What Can Drain A Car Battery When It’s Off?

ways to find how to stop car battery drains

Having a dead battery is no joke, especially if you get stranded in an isolated place. But if it happens only once, it is easy to pass it off as a fluke. Batteries get drained and die for a lot of reasons, and it is tempting to think that it won’t happen again. But when your car battery gets drained and repeatedly die when the engine is even off,  then there must be something really wrong that you need to deal with. You then ask, what can drain a battery when it is off?

Reasons Why Batteries Are Drained ?

The reasons why a battery is drained even if the engine is off are numerous. Some of these issues are easy to remedy and can be done on your own, but some might need the help of a mechanic.  

Human Error

Many of us are guilty of this, not only once, but maybe even several times - you arrive home from work exhausted. You park the car and forget to turn off the headlights or maybe left the trunk not completely closed. The battery is drained overnight, and when you start the car in the morning, it won’t start. Modern cars give out an alarm when you leave the lights on, but maybe not so for other electronic systems or components.

Parasitic Drain

Parasitic drain is defined as a continuous abnormal power discharge even after the engine is turned off. Frequently, this is due to a short circuit or an electrical device that has not been turned off such as the headlights or an open trunk, both of which could be due to human error.

Some parasitic drain, however, is normal such as when the battery provides power to keep some electronic devices running continuously, such as radio presets, clock and security alarm. When, however, there is an electrical problem, the drain on the battery can be excessive; thus the battery dies.    

Bad Charging

When the battery is not charged properly because of a faulty charging system, your battery can be discharged even when you are driving. What makes the drain problem worse is when car electrical systems such as the sound and lighting system and other electronic devices draw power from the alternator. Even if you give your battery a boost, the drain will persist because of bad charging. You will need a mechanic to solve this problem for you.

Damaged Alternator Diode

Alternators provide power to run the electrical systems of a car such as the ignition, starter, and all the other electronic devices in a car. IN so doing, it also recharges the battery. If the alternator has a defective diode, the battery is drained, and it will die. The defective diode can continue to charge the circuit even when the engine is off, resulting in a dead battery in the morning.  

Old Battery

An old battery normally could no longer hold a full charge. If you consistently have a dead battery, it could be that it is worn out.  Customarily, batteries should be replaced after every 4 or 5 years. If your battery is dead most of the time and your car won’t start, maybe you should look into getting a replacement for your battery.

A battery that won’t hold a charge is a pain in the neck, and assessing the cause of the problem can be difficult. If the cause of the drain is not human error, you will need the help of a mechanic to determine the cause.

​What Should Be Done To Prevent A Battery Drain

preventing the drain issue is the way to last a battery longer.

Fortunately, the ways to prevent a battery from being drained are as numerous as the reasons why it dies. What is important is to identify the cause or the factor that would likely cause the battery to drain and die in the future, and then attend to it immediately.

Factors That Can Cause A Battery To Drain And Die


Extreme weather is a bane to batteries. Unfortunately, there is not much you can do for your car battery during a hot summer spell or the bitterest of winter except to put it in storage or garage. But you need transport, so this option is not even feasible. 

The only thing that you can do to prevent the battery from dying in extreme heat or cold is to keep it properly maintained. A battery that is in good shape will be able to handle the ravages of extreme weather condition better.

​How To Protect Your Battery From Extreme Weather

Weather can cause the battery life expectancy.

1. During the hot summer months, keep the electrolyte topped off. The electrolyte in a battery has a tendency to evaporate because of heat. Low level of the electrolyte has a negative effect on the performance of the battery.

2. Always check the strength of the electrolyte using a refractometer or hydrometer. A continuously weak electrolyte is an indicator that the battery is dying or has reached its retirement age.

 3. Always keep the battery fully charged and the electrical connections clean. This will be a big advantage in freezing weather when the available cranking amperage is less. In a freezing temperature, the capacity of a  lead acid battery is reduced by almost 20%, that is why for every amp count the lower the temperature drops.

Parasitic Drain

A parasitic drain is difficult to pinpoint since you only notice that something is wrong when the battery is almost dead, except maybe if the cause of the drain is a headlight left on. But when the parasitic drain is caused by another electronic device, there is no outward or visible sign that something is wrong. 

A parasitic drain that caused a battery to die does not really do lasting damage to a battery if it happens only once. The important thing is to identify what caused the drain and correct it so that the drain won’t happen again. Every time the voltage drops below a certain level in a lead acid battery permanent damage could occur. It is best to attend to the problem immediately.

The easiest way to find and fix a parasitic drain is by trial and error. Turn off the ignition and disconnect the battery. Then with the use of a test light, look for the drain. If the test light which is connected to the battery terminal lights up, then a system is drawing amps. You will need the expertise of a mechanic to identify and fix the exact trouble. 

Long Storage

Keeping a car in storage for a long time can drain a battery when it is off. If you only plan to leave your car in storage for a short period, all you need to do is to disconnect the battery. Self-discharge is one cause why a battery drains even when it's off.  Even new batteries can slowly drain because of self-discharge. Even though lead-acid batteries have a comparatively low self-discharge rate of around 5% monthly, keeping a car in storage long-term could also do damage. Normal self-discharge could drain the battery to an alarming level.

To prevent your battery from being drained when in storage for a long period, you can do either of two solutions:

● Charge it every now and then.
● Use a battery tender or float charger that regularly charges when the voltage of the battery drops below a particular level.  Although a float charger could prevent the battery from being fully drained while your car is in storage, you still need to monitor condition every once in a while. If anything happens, and the charger fails to switch off, that could also do damage to your battery.


There are several reasons and causes that can drain a battery when it is off. Fortunately, as many as there are reasons, there are also as many ways to prevent or correct it. First and foremost is to keep your battery always in good shape. Also, it won’t take much of your time to check the condition of your battery from time to time. As they say, prevention is better than cure. Remember, a car dying on you when you most need it or when you are in an isolated place is not at all an enjoyable experience.