Why Does A Car Battery Voltage Drop Overnight?

Voltage dropped on a car battery, why is that?

Have you ever experienced that you are ready to go to work, the kids are prepared and dressed for school, and you are all inside the car, but you discovered that the car battery is dead? And you will ask yourself, why on earth this car battery voltage drop overnight? What could go wrong?

Why do you think a car battery loses charge overnight? Well, it is because of two main reasons. First, the battery could be dead, and you can no longer revive it. Or second, there is something inside your car that is draining the charge of your car’s primary energy source. It is called a parasitic drain.

What Is A Parasitic Drain?

A parasitic drain or parasitic draw happens when an unusual and constant flow of energy from the battery still occurs even after you shut off your car engines. Most of the time, the reason is that there’s a short circuit or funny as it may seem, an electrical device that you unintentionally left on like your air conditioner, your radio, the alarm service of your car, your trunk light, and other really hidden electrical devices that you may not always see.

How to Diagnose that it is a Parasitic Drain or simply a Faulty Battery?

There are different methods that we can use for us to come up with the result for the reason why your battery drains overnight. Different tests that you can follow:

– Amp Battery Drain
– Voltage Drop Battery Drain
– Bad Alternator Diode
– Car Battery Hydrometer

1. AMD Test – In this test, you check the distinctive electrical circuits running down the appearance of electrical flow when there ought to be none. On the off chance that you identify the electric surge of flow in the light circuit, you’ll realize this isn’t directly since all ought to be off. Now recall that the channels for your alarm system, your stereo, the air conditioner, even the trunk, and the clock have electricity flowing in them.

How To Perform This Test?

For you to start the amperage strategy test, separate the negative (-) battery link from the cell terminal. The negative (-) battery link is the dark or black link associated with the battery link with the negative (- ) sign alongside it. At that point, connect your computerized meter’s ground (black) test to the negative (-) cell post.

  • Attach the meter’s positive (red one) test to the cell link terminal you merely detached.
  • Installed the multimeter to the greatest perspective on the DC amp scale and switch it on.
  • Presently, check your meter’s presentation. It should peruse 0 amps.
  • Transfer the meter’s control to the following lower perspective on the amperage scale.

Hold transferring to a lower perspective until you distinguish the approach of electrical flow. On a car out a parasitic channel issue, this course is an inside scope of around 50 mA. On a car with this issue, this electrical flow will be greater than the average range.

While performing the test, please make sure that no one would try to crank the motor, turn on any device inside the car like air conditioner or heater, and other small accessories. Doing so will inflate out your igniter or damage your tool.

How To Locate The Broken Circuit

For you to find the flawed circuit, begin hauling out breakers, each one in turn, from every one of your car’s wire boxes. Start on the under-hood combine box, if there’s any. Draw whole of the breakers and see your meter’s perusing. At that point, you can install the wire again. Continue pulling wires each one in turn until you notice the unexpected amperage scanning on your meter goes down to the typical inside range.

When you discover the channel causing the parasitic draw, check the depiction of the channel under the wire box cover. It’ll state something like “Tail, stopping, side marker lights”, or “Blower engine 30-amp intertwine.”

Begin with the switches, assuming any, to ensure they are legitimately working. On the off chance that it is an entryway switch, for instance, do beyond any doubt the switch is yet effectively mounted, and the entryway presses the turn off when shutting.

Ensure the cables in the line are far from hot exteriors and cables and burdens have strong and clean links.

Start with the switches, assuming there’s any, to guarantee they are authentically working. In case it is a passage switch, for example, do without question the switch is yet viably mounted, and the door presses the turn off when closing.

Guarantee the cables in the circuit are a long way from hot surfaces and connectors, and weights have hard and clear links.

Voltage Drop Battery Drain Test

Even though the amp battery channel inspection is the mainstream and dependable procedure for a few times, it is hard to execute on numerous late model wheels. A significant number of these types have a few PC modules that stay online for thirty minutes or progressively after you’ve closed off the motor and expelled the key of the start switch.

So you have to trust that every one of these frameworks will go into rest mode (reserve) before you begin your tests. What’s more, you may coincidentally “stir” at least one modules while endeavoring to locate the flawed circuit.

With this analysis, you’ll examine for the appearance of the lost charge due to the reserve in a line caused by a sudden spike in the temperature on the electrical flow.

Bad Alternator Diode Test

Typically, a faulty one can affect your instrument board lights, headlights, or stereo presentation to gleam or diminish and, some of the time, empty the battery control overnight or few hours after being not in use.

To review for a likely awful alternator diode, change your voltmeter to a deep perspective on the AC (substituting current) energy range.

With the motor running, reach the meter tests to the battery ends—positive (red) meter test to the cell’s positive end, and negative (dark) meter test to the cell’s negative end.

The voltmeter should show or indicate zero AC volts.

Hydrometer Test

It is a speedy method to verify the condition of the energy level and soundness of your car’s battery. It will tell you whether your cell is ultimately energized, needs a load, or at least one cells have fizzled.

The device is a kind of pricey instrument. You can try to check with your best mechanic in town to see the auto battery using this instrument if you don’t have one.

Primarily, if your hydrometer is showing under the 1.265 marks means typically your car’s cell needs to be charged.

How Do You Know That The Battery Is Really Dead?

Aside from the fact that the car functions properly after changing the defective or dead battery by a new one, there are other ways for you to check if your car battery is really the culprit of the everyday dilemma.

A battery’s life span is a maximum of five years only. Don’t believe those salespeople who made you believe it is long lasting. Nothing lasts forever, even the car batteries.

The following are some of the reasons why you can say that your car battery is actually dying (except for the parasitic drain):

  • You have bought the best low-quality car battery in town.
  • Your gluttony for accessories is unexplainable.
  • Your car battery lacks acid, one of the reasons it drains really fast.
  • You’re charging it the wrong since the beginning of time.

Yes, most of the time, it is human error.

But, no. You should check the following as well – because here, it is not your fault but the battery’s fault on his own.

The egg rotten smell and the clicking noise.

I am sure you have experienced that egg rotten smell even just once in your life. It’s a good thing if you have found out the reason why at first try and tried not to experience that again on the next battery purchase.

But if you have experienced it now for the second time, here is why it is happening: that is sulfuric acid that you are smelling. Lead acid car batteries are loaded up with a mixture of sulfuric corrosive and water dissipate that can cause the battery to overheat. This creates rank “spoiled egg” smell.

Now, what about the clicking noise? Try to start your engine. You should hear the engine crank or clang in.

Be that as it may, in the event that the battery is dead, at that point you’ll just hear a clicking clamor. When this occurs, have a go at kicking off your battery to check whether you can restore it. But if it didn’t, then it is the end of your car and its battery’s relationship. You need to replace it.


There are two main reasons why a car battery voltage drop overnight. First, it could be your battery is nearing its end and needs to be replaced and second, the parasitic drain which has been discussed intensively on this article.