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Can You Leave a Car Battery Charger on Too Long?

Can You Leave a Car Battery Charger On Too Long

Overcharging a car battery can kill it. That is largely true. Then again, to fully charge your car battery, you need to leave it on charging overnight. While that seems ok with most electric cars.

But, can you leave a car battery charger on too long? How much time is too long? 

To simply put, a car battery will gradually die if you put it on the charger for too long. In general, the experts recommend not to leave batteries connected for more than 24 hours.

While that is true for most of them but if you need to leave your batteries charging a long time, you are in the right place.

For instance, what type of issues arises when you leave a charger on too long? Is it alarming? Can you overcharge a car battery with a charger? In this article, we’ll try to answer these in detail. Let’s get started.

Can You Leave a Car Battery Charger on Too Long?

Preferably not. If you leave the car battery charger on for too long, it will damage your car battery. As mentioned, overcharging a car battery causes the acid inside the battery to evaporate.

However, this is true when a car battery is excessively overcharged. Otherwise, there shouldn’t be an issue with overcharging if you use the best car battery charger.

Even then, any battery shouldn’t be connected to the charger for over 24 hours. Whether you can leave a car battery connected depends on a few things. If you often use an overnight battery charger, you must keep in mind those.

Thus, the battery will no longer be able to hold the charge and become useless. Overcharging also causes extreme gassing. The electrolyte becomes hot, and it generates both hydrogen and oxygen gases.

The manufacturer nowadays has come up with batteries that can tell when the battery is fully charged. Once those battery chargers detect a full charge, they stop sending electricity to the battery.

Hence, we have some other articles on related problems that you can check-

What Happens If You Leave a Battery Charger on too Long?

When overcharged, a car battery can die, and even more, dangerous things can happen. Basically, the main element in car batteries is lead-acid. It will naturally sustain a certain amount of overcharging.

But if you continuously charge it, even at low amps, the extra charge will eventually boil it to dry. It will boil the distilled water and sulfuric acid as well as damage the internal lead plates. (Source)

This causes the casing of the battery to overheat. In the worst case, the casing of the battery begins to expand and melt. Flammable hydrogen builds up inside the cells of the battery from overcharging.

If the inside hydrogen comes in contact with oxygen, it can become a sitting time bomb. Then it will explode even with a tiny electrical spark. Well, this will happen in the worst case.

But there are still many milder effects of overcharging a car battery. Continuous overcharging can reduce the service life of a car battery. Gradually, it will kill the battery, and reviving the dead car battery may not work. You cannot get the best performance from the battery with a short lifespan.


Can A Car Battery Be Overcharged?

No, it should not. The cells of a car’s battery will begin to fill with flammable hydrogen as time goes on. Furthermore, if the hydrogen contained within the battery comes into contact with oxygen, the battery will behave almost like a ticking time bomb.

Even giant manufacturers like tesla suggest charging up to 90% overnight. It will keep your battery healthy for a long time.

What Could be the Reasons for an Overcharged Car Battery?

There may be several reasons for a car battery to be overcharged. If you know the reason, you could possibly avoid doing them in the future.

When Trickle Chargers Left Unattended

Many people leave the trickle charger unattended and sometimes start the car with a battery charger. This can result in overcharging the battery. As the name suggests, a trickle charger provides a small amount of electricity.

This tiny electricity is fed into the car battery and lets it charge slowly over time. A trickle charger can charge a car battery above 12.6 Volts. It is somewhat like the mobile phone charger. But, a mobile phone charger can notify when the phone is fully charged.

It then stops sending electricity to the phone battery. In this case, the trickle charger mostly cannot detect when a car battery is fully charged. So they are unable to stop sending electricity to the battery.

When the trickle charger is left unattended, it will continuously send electricity to the battery. At one time, it causes overcharging. So make sure the trickle charger you are using can determine whether the battery is fully charged.

Black and Deck trickle chargers are best in this case. They can determine when the battery is fully charged and then switches to float mode. The battery charger again turns on when the battery voltage drops below a certain threshold.

Defective Alternator

The alternator on a car is used to produce and send electricity to the car battery. Alternators are equipped with a voltage regulator which regulates the voltage from the alternator to the car battery. Voltage regulators send the voltage below 14.8V to the battery.

When the voltage regulator glitches, it’ll send the additional voltage to the battery—this causes the battery to overcharge. If the voltage is over 14.8V, it will overcharge the battery and kill it as time goes on.

Human Blunders

In most cases, overcharging is caused due to the mistakes made by humans. As you know, you need a battery charger to charge your car. But you don’t know the right way to do it.

If you set the voltage or ampere in the wrong way, it’ll lead to overcharging. Also, you may often leave your car charger for long. In fact, you do this too often, which causes overcharging.

Defective Battery Charger

A defective battery charger often causes overcharging. Sometimes battery chargers get defective when the setting is incorrect. No matter whether you monitor it or not, the battery will overcharge.

Poor Connection to the Alternator

The poor connection between the alternator and the charger will cause overcharging. Make sure you check the connections if you suspect such an issue.

Significant Signs of Overcharging

Overcharging a car battery can be problematic for car batteries. Can you leave a car battery charger on too long? Ideally, not because you want to avoid overcharging. So how do you know if the battery is overcharged?

Some significant signs denote your car battery is overcharged. Knowing these signs will help you avoid dangers from an overcharged battery.

1. The Battery gets too Much Hot

Generally, a car battery should be cool to touch. Sometimes it can be a little bit hot because the battery is being charged. But, the battery sides get too much hot, possibly the battery is getting overcharged.

If the heat in the battery is increasing, it denotes the battery is taking too much charge. In such a case, you shouldn’t start the car with a battery charger. Make sure you check the battery after driving for 30 minutes or more.

2. Curved Battery Sides

Because of overcharging, the sides of a car battery can become curved. It happens due to the overproduction of gasses inside the battery. When overcharging has been occurring for some time, it results in puffiness and swelling. In this case, you should replace the battery as soon as possible.

3. Battery Leaks

When the battery is overcharged, the hydrogen gas inside the battery builds up. It causes pressure in the plastic housing. Due to the pressure, there may be a small opening on the housing, such as filler caps or vents. The electrolyte will leak through these openings.

4. Acid Smell

When overcharged, the inside distilled water and acid inside the battery boils up, which you may smell. If you can detect this acid smell, it means the battery is overcharging.

5. Low Electrolyte Levels

Due to the overcharging, the electrolyte level in the battery becomes low. As we mentioned, the battery gets too hot when overcharged. It happens because the electrolyte starts to evaporate depending on the levels of heat.

Leave a Car Battery Charger


How to Check the Battery and Alternator for Overcharging?

At first, you need a multimeter to check the battery. Set the multimeter Dc voltage and set it above 15 volts. Begin with your car start and let the alternator start the battery. Make sure to stay away from any moving parts like the radiator fan.

Now open the hood on your car and find the battery and battery terminals. Then connect the multimeter’s red wire to the positive terminal or red wire in the battery. Similarly, connect the black wire with the negative or black terminal.

From the multimeter display, you can get the reading and check if the battery is overcharging. If you get a reading above 14.8 volts, it means the battery is overcharging.

Otherwise, the reading between 13.6V to 14.6V denotes the alternator is working correctly. But, if you get a reading below 13.6V, the alternator isn’t releasing sufficient charge to charge the battery effectively.

How to Fix Overcharging Issues?

First and foremost, if you could sense the overcharging, simply stop charging it anymore. You can check out the voltage, battery amperage, discharge rate, etc.

Sometimes you cannot repair an overcharged car battery. Also, replacing the lead plates is not affordable. But, you can actually rebuild or recondition your car battery. If you want to do it by yourself, there is a way for it.

You’ll need distilled water and Epsom salt to recondition your car battery. These components clean off sulfates from the battery and replenish them with electrolytes. Instead, you can use re-conditioners, but they are not too effective as well as costly.

How do You Know If the Battery Is Fully Charged?

Usually, the batteries used in a car are larger than other batteries used in any electronics. In a car, a large 12V battery is used. When the battery is fully charged, it should only read over 12.6V in an off state.

You can get this reading when the car is running. While measuring the voltage, you are also if the alternator is appropriately working. The voltage reading should be between 13.6V to 14.6V when the car is running.

The newer battery chargers can read this reading and detect when the battery is fully charged. It is so easy to measure the voltage reading in the newer battery chargers. The battery charger itself will tell you when the battery is fully charged.

How Long do You Need to Charge a Car Battery?

How long the charger will take depends on the amperage of the battery charger. Most car batteries include an ability of 48amp-hours. It denotes a 2-amp battery charger would take 48 hours to charge the battery. Similarly, a 4-amp charger would take 12 hours to charge the car battery fully.

Why do You Need to Charge Your Car Battery?

Even a brand new battery slowly dies if you do not charge it for too long. There are lead cells inside the internal cells of the battery. So, these cells accumulate lead sulfates. A car battery needs both charge and discharge cycles for shelf-life longevity.

When you leave it without charging, the battery doesn’t get these cycles. The sulfates crystalize as a result. This issue decreases when you recharge the battery. Some people think alternators will top off the charge on a battery.

But in reality, alternators will not charge a battery that already has lost most of its charge. It is unwise to expect an alternator will charge up a dead car battery. Hence, you need to charge your car battery, especially when storing it over the winter.

Tips to Follow While Charging a Car Battery

  • Charge the car battery in a place with room temperature. Too hot or too cold weather can have an impact on the charging.
  • Charge it in a well-ventilated place.
  • Connect the correct charger with the correct charge rate for your vehicle.
  • Use a smart charger that can measure the voltage and detect when the battery is fully charged.
  • Keep the battery terminals clean.
  • Charge at low amperes. It may take longer, but best for your battery’s life.

Final Words

So, can you leave a car battery charger on too long? Let’s say it for one last time, absolutely you can do that. However, you need to pay a heavy price for doing that. Your car battery will simply die if you overcharge your car battery.