What are the causes of a swollen car battery?

A less common problem with car batteries is their swelling. Some lead-acid batteries, including those with gel, have common swellings that do not endanger their ability to operate at optimum parameters. But in other conditions, an inflated battery is a problem that you need to check and fix, if necessary.

The reasons why car batteries swell most often depend on the season. In winter, the battery swells because of frozen electrolytes, and in summer because of the gases that form as a result of its boiling.

In both situations, a swollen battery can be fixed, but it is not always a good option because, most of the time, the capacity of the car battery to function properly will be diminished, and the best solution for this problem is to replace it with a new battery.

Why do car batteries swell?

  • Manufacturing faults;
  • Condensed or freezing electrolyte that has lost its density;
  • Damage to the battery’s case due to blows or mechanical stress;
  • Short circuit of positive and negative plates;
  • Improper battery charging. High current, or long charging time;
  • A bad or failing voltage regulator in the alternator.

Most of the time the electrolyte is to blame for a swollen battery, either is frozen or boiled. They can bulge on one side or both. The bad thing is that, if the battery has a warranty, most likely it won’t be replaced without costs because it is considered that the operating conditions have been violated.

Why does a car battery swells at low temperatures?

As I have mentioned, in the cold season, the battery can swell because of frozen electrolytes. As it freezes, the liquid increases in volume, bulging the battery sides.

The normal freezing point of the electrolyte is -60°C or -76°F. If the electrolyte’s density drops to the corresponding of 50% of the battery charge, the liquid will freeze at -24°C or -11.2°F. And if the battery is almost fully discharged, the battery can bulge already between -7°C to -10°C or 19.4°F to 14°F.

In winter, the decrease in density is natural as the air temperature drops. Also, because we turn on heating systems and we use the lights more often in the cold season, there is an undercharging. Thus, it is important to control both the battery charge and the electrolyte density in winter.

Why do car batteries swell at high temperatures?

During electrolysis, hydrogen and oxygen are released inside the battery. The gas formation intensity depends on the temperature, and for those gases to be eliminated, gas exhaust system is used.

Thus, a battery swells because these gases are not eliminated. Here the exhaust system is to blame because it is clogged with dust, dirt or impurities. Another cause could be an active formation of gasses because of high temperatures or a malfunction.

But why does this process of gases inside the battery alter?

  • Battery plates are short-circuited.
  • Battery was charged with too much high current.
  • Battery overcharge because of a failing alternator voltage regulator.
  • Overheating cause the electrolyte in the battery to boil.
  • Clogged gas exhaust system.

Can a swollen battery be fixed?

Although there are some fixes for this problem , they are not recommended. If the battery body is cracked, or the plates are damaged, then the only solution is to buy a new battery.

For a frozen electrolyte, defrosting and charging the battery can eliminate bulging. For that you will need to take the battery to a room temperature of about 19-22 degrees Celsius or 66-73 degrees Fahrenheit.

Leave the battery there until it completely warms up, and charge the battery with low current, no more than 10% of the battery capacity, for example 6 amperes for a 60 ah battery.

If at the beginning of recharging the electrolyte boils, then there is a strong symptom that the battery’s plates are damaged

A high voltage in the battery terminals and battery walls that are not swollen are the signs that the battery may have recovered. After that check the battery to see if it has no cracks where electrolyte might leak.

After that check the battery’s voltage, it should be around 12.6 V, if it’s below 12, then the plates are damaged.

A clogged gas exhaust can be cleaned since the holes are usually clogged from the outside, and you can do this without removing the battery. Also the pressure that hydrogen and oxygen cause when they come out is not that high to cause an injury or an explosion.

But even if you manage to recover the battery after cleaning, recharging or defrosting, it will not work as before, so the best solution is replacing with a new battery.

How to prevent the car battery from swelling?

To avoid battery bulging as much as possible you need to carry out preventive measures such as cleaning the battery terminals, cleaning the gas exhaust system. In colder temperatures, avoid as much as you can battery discharge.

If you don’t use the car often in winter, the best thing to do is to remove the battery and put in a warmer place. Starting the car often is another good way to keep it functional in lower temperatures.

Try to make a habit of checking the car battery regularly, especially for current leaks. Also, don’t forget the alternator.


A swollen battery is a problem that you need to take care of quick. Continuing to drive with this battery issue is not recommended because it can harm your car, especially if the acid is leaking on your other parts. The best thing do do is to replace a bulging battery, because, as I said above if you do manage to repair tit, it won’t work as before.

Scroll to Top