Causes of Overnight Car Battery Voltage Drop

C:\Users\KELECHI NWOKE\Downloads\Dead Car Battery.jpg

One of the most annoying things a car owner can experience is leaving your car with a full charge one day and trying to start it the next day only to discover that the car won’t start because the battery’s voltage has dropped so low during the night. This is certainly not a normal occurrence. It is an indication that something is very wrong somewhere.

In this article, we will be looking at this issue and other related ones. The aim is that at the end of this article, you will have a better understand of your battery, what can cause the sudden drop and how to prevent this from happening.

What is Voltage?

C:\Users\KELECHI NWOKE\Downloads\How to Check Your Battery's Voltage.jpg

To ensure that everyone will understand this article, we will start by looking at what we mean by voltage in general. Voltage is a word used to describe the pressure that a power source uses to push current also known as charged electrons through a circuit to get a task done. This is very closely related to the term “potential difference” which is used to describe the difference in the energy levels between two points. This tells us the amount of energy that potentially exists to move electrons from one of the points to the other.

When looked at together, one can describe the operations of voltage and potential difference with the example of a water tank. The bigger and higher a water tank is, the more forceful the flow of water will be when the tap is opened. As the water gets depleted, the force of the flow also reduces.

The water in this case represents the current or charged electrons which will deplete with use, causing a reduction in both voltage and potential difference. You can learn more about this here.

Understanding Car Battery Voltage?

From our definition above, you will hopefully have a better understanding of what we will now be looking at. Your car battery should have different voltage readings depending on its state. We will explain this.

When Fully Charged

When regular car batteries get fully charged, its reading will be about 12.6 volts. These will usually be described as 12 volt batteries. Note that this should be the reading when it has been fully charged but not in use. Basically it is sitting idle at this point.

When Starting a Car

When you start your car, it draws heavily on the current reserve and depletes it. If you immediately take its reading, it should give you something like 10 volts. It can be higher or slightly lower depending on factors like weather conditions. This will however only be for a very short while as the alternator will kick in once the car has started.

When the Car is Running

Once the vehicle is running, the alternator will kick in and take over the task of powering the car while also recharging the battery. If measured at this point, the battery reading you will get will be anywhere between 13.5 to 14.7 volts. Remember that this is not a valid reading since the alternator is actively charging it.

What are the Implications of Sudden Voltage Drops?

C:\Users\KELECHI NWOKE\Downloads\What Causes Voltage Drop.jpg

The implications are pretty straight forward. It means that your vehicle will not receive the quality of power it needs to function fully. You will not be able to get it started easily. You may find yourself stranded on the road, looking for another car to help you jumpstart yours. This can most certainly be a source of great inconvenience.

Aside from this, there is also the risk of electrical fire depending on what is actually causing the drop. Let’s consider some factors that can be responsible for this.

What Causes it?


The first thing to look at is the age of the battery. Any battery that has served for about four years can begin to lose charge very quickly. This will be chucked down to simple wear and tear that comes with aging. The solution is to get a new one.

A Hidden Drain Source

If age is ruled out, then the next thing to look for is a hidden drain source. If you leave your headlights on overnight, you may wake up to a drained battery. The same will happen if there is a light left permanently on somewhere in your car. Check for this to ensure you did not mistakenly leave something on through the night.

Faulty Wiring

If all of the above have been eliminated, the next thing to check for will be faulty wiring which may be causing shorting or triggering a drain source. This is something that may require the services of an expert.

How Can it Be Prevented?

Now that we have looked at some possible causes, how can we solve it? Obviously, the solution will depend on the problem. If the problem is the age of the battery, then the only solution as already stated above will be to get a new battery.

If you discovered a drain source like a light left on even after the engine had be shut off, then you will ensure you check all of these before you shut down the engine and turn in for the night.

If it is a case of faulty wiring, you should get an expert to take a good look at your vehicle’s electrical system. These are just the basics. There are a number of other things you can do to at least keep an eye on your car battery’s life. All of these and more are listed here:


Your car battery’s health is of utmost importance. Ensuring this will save you a lot of stress, funds and disappointments. Do the following constantly to monitor it so you can easily spot any issues:

  • Frequently inspect the battery visually to ensure there is no swelling, buildup or discoloration at the terminals.
  • Look for any leakages. If you find any and cannot fix it, call a qualified personnel.
  • Run regular terminal, voltage and hydrometer tests.
  • If you cannot do any of the above, take your car for constant checkup.