9 reasons why car batteries drop voltage overnight

If you own a car, there is a time you might have experienced your vehicle failing to start in the morning. This can bring a lot of inconveniences, especially when you don’t understand what the cause is. Many issues can cause a car to seize in the morning or start to crank instead of starting slowly.

In most cases, a dead battery is the leading cause of this failure when the voltage has dropped overnight, and the battery lacks enough fluid to start the engine.

Have you ever wondered why your car’s battery discharges at night? Although electrical accessories that might be left on overnight might drain the battery, there are possible causes.

Why does car battery voltage drop overnight?

A car’s battery can drop overnight for several reasons. The primary reasons your car’s battery can discharge after even shutting the engine, including electrical systems, might be affecting the battery power, a parasitic drain, or the battery being too old.

Causes why car battery voltage drops overnight

Your car discharges overnight for the following reasons.

1. Accidentally leaving the car ignition on

If you leave your car’s ignition system on overnight, it can drain significant amounts of power from the battery. Lights on can drain the power in just a short time.

2. Leaving electrical accessories on

Cars have several electrical accessories that might be left on accidentally. These include radio and the interior dome lights. These accessories can drain the power when left on and even when turned off.

3. Batteries often self-discharge

Batteries do not guarantee 100%charge storage. Typically, batteries can discharge themselves even when no appliance is connected.

However, this drain is not a significant one since a 55AH battery can lose about 0.1 percent for a night

4. Poor connections at the terminals of the battery

Having batteries poorly connected at the terminals can interfere with the charging system of the batteries. When the batteries are not connected, they will charge slowly, so you might not get any charge to start the car in the morning.

5. You might be taking your car on shorter trips.

Short trips are not suitable for your car’s battery. It takes a long trip for your car’s battery to fully charge. Therefore, when you take the car for short trips and leave it in the parking for a long time, the battery might charge as required.

6. A failing or bad alternator

An alternator with good working conditions should produce about 14.5V measured across all tribals of the battery measured with a digital meter while the car is at rest. If you find that the voltage is lower or high, it is a clear indicator that the filter is faulty.

7. Aging or old battery

A discharging battery is a sign that it is aging. The ability of batteries to hold a charge diminishes with battery age. This means that these old batteries have a high chance of losing charge overnight.

Test your battery and confirm if it can deliver the amount of CCA that it is meant to. If the battery can only provide a portion of what it was designed to, then the battery is failing and needs a replacement.

8. Parasitic drain

The other common cause of power battery discharging overnight is a parasitic drain. Parasitic drain continues to drain the battery even when the engine is off.

Even if the engine is turned off, the car battery powers some components like anti-theft features and the clock. This drain occurs when the battery sends a charge to other parts that do not need to be charged after powering off the engine.

This can be due to several factors, including leaving the headlights on and cabin and glove box lights overnight; parasitic drain can also result from bad wiring, poor installation of a car battery, and defective fuses.

9. Temperature extremities

When the surrounding temperatures tend to be too cold, they can affect the car battery if it is left outside. Leaving a battery for long during cold times, the cold will affect the battery’s chemical reactions.

On the other hand, hot temperatures can also affect the car battery’s functionality. High temperatures will make the car battery’s chemicals evaporate therefore draining it.

This means that keeping the car in a well-conditioned place is vital if you live where temperature extremities are high.

Is it normal for a battery to drop voltage overnight?

No, Car batteries should not lose their charge overnight. While some bit of a volt might be lost, the loss should not be so big to be noted. If you realize or note that the car battery is draining overnight, then there is something wrong that is draining the battery.

How much voltage should a car battery drop overnight?

There is no rule to determine this, but a car battery should not fall below 0.3 V overnight.

How to fix the Car Battery from Draining Overnight?

To fix the problem of car discharging overnight, you should

1. Switch off all electrical appliances

Electrical accessories like the audio system might draw significant amounts of power if left without switching off.

2. Replace the car battery

Get a qualified auto technician to test your car battery to ascertain if it can deliver the required powerPower. If you see any signs of failure, like the slow start of the engine, you will have to replace it immediately.

3. Connect your car battery to a maintenance charger

You need to connect your car battery to a maintenance charge if you drive with many accessories switched on.


Starting a day with a car not starting can be so frustrating, more so when you have to get somewhere very first. Experiencing power drains overnight is normal only if the drains are not significant.

If you notice volt drops, you need to check the car to see if there are any electrical accessories left on because it is the most common power drainer.

Moreover, if you drive with several accessories on, they might slow the battery’s power intake and even drain the battery flat. Therefore ensure you connect to a maintenance charger.

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