Why Your Car Battery Keeps Dying [Explained]

There’s probably nothing more annoying than to be running late, turning your car key, and finding that absolutely nothing happens. Congratulations, you’ve got a dead car battery!

There are many reasons why you experience absolutely nothing when you turn the key… well, except for frustration.  I’m going to cover 14 of them with a bonus tip at the end.

If you’re just here for the quick answer, I’ll fill you in with the following:

A car battery will keep dying in short order primarily due to excessive and irreversible lead sulfation which has built up over time as a battery spends time in a state of partial or full discharge. An abnormal parasitic draw from the car’s electronics can also kill a car battery.

Hopefully, the following will keep you stress-free in the future and maybe even correct some habits that you didn’t know were secretly destroying that battery that lets you enjoy the road.

Let’s get started!

1. Bad Alternator / Voltage Regulator

The alternator and voltage regulator are responsible for charging your battery up immediately after your car battery releases a massive spike in amps to start your car.

If the alternator is bad, it will simply not produce the electricity needed for the voltage regulator to pump into the battery.  If the voltage regulator is bad, it might be pumping too little or too much current into your battery. Both of which will kill your battery over time. 

2. It’s Just Old

Let’s face it, batteries have a shelf life and a natural lifespan.  There’s only so many times they can be used to start your vehicle, deal with the extreme temperature swings and inconsistent charging before they give up the ghost. 

If you’re the average driver, you’re likely to get 3 years out of your car battery.  If you’re lucky you might get five. I’m currently at 5 myself, so I had better start budgeting!

3. Parasitic Draw

When you turn off your vehicle you might be imagining that your car battery gets to take a nap until you decide to drive again.  That couldn’t be further from the truth!

Anti-theft systems, the clock, radio, and that kids DVD player you left plugged in are all taking their slice of the pie from your car battery.  Over night, or even for a few days it probably won’t be a big deal if you have a decently healthy battery. But after you start getting passed a week or two or three (depending on your vehicle) you’ll see a real difference in your battery’s health.  

Have you ever noticed that when you get back from vacation your car battery seems to work harder to start?  It was being slowly drained by your car’s computer systems and anything you left plugged into the charging ports. 

4. Faulty Relay Switch

This goes hand in hand with the previous number.  If you have a faulty relay switch in your car’s wiring system that stays in the “on/open” position, then whatever it leads to will be sucking the energy out of your battery without your knowledge or consent.  

5. Naturally Self-discharges

This is probably the most innocent one out of the group.  Yes, batteries can naturally just lose their charge by doing nothing.  This rate should be slow on a healthy battery that doesn’t have any internal damage or corrosion, but it happens nonetheless.  

Self-discharge is the reversal of the chemical process inside of the battery when it is not connected to a charger. Instead of keeping the energy stored chemically, it slowly releases it to the surrounding environment electrically.

While parasitic draws sip energy out of your battery when the car is off, even if you disconnected the battery it would still lose its charge — only at a slower rate. 

Just think of all of the batteries sitting on the shelves at the auto store.  Unless it’s a store that’s very conscientious about keeping their batteries topped off for the final customer, I would bet that you would find them all around 50-75% charged right off the shelf.

These batteries were made in China, fully charged, shipped across the ocean on a freighter, sat in a storage unit, were transported across the country, and sit on a shelf for 6 months before being purchased and now have 25-50% of their energy missing.  It happens and there isn’t anything we can do about it.

6. Low Deep depth of Discharge Too Many Times

If you’ve decided to use your car battery like a golf cart battery by “deep cycling” it, you are headed for a permanently dead battery in short order.

Car batteries are engineered to be starter batteries.  They have thin lead plates with lots of surface area and are designed to release high bursts of amperage in a short amount of time to turn over your car’s engine and electronics.  They are not designed to power your TV with an inverter for hours on end. If you do that, at least do your car battery a favor and leave the car running to let the alternator do most of the work.

A new car battery can only tolerate being fully discharged about 10-12 times before it won’t hold a charge.  A deep cycle battery (though also not healthy for it to fully discharge) can do this 150-200 times due to thicker lead plates and less surface area.

7. Sulfation within the Battery

Anytime the your car battery is not at 100% charged, sulfation is occurring within the battery itself. 

This means that little crystals are forming on the lead plates and creating a “scale” on them.  This scale inhibits the flow of electrons and doesn’t allow the battery to charge properly or release electricity properly. 

Plate on the right is sulfated with a scale of crystals over it. Photo courtesy of Wikimedia Commons.

If you are leaving your car unattended for an extended period of time (over the winter or even more than a month) it is crucial to invest in a battery maintainer to keep your battery topped off. 

Unlike a trickle charger that will slowly pump current into your car battery which will lead to overcharging, a battery maintainer operates in “float mode” and only gives the car battery the energy it needs as it naturally self-discharges. 

I have owned this Deltran Battery Tender® from Amazon for 5 years and it has never let me down for charging and maintaining my car battery or battery bank for emergency backup power.  They are gentle and never lead to overcharging a car battery to the point where it off-gasses. Best yet, they can be left connected indefinitely for long-term storage.

8. Sediment on the Bottom Creating a Short

Similar to sulfation is the sediment that builds on the bottom of the car battery and can lead to the end of your car battery’s life.

This sediment is basically the plates within the battery naturally flaking through the charge/discharge process, temperature fluctuations, and vibrations. 

These flakes can conduct electricity and start to pile up on the bottom of the battery.  Since car batteries have thin plates with lots of surface area for releasing high currents of energy in short periods of time, the plates actually extend down right near the bottom of the battery… right into this pile of conductive material.

When the sediment piles up high enough and starts to create a bridge between the plates an internal short is created inside the battery.  This will cause the battery to fail to hold a charge and release its energy internally without anything happening externally to it. 

When this happens, it is definitely time for a new battery!

9. Low Electrolyte

Constantly being exposed to the hot temperatures above the pavement, the hot temperatures under your hood, and the internal heat created by discharging your car’s battery can lead to the evaporation of the electrolyte solution within the battery’s cells.  

As this solution evaporates and is never replaces, the lead plates within the battery will be exposed to air and this will permanently damage them.  

It is best practice to check your battery fluid (if you have a flooded lead acid battery) quarterly and really it doesn’t take much time.  Simply clean the area around the cap with a damp rag, and vacuum up any bugs or debris. Then pop the cap off and inspect the levels within each of the cells. 


The level should come up to about ¼”-⅛” below the fill tube that extends down into the cell and has a “v-notch” cut out.  

Always charge your battery before you add distilled water to replace the electrolyte unless the plates are exposed.  Then, only add enough distilled water to cover the plates by ⅛” and proceed to charge. 

How to fill a car battery with distilled water.  Replenish the electrolyte when low.
The red line is the edge of the slotted fill tube. The blue is the water curve beneath it. The green is the 1/8″ space between. You can see the lead plates at the bottom of the hole, and the slot for the fill tube at the top of the hole.

Once it is done charging, fill to the appropriate level with distilled water. I then hook my charger up again after the water has been added.

Now, if you have a sealed battery (one without the caps to check the fluid), you won’t be able to replace the fluid inside the battery. 

These batteries are maintenance-free, but typically have a shorter lifespan since they cannot be topped off and they require a more sensitive charge since they are more prone electrolyte loss through a higher charge.

10. Corrosion at the Terminals

We’ve all seen the bluish-green and white flaky stuff that builds-up on the terminals of a car battery.  Topically, this corrosion creates increased resistance at the terminal connections and doesn’t allow the battery to receive current properly from the alternator and voltage regulator.

Over time, this will result in a series of impartial charges which will lead to sulfation inside the battery and we’ll be back to point #7.

In fact, when you see corrosion on your car battery’s terminals, you can almost guarantee that the issue started within the battery and worked its way out.  You’re only now seeing the visible symptoms of a problem that has been occurring for a while within your battery.

I have a full article here on understanding the corrosion on your car battery terminals, what to do about it, and what the issue is depending on what terminal has the corrosion.  

11. Loose Cables

Loose cable connections at the terminals are similar to corrosion.  They don’t allow current from the alternator and voltage regulators to flow properly into the battery, and the loose cables also create excess heat (through unnecessary lost energy from the battery) as it tries to power your car.

Remove the cables, clean them up with a damp cloth or wire brush, and make sure to get them connected tightly! 

12. Short Drives

This is a human behavior that will kill your car battery in a hurry.  If you are frequently taking small trips (5-10 minutes) and nothing more, you will gradually wear down your battery through improper charging. 

Every time you start your car, your battery basically loses 1-2% of its energy to turn over the engine (depending on the temperature). If you only drive for a few minutes the alternator doesn’t have enough time to fully replenish that loss.

Then, when you start the car up again to drive home in 5 minutes, it also doesn’t have enough time to replace the other 1-2% that you just lost.  

Over time, you’ll have your battery in a constant state of undercharge and we’ll be right back to point #7 with sulfation.

13. Extreme Temperatures

Extreme temps on either end of the spectrum can be the final nail in the coffin for an already marginal battery.  

The heat coupled with a drop in humidity will cause the electrolyte solution within a battery (flooded or sealed) to vent out at a high rate.  Unfortunately for sealed batteries, the electrolyte cannot be topped off.

Extreme cold makes a battery work harder than it does at room temperature to achieve the same result of starting your car’s engine.  The electrons just don’t want to flow as readily and the battery has to discharge itself more to get them moving.

Freezing temperatures, especially sub-zero, can also physically damage your car battery if your battery is not fully charged.

The more your battery is discharged, the more the electrolyte in the battery gets heavy on the water side of the water to acid ratio.  This watered-down electrolyte is more susceptible to actually freezing as the freezing point rises and gets closer to 32 degrees Fahrenheit.  

Once a battery freezes internally, you will be in trouble.  The expansion of the ice within the battery has likely ruined the plates inside.  Do not try to jump a frozen battery! Bring it inside to thaw as long as the case isn’t cracked.  Once thawed you can test the voltage with a voltmeter and check the specific gravity of each cell.  

14. Adding Something to the Electrolyte that Shouldn’t be in There

This one is especially discouraging if you’ve actually taken the effort to keep your batteries topped off with water.

It is recommended to use only distilled water in your car batteries, as distilled water is the process of collecting evaporated water. Due to this, hard minerals such as iron and calcium are not present and the water is as pure as can be.

If you add tap water, sure, it will work for a time. But the long term damage that is caused by adding water that has trace amounts of chemicals and mineral deposits are not ideal for the life of the battery.

At less than a dollar, a gallon of distilled water is great to keep in the garage.

Also, popping the caps and allowing dirt, bugs, and debris into the battery cells won’t do anything to help your efforts, so make sure to carefully pop them open only after you’ve cleaned around them with damp cloth and vacuum!

Bonus Tip:  Make sure it isn’t the starter!

If you go to turn your key and you don’t even hear the battery struggling, you might indeed have a fully discharged battery.  You also might have a perfectly healthy battery and just a starter that went bad.

I’ve had a few of these fail over the last 20 years and they tend to just happen out of the blue.  The battery will be fine going to the store, and when I come back to the car all I get is a single “click” when turning the key.  

Sometimes you can get an extra start or two out of your starter by giving it a gentle but firm tap with a piece of wood or the tire iron while someone turns the key. Just remember that you had better not turn the car off before you get to the repair shop or you might not get another chance!

To rule out the battery being the culprit, turn on the headlights and the brights.  See how bright they are. If they look bright and vibrant, it is probably not your battery.  

Then, have someone turn the key when you’re watching the headlights.  If they don’t get considerably dimmer when the key is turned then you are probably looking at the starter being bad.  

Getting the starter to work by taping on it while someone turns the key will positively confirm your suspicions.  

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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