live. \$\endgroup\$ – Dmitry Grigoryev Jul 12 '17 at 12:32
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Car battery is containing a pretty dangerous chemical called "sulfuric acid". It is dangerous by itself being a highly corrosive chemical. Its vapors are toxic. When it is contacting with some other materials some other toxic vapors/gases might be produced.
This is not a problem when the battery is intact and not leaking. But can you guarantee it? Better use the car battery where it is supposed to be used. In the car only.

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    \$\begingroup\$ onions cause tears because of sulfuric acid, and we survive; i don't think it's easy to be harmed by such vapors without realizing it long before damage is done, absent some explosion. \$\endgroup\$ – dandavis Jul 6 '17 at 21:44
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    \$\begingroup\$ @dandavis When a person is sleeping (and that is the question) they have a limited capability for early detection of a danger. \$\endgroup\$ – Eugene Sh. Jul 6 '17 at 21:48
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    \$\begingroup\$ @dandavis According to Wikipedia, onions cause eye irritation due to syn-Propanethial-S-oxide, a quite different chemical. I really don't think onions are any proof that sulfuric acid vapors are safe (although I can confirm the latter stings). \$\endgroup\$ – marcelm Jul 6 '17 at 22:00
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    \$\begingroup\$ @dandavis 1) that says that sulfenic acid is involved with creating the syn-Propanethial-S-oxide; but the acid stays in the onion, it's the second chemical that enters the airs and irritates the eyes. 2) sulfenic acid is not sulfuric acid. Just because it says a similar word on the page does not prove your point. \$\endgroup\$ – marcelm Jul 7 '17 at 7:13
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    \$\begingroup\$ Sulfuric acid vapors are corrosive and toxic - not to say non-existent at room temperature: 0.001 mmHg at 20 °C. \$\endgroup\$ – Dmitry Grigoryev Jul 7 '17 at 12:06
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The simple answer to your question is yes it is safe to sleep in a room with two (or more) car batteries. However, if the batteries are being charged (and discharged), then most likely it is not safe. It depends on how good the ventilation is, and how much charging is being done.
To keep the risks to a minimum: maintain good ventilation, use a slow charging method, and keep the batteries away from direct sun light and/or other sources of heat.
A wooden or metal shield is recommended, for protection against possible battery explosion.

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I have been sleeping about two meters away from a set of two 125Ah deep-cycle unsealed lead-acid batteries that are constantly being discharged and recharged. The charger is an IOTA universal input (90-270 volts) which outputs up to 40 Amps at up 27 volts (24-volt system). Most of the time, there has been no problem, and I have had this arrangement for about two years now. However, if one chooses to do likewise, be sure to check the water levels in the cells. They ran low once, and for about two or three days, the entire room smelled awful, something akin to so-called "blue smoke." I thought something in the electronics of the charger had gone bad initially, but the charger turned out to be fine--it was just the batteries.

I have had some concerns about possible lead exposure over time, but do not know if these are valid. The room is fairly well ventilated most of the time, and when the batteries are not actually deep-cycled, the charging on them is gentle. Nonetheless, sometimes, as is normal with such batteries, we hear the bubbling during charging.

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If you really do need to do have them indoors, consider catalyst vent caps. These recombine the hydrogen and oxygen back to water so it does not vent. As they are are sintered filter they also catch the acid vapor. They also prevent H+0 sitting inside the headspace (as happens with sealed gel cells), as they will react the gas space inside the battery, as well as the venting gas.


I avoid batteries sharing a space with anything else. Even in a shed, They should be isolated in an externally vented box or area.

Primarily this is because they always manage to outgas/leak or spill acid. Carpet, clothes, metal, and anything electrical get trashed by even small amounts of acid vapor.

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  • \$\begingroup\$ Of course if you connected a fuel cell to the vents, it would make power from the hydrogen, which you could use to charge the battery, making more hydrogen for the fuel cell, and it would never go flat as long as you didn't draw more power than the fuel cell makes. \$\endgroup\$ – Henry Crun Sep 8 '18 at 3:02
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while charge/discharge ing that is a kill yo self NONO. for storage of healthy batteries , ok . Swollen ones, keep caution.

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Sure. A friend of mine who has long been enthusiastic about renewable energy sources (more for the technology than the ideology) had a large lead acid battery set under his bed for years for his solar and the wind turbine. When others expressed surprise, he would reply, "well where do you keep yours?".

Two car batteries is small potatoes.

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    \$\begingroup\$ And a lot of people smoke cigarettes without getting cancer. \$\endgroup\$ – pipe Jul 6 '17 at 22:23
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    \$\begingroup\$ I once did an entire blotter sheet of LSD, and I'm fine. And so am I. And so am I. And so am I. And so am I. \$\endgroup\$ – Wesley Jul 7 '17 at 17:07
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    \$\begingroup\$ The point is, car batteries aren't dangerous. \$\endgroup\$ – Ian Bland Jul 7 '17 at 17:13
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    \$\begingroup\$ @IanBland The point is: They are. \$\endgroup\$ – Wesley Jul 7 '17 at 17:30
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    \$\begingroup\$ Your advice is dangerous, alas. I can confirm from painful & health affecting personal experience that sleeping in a room where a lead acid battery was charging caused severe inflammation of my mouth and throat - and probably to some extent my lung "input" area. In my case the battery probably "gassed" badly during charging. It MAY not cause problems on many occasions but I can attest that the possibility exists. The mechanism is uncertain. eg generated Hydrogen may have assisted vapourisation of liquid droplets in excess of the vapour pressure. Regardless of HOW it happens. \$\endgroup\$ – Russell McMahon Jul 12 '17 at 12:02

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