Why Does Car Battery Die when not Used? (The Facts)

Please, raise hands who have never experienced the annoying, dreadful, panicking feeling that happens when your car, a random, cold, and rainy morning, stubbornly refuses to turn on because its battery died out overnight.

Knowing how frequent and awful this event is, I decided to summarize in this article everything you need to know in order to avoid that your battery unexpectedly forsakes you.

Without being too technical, I will cover the reasons why batteries dye out and what can you effectively do to preserve this fundamental component of your car always efficient and ready to go!

Why Does Car Battery Die when not Used?

A car battery might die when not used because of 4 reasons: alternator malfunctions, age, system absorption, or wrong user habits.

Let’s start by admitting that discharging a car battery is a very common event, especially in the summer when hot temperatures shorten (unused) batteries leaving our cars without a rather simple, but critical component for assuring a smooth ignition of the engine.

Most people know that when the car does not start, one simple solution is to try to connect the battery to a recharger and see if this solves the problem. Most of the time it will likely do.

Easy problem, easy fix. Right? Yes, but on the other hand understanding why it happens is something that not everyone does when the battery is low and you desperately need to go to work.

In emergency situations, you can also rely on battery cables to restart your vehicle, as I explained at the end of this article on batteries life, but if the battery life is continuously low and is always discharged then you have to understand the reason behind the malfunction.

Here follow the checks (and the reasons) you should do when the car does not start and the suspicion falls (rightfully so) on the flat battery.

One: exclude alternator malfunctions

If replacing the car battery is the simplest solution for a car that does not start, this will not solve the problem if the malfunctioning part is not the battery itself but the alternator: the part of a car’s engine designed to constantly recharge the battery when the vehicle is on. 

If the battery is discharged because the alternator not charging, a warning light on the instrument panel will almost certainly come on. The test is simple: with the engine running, just measure the voltage on the battery poles with a multimeter (or tester): if it exceeds 14 Volts then the alternator works.

Trending Video: How to Easily Bring Back to Life any Old Car Battery and Save Tons of Money (click to watch)

Two: check battery age

If the car’s alternator works, the next step is to try to figure out for how many years the battery has been connected to the car. If you do not have any invoice receipt don’t worry, it is likely that the dealer or the workshop has engraved an installation date on the battery itself.

Now, establishing the life span of a battery is a difficult process, as it depends on the type of battery, the operating temperatures, and the frequency of use of the car.

On average, however, we can consider that a battery could easily last up to 5 years. Therefore, if your battery starts to be faulty and is older than 5 years, then a substitution might be due (prices start at $40 and can go over $120 depending on brand and characteristics). In such a case, just make sure to dispose correctly of the previous battery and use caution in the change operations.

On the other hand, if the faulty battery is recent, then it is time to test whether the car’s electrical system absorbs too much energy when the vehicle is turned off.

Three: car’s electrical system absorption

Checking if any aftermarket electrical device or accessory is stressing the battery until it is discharged overnight is very easy: you can just use a simple multimeter again. 

Note: For delivering the best results, the ideal would be to recharge the battery before taking the test to be in conditions of normal use. Also, you need care in setting the instrument to draw current (read the manual) and connect the leads of the multimeter in series to the battery.

With the engine turned off, disconnect one pole of the battery and connect the two leads of the multimeter between the disconnected cable and the pole, as if it were a bridge. If the absorption that you read on the multimeter in thousandths of an Ampere (1/1.000) is close to zero, then there is nothing that “steals” too much current with the engine off.

On the other hand, if the value is already in the order of 500 mA or more, then the investigation in the workshop must be carried out. Most of the time it is enough to individually disconnect the fuses of the alarm, stereo to isolate the cause that discharges the car battery.

Clearly, by going to the workshop, specific tests on the car will be able to establish whether the internal elements are intact and if the car’s electrical system is faulty.

Also read: Does Car Battery Brand Matter? Which One is the Best?

Four: user habits

Finally, if the car is shared by several people, often the cause of the low battery is to be found in bad habits: it is not advisable for instance to keep the radio on when the car is off, forget the emergency lights or arrows on or use the car for short journeys a few times a week.

Which Elements “Steal” Battery when Car is not in Use?

Now that you learned that individual components of the car can dry the battery out, we can list which, as per our experience, are the accessories that most likely can steal away the energy from a charged battery.

Those pieces of equipment are likely to affect dated cars the most since, as technology progresses, manufacturers have equipped our cars with mechanisms that turn automatically off some accessories when the vehicle is switched off to preserve batteries.

1. Lights, emergency lights, arrows and radios. It might seem obvious, but those are the single and most likely probable tools that, if left on, might easily drain your battery throughout the night.
   
2. Alarms. This is a double problem because an electrical demanding or faulty alarm system can entirely wipe out the current contained in the battery and, as you correctly guessed, stop working when needed the most (i.e. like when somebody tries to steal your car). It remains to be seen whether the theft will be able to run away with a dead battery or will he decide to push it down the road. For these reasons, if you realize that your alarm absorbs too much energy, consider changing it at once.
   
3. Small electronics. Is it true that those accessories are normally unplugged from the electrical circuit once the vehicle is turned off and the keys are unplugged from the dashboard, but they can still make the battery pay a huge toll if used on a car that has been switched off. Those are multiple USB sockets, portable vacuums, portable fridges, car ionizers, and portable hairdryers. Battery-wise it is better to use them with discretion, or better, not at all.  

Also read: Are Sedans Dying? Will They Die Completely?

After How Many Days a Car Battery Usually Die when not in Use?

If on one hand, it is quite difficult to say when a car battery will effectively die, one can determine the residual life of a battery depending on a series of circumstances. 

The first of them being the climate of the place of residence: if you live in a country where the climate is not extreme, you can easily rule out “temperature” as the most important reason underlying the shortened lifespan of a battery. 

The second element most important element is the age of both the car and of the battery. The risk of having a flat car battery increases with the age of the same and the absorptions present in the car. 

After this quick explanation, we can affirm that in normal temperature conditions, efficient vehicles mounting completely charged up batteries that are new (i.e. up to 4 years) can easily last for 3 to 4 weeks without leaving any worries for the restart of the car.

On the other hand, if we are dealing with an old vehicle or battery with an age greater than 4 years, such risks increase accordingly. This means that even new batteries if mounted on very old vehicles with leaking electrical systems (i.e. accessories that absorb energy when the car is turned off as we explained here above), can dry out before the 3 weeks span we just mentioned.

Notwithstanding all the above, to limit the damage to the batteries will suffer in a long stop of the vehicle, it is better to take precautions. In addition to making sure you have turned off all systems that can absorb energy, such as lights or radios, you can act as follows.

For Older Cars

For older cars, with traditional free acid batteries, it is possible to disconnect the negative pole. Obviously, when reconnecting, there may be some things to set (i.e. the clock, the radio, driving profile, etc.).

For Newer Cars

With newer cars the situation is more complicated:

1. If the car does not require the registration of the battery in the BMS (Battery Management System) then you can also disconnect the negative pole in this case as well.
2. If, on the other hand, the Start & Stop system requires the battery registration, you must not disconnect it: you risk not having it registered at start-up, remaining really on foot even if the voltage is good.

It is also true that in such cases, the AGM and EFB batteries installed on vehicles equipped with Start & Stop, will tolerate deeper discharges and wear less than the classic ones. 

Also read: Can a Car Battery Die while Driving? How to Prevent it?

How often Should You Drive Your Car to Maintain Car’s Battery Life?

As a general rule, a once-a-week ride for at least 20 minutes (15km or so) should suffice to keep your battery healthy and sound.

On the contrary, in case if your car has not been used for some weeks, the advice is to start the engine and keep it at a speed of about 1,500 RPMs for 15 minutes or at idle for 20 minutes. 

The effect of vibrations

Talking about the direct effects that driving has on batteries, please take into consideration that vibrations damage batteries more than you can imagine.

Even if most modern batteries have more resistant lead-calcium elements, if the battery is not mounted firmly, it will not last long. Not tightening well the fixing brackets is the most frequent risk when mounting a battery for the first time. The same happens when an oversized battery is mounted but not properly attached to the custom-made straps of the original battery. 

What Makes Car Battery Die Faster when not in Use?

In addition to environment temperature, age, aftermarket accessories, effective wear and tear of the electric system and of the battery itself, there are other problems that may affect battery duration: the first is Sulphation and the other is Battery Self-Discharge.

Sulphation

During the discharge process, the lead plates (electrodes) combine with the acidic electrolyte solution to create lead sulfate crystals. These crystals are deposited on the surface of the plates themselves, not allowing the electrochemical process to take place correctly, thus causing the degeneration of the battery performance. This situation is indicated by the term sulphation.

The recharging process of a battery would normally lead to the reabsorption of these crystals in the electrolytic solution, with the consequent restoration of normal conditions, but the excessive accumulation makes it difficult to dissolve these crystals to the detriment of the correct functionality of the battery.

In such cases, one can apply a forced process (i.e. by a specific recharger) called desulfation which returns the initial density of the electrolyte solution through the application of particular current pulses that disaggregate these crystals (breaking of the molecular bonds between lead ion and acid sulfate ion) allowing the battery to restore its conditions.

Battery Self-Discharge

Self-discharge instead is a continuous process, which occurs even if the battery is not connected to anything. The self-discharge rate depends on different things, such as the ambient temperature, the type of battery, and whether the battery electrolytes liquids are high enough. 

In particular, a temperature above 55°C causes an even faster self-discharge process. These temperatures can be reached if the battery is stored in a garage or shed in hot climates. Thus, for long-term storage, it is preferable to maintain low storage temperatures. Consider that an AGM battery stored at 0ºC maintains 90% of its capacity for approximately 6 months. The same battery, if stored at 40ºC, loses 50% of its capacity in 4 months. 

Note: there is a widespread misconception that a battery will self-discharge quickly when stored on a concrete floor. This concept was true thirty years ago when the battery containers were made of rigid rubber and the humidity of the concrete led this kind of battery to discharge directly on the surface of the floor. However, modern battery boxes are made of polypropylene and thus can be stored on concrete floors without the risk of excessive self-discharge.

How to Keep a Car Battery from Dying When not in Use?

If you plan to set your car aside for some weeks, the best way to keep your battery from dying is to recharge it once every while with a dedicated battery recharger.

If you think about it, recharging the battery every time the car is stopped in the garage will be a daily habit with electric and hybrid cars. The same should also be for traditional cars when they sit in the garage for days or are driven for short trips where the battery cannot recharge on its own.
Buying a good charger suited to the characteristics of the battery and the car will certainly help prevent a sudden discharge by extending the battery life (you can buy a decent one for $45). It is better to carry out this operation every 4-6 weeks, for a maximum of 24 hours.