What Can Causes A Car Battery To Die Overnight?

Several factors that you are killing your auto battery.

I have often wondered what causes my car battery to die overnight until I made the effort to do a thorough research on this issue. The result of my research is what I’m sharing with you today.

What would causes a car battery to die overnight? There are actually many causes for an overnight battery death, but the most common cause is an electrical drain that could be caused by lights left switched on such as a dome light or parking lights, among others. These are electrical circuits that get around the ignition switch and are drawing power even when the ignition is off.

Other more obvious reasons why battery dies would be a power door or window which is not properly closed, a radio left running or the light in a glove compartment which is not shut down completely.

The good news is, these causes are avoidable and easy to correct. The bad news is, you might need the help of an electrician to correct them especially if the reason why the lights are on is that of a malfunction somewhere in the electrical system of your car.

You must remember that these lights and electrical accessories are supposed to be turned off automatically, and when they don’t, something is definitely wrong.

Less Obvious Reasons For A Parasitic Drain

By the way, the technical term for such a situation is a parasitic drain, and one less obvious reason for it is an alternator diode that’s not working. The alternator of your car is meant to recharge your battery as well as provide power for the electrical system.

If the diode of the alternator is malfunctioning, it can continue to charge the circuit even after you shut off your engine. That’s why you are left with a dead battery and a car that refuses to start in the morning.

Other less discernible causes of a battery drain would be a broken switch of the trunk light or an electric seat, or a grounded cigarette lighter. These may cause a slow drain but will still cause your battery to die, especially if it is left unused for a long time.

How To Check The Reasons For A Battery Drain

Like everything else in this life, batteries do not last forever. It has a life expectancy or an expected useful life. But when a battery dies prematurely and unexpectedly, especially at the time when it is most needed – in the morning when you are in a rush to go to work, then you really need to read on.

If your battery is new, but it still won’t start your car in the morning, then it really must be because of an electrical drain. The best way to find out which of the various electrical circuits is causing the drain is to inspect them one by one,

1. Start with the more obvious, the lights. Check if the dome light is off. If it is, proceed and check out the trunk, console, glove box, and engine compartment.

If the bulb is hot once you open any of these compartments, then most probably there is a short on the switch and it needs replacement.

2. Next in line for inspection are the control switches for the electric seat. Sometimes the switch can become weak and sticky and it remains engaged so that the seat motor continues to draw power even when the engine is switched off.

3. If the seat motor control switch is working properly, your next bet would be the cigarette lighters. If the lighter is thrust down, still hot and has a burning smell, in all probability it is the culprit. All you have to do is pull out the lighter and find out if it has really gone bad and needs to be replaced.

4. Car radios automatically shut down once the ignition key is in off position, not so with CD players. If a CD is stuck inside the player in an eject or load mode, the motor continues to run and draws power from the battery.

If you listen closely and you hear a soft whirring sound, the motor is running and there’s a CD stuck somewhere inside. Pry out the CD and you’re home free.

If you can’t pry out the CD, disconnect the fuse of the CD player and have it repaired or maybe it already needs a replacement.

5. If still, it’s not the CD player, you may need to check the control switch for the door lock. There are instances when grime and gunk will cause the switch to get stuck in either the lock or unlock mode. This would result in the door’s lock actuator to remain active and continue to draw power.

Check the control switch and see to it that it remains in the neutral position. If the switch is damaged, you might have to replace it.

6. If your car has a light in the vanity mirrors on the sun visors, this too needs to be checked since this can get stuck up and cause a drain in your system.
Draw down the sun visor slightly and observe if the light is still on even when the visor is up. If it is, check the switch if it is working and repair it if needed.

7. Modern cars have what is called a Body Control Module (BCM) or Body Computer which monitors and controls the different electronic accessories in a car. The BCM could short circuit and this will cause the various electronic accessories in your car to continue to draw power from the battery even when the engine is off.

The best way to check if it’s the BCM that’s causing the drain on your battery is to simply locate the system’s fuse and remove it If your car starts then you’ve found the problem.

The Alternator As A Cause Of A Battery’s Death

As mentioned earlier, the alternator is supposed to charge your battery when the engine is on. However, a malfunctioning or damaged alternator diode can cause your battery to die overnight.

The alternator diode is a component of the rectifier assembly that transforms the alternating current(AC) output of the alternator to direct current (DC). The power output of the alternator passes through the diodes in the rectifier assembly prior to flowing out to the car’s electrical system and battery. As a consequence, the diodes get hotter when the charging load is higher.

Under normal charging load and driving conditions, diodes won’t get burned. But, short, infrequent trips, especially at night when the car lights and other electrical accessories are working, increase charging ,load. This could considerably shorten a diode’s useful life.

There are six diodes in an alternator. If the diodes malfunction, the alternator’s charging output is reduced. If there are only one or two damaged diodes, the alternator could still provide sufficient current for the car’s electrical needs, but not enough to fully charge the battery or sustain a higher load. Over time, such a condition could run down the battery.

A malfunctioning diode also causes a battery drain when the car is parked. Since diodes allow current to flow in one direction only, a leaky or failed diode keeps the charging circuit running even when the engine is switched off, thus draining the battery and causing it to die overnight.

Ways To Check For A Faulty Alternator Diode

Checking for a possible malfunctioning diode in an alternator will require the use of a voltmeter or voltage meter. This is an instrument used to measure the voltage between two points in an electrical system.

How to use the voltmeter to check for a possible faulty diode:

● On the AC voltage scale, turn on the voltmeter on a low setting.
● Switch on the engine to around 1200RPM and put the meter probes in contact with the battery terminals; the red probe (positive) to the battery’s positive terminal and the black (negative) probe to the battery’s negative terminal.
● The voltmeter should have a 0 AC volt reading

If there is even a slight AC voltage reading on the voltmeter, it is a sign that there is a failed or malfunctioning diode. You should either replace the rectifier of the alternator or the alternator itself.


What causes a car battery to die overnight range from the avoidable, such as ensuring that all the electrical accessories of your car are turned off, to the inevitable such as a faulty alternator diode. Fortunately, there are ways to find out what exactly is draining your battery and remedies for it.

Hopefully, this article has given you a clear idea on why your car won’t start in the morning, and how to find out what is causing it. A car that won’t start in the morning is not exactly a good start, especially on a workday.