Why Does My Car Battery Voltage Drops Overnight?

Key Notes:

  • Voltage drop overnight can be caused by parasitic drain or equipment faults.
  • Under normal conditions, you should see voltage drop over the course of weeks.
  • An ammeter can help to determine if the problem lies with the battery or the car

Voltage drop is not uncommon in car batteries. But it should happen over a long time.

The voltage drops over a night is not sustainable nor normal. This type of voltage drop can be caused by many things like a parasitic drain to equipment faults.

In this post, we will go from basics to what to do when you see voltage drop overnight. As there are many factors in play, we will discover them one by one.

Why Does Voltage Drop Happen?

Inside the battery, there are two poles. One is called cathode another is called the anode.

The positive and negative charges reside in these two poles, when they move from one pole to another it generates electricity. As the charge moves from one pole to another, it reduces the electric potential in the battery.

In layman’s terms, it reduces the voltage. So, the voltage drop is a very natural thing to occur.

As you keep using the battery more and more, the charges in both poles will go toward getting leveled. Once, they get leveled, the battery won’t be able to produce electricity anymore without recharging.

Voltage drop is a fundamental mechanism that takes place to generate electricity. The problem arises when there is too much voltage drop compared to electricity consumed in a car.

How Much Voltage Drop Is Acceptable?

Voltage drop should happen too slow to notice any difference overnight.  As voltage drop is a natural mechanism that takes place when producing electricity, it is inevitable.

But how much voltage drop is acceptable? Should it be noticeable overnight? The answer is no. It should not be noticeable over the span of a few days let alone a night. The difference in voltage should appear to be noticeable over the weeks.

If you notice such a fast drop in voltage, you can assume something is wrong. It can either be the wiring, electrical equipment, or the battery. In case there is a noticeable drop within a night or a span of 2-3 days you should take your car to a professional.

When The Car Battery Voltage Drops Overnight

In most cases, the car battery voltage drops overnight if there is a parasitic drain.

What is a parasitic drain?

A parasitic drain is when the electrical system in the car keeps pulling power from the battery. It can continue pulling power even when the car is turned off.

Consuming power from the battery even in a turned-off car is normal. But in normal circumstances, it happens so slowly that it is hard to notice.

If the system consumes too much power overnight, then you will notice a big voltage drop.

Different Voltage States

Voltage AmountVoltage per cellState of charge
12.72.12100%
12.52.0890%
12.422.0780%
12.322.0570%
12.22.0360%
12.062.0150%
11.91.9840%
11.791.9630%
11.581.9320%
11.311.8910%
10.51.750%

How to Diagnose Car Battery?

There are several ways to test the battery. The easiest way is to remove the ground connection of the battery once you are done. Keep it that way for the night.

In the morning connect the ground connection and check if the car starts properly. In case it does not start, then there is a problem with the battery.

If the car starts, then you can rule out the battery itself from the list of the suspect. The next thing you should do is to take an ammeter and check the current flow when the car is idle. It should be less than 20 milliamperes.

To do this test you will need only an ammeter, then connect its ground to the ground pin and the red pin to the connecting wire. The current flow should be shown on the ammeter’s monitor.

In case, the resting current flow is more than 20 milliamperes, then you should take your car to a professional for a more intensive checkup.

Frequently asked questions (FAQs):

How many volts should a car battery drop overnight?

A car battery should not drop volt in any noticeable amount. The voltage should drop over a long-time span.

Why is my car battery losing charge overnight?

There could be many factors at play behind this incident. It can cause by old batteries, there could be equipment failure, parasitic discharge, or something more.

What causes car battery voltage drop?

The voltage of a car battery drops as much as you use. If you keep lights or any electrical equipment on over the night, it will drain the battery. As a result, the voltage will also drop.

How much voltage drop is acceptable overnight?

The amount of voltage drop overnight should not be a noticeable amount. In case, your car battery voltage has dropped a noticeable amount over a night, you should see a mechanic immediately.

Why does my car battery die after sitting for a few days?

There are multiple computers, clocks, and other electrical equipment running inside of a car. A few of them draw power even when the car is turned off. Due to their power usage, car battery usually dies after sitting for a few weeks.

Bottom Line

Voltage drop is nothing to be worried about as long as it is within the limits. If the drainage is too much then you will face all sorts of problems. So, the best course of action would be to do a primary diagnosis yourself if you know the know-how. Then take the car to a professional.

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