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Can You Leave a Car Battery Charger On Too Long?

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When it comes to overcharging a car battery, a few variables are involved. If you own a vehicle, you must know the right way to recharge your car battery. But what type of safety issues can occur if a car battery is overcharged? Should you worry about it? Can you leave a car battery charger on too long? In this post, we will try to discuss this issue in detail.

What happens to an overcharged battery

In simple terms, a battery can die from being overcharged. But this is just one thing that can happen when a battery is overcharged. In fact, more dangerous things can happen. The extra charge in the battery will boil the distilled water and sulfuric acid. The casing of battery will become hot, and in extreme cases it can begin to swell and melt

Inside the cells of your car’s battery, flammable hydrogen will build up. And the battery will become almost like a sitting time bomb if hydrogen inside it comes in contact with oxygen. A tiny electrical spark is enough to cause the battery to explode. However, this is not a very likely scenario.

There are some milder effects of overcharging a car battery. If a car battery is overcharged, its service life will be significantly reduced. If overcharging continues for days, it will eventually kill the battery. And when a battery has a short lifespan, you will not get the most of it.

Why you need to charge your car battery

When you see a brand new battery on a display shelf, the first thing you need to know about the battery is that it is slowly dying. In the internal cells there are lead cells and these cells gather lead sulfates. Charge and discharge are two cycles of a battery, and it needs these two cycles for shelf-life longevity.

If you do not charge a battery for too long, it does not get these two cycles. As a result, the sulfates crystallize. Sulfation decreases when you recharge an idle car battery.

Causes of overcharging

A car battery can be overcharged for several reasons. If you know the causes that lead to this problem, you can try to avoid them. Now we are going to discuss them.

Failing voltage regulator

The failure of alternator voltage regulator is one of the main reasons of overcharging. Almost every modern vehicle comes with a voltage regulator. As the vehicle runs, this device regulates electrical charge. If it fails, your car’s battery will get overcharged, because it will receive lots of electrical charge.

Human error

Human error, in most cases, causes overcharged batteries. You may need to use your car’s battery charger, but you may not know the right way to do it. An improper setting of ampere or voltage can lead to an overcharged battery. In fact, it happens too often.

Your car’s battery may get overcharged if you leave the battery charger unattended for a long time. While the newer versions of battery chargers can prevent overcharging, older models lack this feature.

Faulty chargers

Overcharging of a car battery can also be caused by a defective battery charger. If the settings are incorrect, your car’s battery charger will get overcharged, regardless of whether it is monitored or not. Can you leave a car battery charger on too long? Yes, you can. And it can happen in several ways.

Flooded lead-acid batteries

If your car is equipped with a flooded lead-acid battery, you must know what happens if the battery is frequently overcharged. First of all, the battery acid will boil when the battery is overcharged. The battery housing will become weak, making the battery vulnerable to swells and cracks.

Electrolyte will evaporate due to the excessive heat generated by the power cells. If you do not refill the battery electrolyte, the battery will easily get overheated. All these factors will reduce the service life of your car’s battery.

Sealed lead-acid batteries

The same thing will happen to a sealed lead-acid battery when it is overcharged. In case of a sealed lead-acid battery, there is a higher risk of danger due to the type of the casing. This risk increases when the acids begin to boil 

The vents of a sealed lead-acid battery expel pressure, and that is why in many cases the battery housings are bloated. However, this feature is useful because it prevents explosion. Due to this feature, the battery may be able to deal with extreme heat.

Telltale signs

Can you leave a car battery charger on too long? Yes, and there are some signs that indicate that your car battery is overcharged. As a car owner, you must know these telltale signs so that you can avoid dangers. Below are some of these signs.


Heat is generated by your car battery when it is being charged, and that is normal. But if you notice too much heat, probably the battery is overcharging. If the battery case is extremely hot, the battery is certainly taking too much charge.

Battery leaks

Hydrogen gases build up inside the battery when the battery is overcharged. And when it happens, the plastic housing receives some pressure. If there is any tiny opening on the housing, the electrolyte will leak out through the opening. The filler caps and vents are two openings that releases electrolyte.


If the housing or casing of a car battery is swollen, it is probably because the battery is overcharged. It happens because of the quick production of hydrogen inside the battery. This swelling is the result of the accumulated gas inside the battery.

Can you leave a car battery charger on too long? It can happen, and swelling is one of the most common signs of an overcharged battery. The battery housing bulges and in extreme cases it splits.

Low electrolyte levels

If the electrolyte levels are low, it probably means that the battery is overcharged. As we have mentioned earlier, too much heat is generated by a battery when it is overcharged. And when that happens, the electrolyte begins to evaporate. The quickness of the process of evaporation depends on the levels of heat.

Fixing the issue

If you notice signs that your car battery is being overcharged, simply stop charging it. You can use a voltmeter to check the level of charge. You can know the overall condition by testing the battery amperage, voltage, and discharge rate and recovery time.

An overcharged car battery can not be repaired. However, such a battery can be rebuilt. Rebuilding a car battery actually means reconditioning it. You can go to any automotive technician to get this service.

Final thoughts

So, can you leave a car battery charger on too long? Let’s come to a conclusion now. Yes, you can do that, and you may have to pay a huge price for that. In most cases, if you overcharge a car battery, it will simply die.

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