Why do batteries explode?

Why do lead-acid batteries explode?

Why do lead acid batteries explode?

During charging all lead-acid batteries produce hydrogen & oxygen which is evolved by the breaking up of the electrolyte into hydrogen & oxygen. Towards the end of charge, the rate of production of hydrogen & oxygen gases increases. It also increases if the battery is overcharged or charged too rapidly.  Flooded batteries always let out these gases through the vent plugs. An ignition source near this area where the concentration of hydrogen gases is over the explosive range of 4%  increases the risk of explosion. If a path exists into the battery like in a vented battery, the flame may continue into the battery casing, igniting gases that may be inside, thereby increasing the pressure inside the container & causing the container to explode. The explosion reaction is 2H+ O2 => 2H2O + Heat. In SMF batteries the flame can’t enter the container as it is sealed. The vent-valve releases very little amounts of gas that is insufficient to ignite.

Hydrogen gas being lighter than air easily disperses into the atmosphere. If the area around the battery is enclosed without any opening (like 8 batteries inside a golf cart battery box without ventilation) these gases can easily result in a powerful explosion that could have the whole golf cart in flames, if there is a small spark (such as from a brushed motor kicking in or due to loose connections on the battery terminals).

Lead–acid batteries can explode during overcharge and gassing and when the percentage of hydrogen gas evolved exceeds 4 % by volume. Oxygen and air form an explosive mixture with 4% hydrogen. Hydrogen is an odourless, colourless & a highly inflammable gas. Possible causes for a battery to explode:

  1. Spark near battery which is under charge
  2. Frayed cables on battery terminals
  3. Tracking across wet battery lids during charging
  4. Sparks or fires, near batteries which is under charge inside the battery room

Spark near battery which is under charge

Poor quality chargers have bad voltage regulation and can cause overcharge. This leads to evolution of hydrogen gases, which normally occurs only at the end of charge. Hydrogen can accumulate within a cell, if the vent plug is blocked by dust. Or when gassing is vigorous with the rate of venting being slow compared to the rate of gas generated. In the normal course, the porous ceramic or plastic vent plug with hole provided allows the hydrogen,  to diffuse out naturally. 

  • Density of hydrogen gas – 0.000089 g/L
  • Density of oxygen gas     – 1.42 g/L ( 16000 times heavier compared to hydrogen)

The  explosion/fire is happens  in spite of hydrogen being 16000 times lighter. Hydrogen can accumulate only when the area around the battery is enclosed. Even a small spark can lead to the battery explosion. 

If the vent plugs on the battery are dirty & clogged from dust the gases can accumulate inside the battery & any spark near the battery will cause the hydrogen gases around to catch fire which will be propagated into the cell leading to the battery exploding & sometimes the lid could blow out. Remedy is to ensure the battery vent plugs are not blocked. Always remove the vent plugs in a flooded battery during battery charging.

Frayed cables on battery terminals


Always ensure the cables connected to the battery terminals are good without any frayed wire ends at the crimped ends of the terminal connectors. Frayed ends are a source of sparks and can easily cause the battery to explode if the battery is housed inside a closed battery container. Brushed motors near the battery spark everytime they kick in. SO ensure the brushed motors are separated from the battery well. Sometimes the battery cable may be frayed at the point where it enters into the battery box at the point of contact of the metal container. Check to see that the cable is not at a sharp angle at the entry point.

Tracking across wet battery lids during charging

It is inevitable that some amount of battery acid bubbles out during battery charging. Especially at the end of charge the evolution of hydrogen is rapid and causes the bubbling & acid spills on the lid of the batteries. Leaving cables tracking over such spills is not a good thing as it causes earth leakage voltage. Always wipe clean any acid droplets after the charging is complete.

Sparks or fires, near batteries which is under charge inside the battery room

Hydrogen gas will accumulate inside the room when ventilation is poor. The gas collects near the ceiling and builds up to 4% by volume. This is an explosive mixture and is triggered by even a tiny spark. When gas accumulates inside a cell during charge and is not vented out promptly, hydrogen can build up inside the room. Hydrogen gas becomes explosive at a concentration of 4 percent. A tiny spark at the battery terminal or connector will trigger an explosion causing considerable damage. Battery charging rooms should never be in closed rooms. There should be adequate ventilation inside the battery room. Hydrogen being much lighter than air, easily escapes through the ventilation.

  • Avoid metallic tools near the batteries to avoid accidental sparks. Use insulated tools- wrapping electrical insulation tape over the handles of the spanner is very simple.
  • Wash the vent plugs with soap & water if the battery is used in dusty environments like on a dirt track in a golf course. Golf cart battery explosions are not unusual.
  • It is best to reduce the charging current towards the end of charge. Most modern battery chargers do this automatically.

Good battery manufacturers use a microporous flame arresting ceramic disc inside the vent plug. You can check to see if your vent plug is having this by looking into the hole. You would see a white disc.

Why does my battery smell like rotten eggs?

Overcharging in a lead acid can also produce hydrogen sulfide. Hydrogen sulphide is a colorless, highly poisonous & inflammable gas with a strong smell of rotten eggs. This gas also occurs naturally in rancid & decaying food or organic matter. If you find the smell coming near your battery, simply turn off the charger & stay away from the smell until it completely clears up. Remember this is only a result of overcharging your battery & does not happen all the time.

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