I read that when charging a battery(charger to battery) or battery to battery(I imagine) a toxic chemical/gas is released from the battery. As such you should ventilate and not be in the home whilst battery charges.

I’m going to try the battery to battery method and want to leave it in the car. My concern is the chemicals that come out might get absorbed into the seats or react with something in the car and I will inhale it later etc or it might react with a cleaning chemical etc. Not sure what chemicals come from the battery but is it a concern or after charging for a while with windows closed(as I cant be around for too long)will the chemicals go quickly once u open the windows?

  • In a lead acid battery, hydrogen gas is created during charging, which is highly explosive. Best to charge a battery where there is plenty of ventilation, all my batteries are under the hood in the engine compartment, I leave the hood up when charging.
    – Moab
    Oct 3, 2020 at 21:56

3 Answers 3


You should never store or especially service a battery in the home or car. The acids do create toxic gasses and explosions are even possible.

  • 1
    Do u mean don’t even put it in the boot or cabin where it’s disconnected?? How are you going to transport it then? Oct 3, 2020 at 22:26
  • Transporting and storing are different. Lead acid batteries can do some bad things. AGM batteries are safer because they don't have the acid do deal with.
    – Jupiter
    Oct 3, 2020 at 23:30

If the battery is not defective and the charging voltage and current are correct, there is no release of non-toxic hydrogen. If hydrogen is released ,you are breaking down water and it would need to be replaced. I have not added water to a lead/sulfuric car battery in 40+ years; ie. I have not had a car battery release hydrogen in 40 years.


The only two concerns are (1) battery acid and (2) hydrogen gas.

Hydrogen gas is explosive, but to explode you need 4% of it in air. This means that in a 30 square meter, 2.6 meters high room, you need 0.04*78 = 3.12 cubic meters of it. Hydrogen density is 0.08375 kg/m3, so you need 0.26 kg of hydrogen.

Water is H2O, so to cause an explosion, you need (16+2*1.008)/(2*1.008) * 0.26 kg = 2.32 kg of water to be completely separated to hydrogen and oxygen.

Car battery is 7 liters in size approximately. However, not all of that is water-acid mixture. If we assume 25% of the size is battery casing, and of the rest, 50% is lead plates and rest is water-acid mixture, we have 0.75 * 0.5 * 7 l = 2.63 l of water-acid mixture.

Battery acid is 37% sulfuric acid, rest water. So a quick approximation is that it's 2.63 l * (1 - 0.37) = 1.66 l of water.

I don't think there's enough water to cause an explosion hazard if charging a battery indoors, even if all the water was turned into hydrogen so quickly that your ventilation system can't clear away the hydrogen (which would require all of the water being electrolysed in way less than an hour).

The main hazard is that if the charger you have is faulty (or too simple, not microprocessor controlled), theoretically it's possible that the bubbles splash acid around, and if the vents release hydrogen gas, some small amount of acid could get away from the battery. This won't happen with an AGM battery. But with a flooded battery it's theoretically possible that you get battery acid on the floor or wherever you happen to have the battery. If you notice it, the solution is simple: use sodium bicarbonate water solution to neutralize it. If you don't notice it, theoretically it's possible it could cause acid burns to your fingers if you touch it.

The best bet is to charge your battery at the car. But if you for some reason can't have electricity there, bringing it indoors is not a criminal offense. It may be useful to charge it in bathroom, so if any acids leak you can dilute them to practically no acid left simply using water -- maybe adding some sodium bicarbonate there to neutralize it if you want to do a perfect job.

Also, if charging a battery, do something to prevent localized hydrogen explosions (so that the entire area doesn't have explosive concentration but a small amount of air near the battery has an explosive concentration): remove the AC side of the charger first, and only then remove the DC side to prevent sparks, or alternately if charging on the car connect the charger negative lead very far away from the battery to some metallic grounded object -- or both.

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