Blog Car Care Advice Why Your Car Battery Won’t Charge (With Solutions)
Car Care Advice

Why Your Car Battery Won’t Charge (With Solutions)

Looking for a mechanic near you for maintenance or repair? AutoNation Mobile Service brings the shop to you. Get a free instant quote today.
Get a Quote

Having trouble with your car battery?

If your car battery won’t charge, there won’t be enough power supplied to the ignition system, and your car engine won’t start. 

So, why won’t your car battery charge?

And more importantly, what can you do about it?

In this article, we’ll go over all the possible reasons why your car battery won’t charge. We’ll also point you to an easy solution to car battery issues and cover some battery-related FAQs.

This Article Contains

Let’s get started.

8 Reasons Why Your Car Battery Won’t Charge 

A battery that won’t charge can be frustrating, and your first instinct would probably be to replace it. However, it’s first important to determine why it won’t charge as replacing the battery may not always be the right solution.

Let’s go over some of the possible causes to help you determine what to do:

1. Faulty Alternator 

The car’s alternator charges the battery when the engine is running. 

If you have a bad alternator, your car battery doesn’t get charged properly.

While the alternator can last much longer than a lead-acid battery, it still has a limited lifespan. It may need replacing once every 7 years or every 80,000 miles.

What are the symptoms of a bad alternator?

A faulty voltage regulator can also mismanage the current between the alternator and the battery.

2. Wiring Problems

Engine vibrations may loosen battery connections. And a loosened or damaged battery cable can reduce the electrical contact between the alternator and battery. 

Other wiring problems like an ungrounded neutral can also create electrical problems in the charging system.

3. The Headlights Were Left On

Leaving the headlights on is a common culprit if you have a flat battery

Remember, if left on, the headlights will draw power from the car battery even when the engine is off, draining it entirely. 

4. Power Drain On The Battery

Car accessories, like interior lighting or Bluetooth kits, generally shut down when the engine is off. If these don’t power down correctly, they can drain the battery. 

Some components may require a live connection to retain specific settings and can create an abnormal draw on the battery, and result in battery failure. 

5. The Battery Is Too Old Or Damaged

The car lead-acid battery typically lasts 3-4 years. 

When a battery is too old or damaged, it can bloat, develop cracks and leak battery acid, or fall prey to corrosion. Extensive corrosion on a battery terminal reduces electrical connectivity and charging ability.

Older batteries can also suffer from sulfation, where the internal plates in the battery cell are damaged. Every lead-acid battery is subject to sulfation, though you can reverse it at earlier stages. 

6. Damaged Alternator Belt

The alternator belt (also known as the serpentine belt) drives the car’s alternator

It allows the alternator speed to follow the engine speed.

Sometimes, the belt can get loose or frayed, slipping off the alternator pulley. 

If this happens, the alternator can’t keep up with the power needed by the engine. 

7. Faulty ECU

Most modern vehicles have an onboard computer called the ECU (Engine Control Unit), which manages each electrical system in the car.

A faulty ECU can lead to problems with the charging system. 

Errors in the ECU can come with symptoms like engine stalling or the Check Engine Light turning on.

If you suspect a bad ECU, get a mechanic to diagnose it immediately.

8. External Battery Charger Issues

If you use an external battery charger, make sure it’s connected correctly. Ensure that the positive lead goes to the positive terminal and the negative lead is on the negative terminal. 

Some car batteries require a specific charger for charging, so the battery charger may not be the right type or could have even developed faults. 

Any mix-ups here and your battery won’t be charged effectively.

As you can see, there are tons of different reasons why your car battery won’t charge.

So, what’s an easy way to get things diagnosed and fixed? 

An Easy Solution To Your Battery Charging Problems

A bad battery might be obvious, but determining the root cause of battery failure can be difficult. Your best bet is to get your entire electrical system inspected to be safe.

In this case, find a good automotive technician to deal with your charging issues. 

Make sure that they are:

And you’re in luck because AutoNation Mobile Service ticks all those boxes!

AutoNation Mobile Service is a convenient mobile vehicle repair and maintenance solution. 

Here’s why you’ll want to consider them first for your repairs:

For an accurate estimate of starting and charging repair costs, just fill out this form.

Now that you know what causes charging issues and how to fix them, let’s go over some FAQs. 

Here are the answers to some battery-related questions you may have.

1. What Happens When The Car Battery Won’t Charge?

If your car battery won’t charge, your car won’t start.


There isn’t enough power supply for the spark plugs and ignition system to engage. 

2. How Does The Car Charge The Battery?

The standard car battery is a 12-volt battery (12V battery) with six cells. 

Each battery cell holds 2.1 volts at full charge.

When you turn the key in the ignition, the battery routes the voltage to the starter motor, and the engine starts. The battery also provides the initial spark to the spark plugs in a petrol engine or powers the glow plug heater in a diesel engine (in subzero temperatures).

Once the engine is running, it drives the alternator via the alternator belt, converting mechanical power to electrical power, thus charging the battery.

3. How Can I Check For A Flat Battery?

If your car won’t start and the headlights are off, turn them on to check for a dead car battery. 

Here’s how to tell:

If the headlights provide full brightness, the issue isn’t the battery, it’s likely a bad starter or faulty wiring in the electrical system.

If the headlights don’t turn on or are dimmer than usual, you might have a bad battery.

4. How Does A Mechanic Check The Battery Charge?

Your mechanic can use a basic voltmeter to check the battery voltage. 

Here’s what they’ll do:

The reading for a fully charged battery should ideally be 12.6V +/- 0.2V

If it’s at 12.4V, it’s still considered a healthy charge but isn’t functioning optimally.  

If it’s 12.39V or less, the battery isn’t fully charged.

If it’s over 12.9V, your battery likely has excessive voltage.

The voltmeter only provides the battery’s ‘state of charge’ — which indicates the level of charge a battery has relative to its capacity. At over 12.4V, a car battery capacity is at 60% or more.

For a more comprehensive view of battery health, they may use a battery tester that has a load tester to monitor battery voltage and current output.

5. Should I Replace A Battery That Can’t Hold A Proper Charge?

Yes, you should have it replaced with a new battery.

If the battery can’t charge up properly, it’s bound to fail at some point. 

It’s better to be proactive and replace it now than to end up with a dead car battery when you least expect it.

6. How Does A Mechanic Check If The Alternator Is Working?

Your mechanic will use a voltmeter and the process is similar to checking the battery charge. However, this time, the engine will be running

Your mechanic will connect the voltmeter to the battery. The voltage reading should be 14-15V for most cars. If it’s less, it means the alternator isn’t generating enough power for adequate battery charging.

7. If The Alternator Is Faulty, Will A Jump Start Work?

Yes, if you have a faulty alternator, a jump start will work (whether from a jump starter or a donor car). 

Just make sure the jumper cables are connected correctly.

However, if your alternator is faulty, your battery won’t last long, and your car will eventually stall. You might be able to drive a short distance if all unnecessary electrical systems are off. 

That’s why it’s just better to get a mechanic to come over and handle your alternator and battery issues.

8. What Is A Battery Maintainer?

The battery maintainer (or battery tender) maintains the charge in a good battery. It can act as a float charger to bring voltage levels back to optimal if they drop. 

It’s typically left connected to a stationary vehicle that is seldom driven, or used to deliver sufficient battery power for a car parked overnight. 

However, it’s important to note that it can’t recharge a dead battery.

9. Is A Deep Cycle Battery The Same As A Car Battery?

No, these batteries are different. 

While they’re both of the lead-acid variety, a car battery is designed to provide a short, high current burst of power to start a car’s engine. Only a small charge is used, which is later replenished by the alternator. 

The deep cycle battery is designed to deliver sustained power with a lower current draw over extended periods. It’s typically used in boats and is also called a marine battery.

10. If My Battery Isn’t Dead, Why Won’t My Car Start?

If it’s a relatively new battery that hasn’t been used for days or weeks, it might have just lost all its charge and just needs recharging. 

Alternatively, a blown fuse could stop the car from starting. 

Fuses can become brittle and worn with age, and cranking a cold engine may blow them. 

A problem in the fuel management system, like a clogged fuel filter, could be a culprit too.

Final Thoughts

There are a lot of reasons why your car battery won’t charge, and determining the exact cause is your best bet at finding a solution that works. 

That’s why, the next time you have battery issues, save yourself the hassle of troubleshooting things yourself. Contact AutoNation Mobile Service, and ASE-certified technicians will be at your doorstep in no time, ready to do the figuring out for you!