Last Updated on February 28, 2023 by Bernard Juchli
It turns out your newly bought alternator is not the problem. So, why is the alternator not charging battery but alternator is good? This publication makes explanations that are not far-fetched for this problem.
When the alternator does not charge the battery, your vehicle may not start. This is because the battery received no juice during the recent driving to be able to start
It is possible that the alternator is not faulty. As such, a replacement alternator is a waste of money and time. You want to tell the actual fault to be able to get the car battery charging again.
Reasons for alternator not charging battery but alternator is good
This section explains the common reasons your battery will not charge even though diagnosis confirms that the alternator is working:
Failing battery terminals or wirings
The battery terminals or wirings can be the reasons your new working alternator is not charging the battery.
Car batteries typically use 3-4 wires, helping in power transmission from the alternator to charge the battery.
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The battery terminals degrade with time and corrode, preventing the battery from charging. Poor or loosened wiring connections can also be the reason the alternator will not charge the battery.
You may have to check your battery regularly to know if it is charging or install an automatic controller with remote notification.
Fortunately, you can tell if the problem has to do with the battery terminals or wirings. This diagnosis may be tougher on a modern vehicle since the battery is embedded in tight areas within the engine bay.
You would require vehicle scan tools and the correct repair manual for this complex diagnosis.
Remove the plastic coverings to access the battery, depending on your car. Refer to the automaker’s handbook for guidance.
Visually inspect the battery terminals and wirings to know if it is the reason for the alternator not charging battery but the alternator is good.
How to fix
You want to inspect the drop in voltage along the wires with a voltmeter or a multimeter:
- Set your multimeter to voltage.
- Put the car engine on idle.
- Measure the voltage with each wire.
If the voltage reads more than 0.2 volts in any wire, it indicates a problem with that wiring connection.
After confirming that the wiring or corroded terminals connected to the battery are the problems, do the following to fix it:
- Clean the corroded surface using sandpaper and a wire brush.
- Try to tighten any loose connections.
- Replace the wiring (if damaged).
You should get a voltage drop of 0 or below 0.1 when fixing the wires.
While you are focusing on the alternator, the battery could be the culprit. It is either the battery is aged, weak, and can no longer hold a charge or after-market accessories are draining the power faster.
Accessories, including radio, GPS, Bluetooth, horns, music system, and charging outlets can cause battery draining if not properly connected to the electrical system of the vehicle. Turning on all the accessories at once can also drain the battery faster.
You can tell if the battery is the reason the alternator is not charging the battery but is working well.
You need an automotive multimeter or a voltmeter to determine the current of the battery to tell the condition.
Typically, you should get voltage between 12.5 to 12.8 volts when the engine is off. Turning on the engine should not also exceed this point unless the battery is bad. Do the following:
- Turn off the lights and other accessories in the car. Meanwhile, if the car lights will not turn off, here are the causes and solutions.
- On the voltmeter, connect the positive lead to the positive (red) terminal of the battery, and the negative lead on the voltmeter to the negative (black) terminal of the battery.
If the voltmeter reads below 12.5 volts, the battery is bad. If it is a new battery, the power may have been drained.
How to fix
If you have diagnosed the problem of the alternator not charging to be the battery, you need a replacement battery.
You can try jump-starting the battery to recharge for you to drive the car. If you experience melted jumper cables, here is how you can fix them.
Blow alternator fuse
The alternator fuse is located in the engine bay, connected to the power cable of the alternator. The alternator fuse is responsible for protecting the alternator from high voltage or current overflow that can damage it.
A surge in power can damage the fuse. The alternator fuse also brittles, wears, and damages with time. When failing, a blown fuse prevents the current from flowing to the battery from the alternator, stopping the battery from charging with the alternator in good condition.
You want to regularly check the battery charge controller and the health status of your car battery before inspecting the fuse.
You can tell a blown alternator fuse is the cause of the alternator not charging the car battery even though in good condition. Do the following:
- Turn off the engine and open the bonnet.
- Find the fuse box using the automaker’s manual to tell the correct location.
- Check the fuses in the box carefully following the diagram in the manual because some vehicles use as many as 40 fuses or more.
- Find the fuse connected to the alternator and remove it using a fuse plier.
- Inspect the fuse.
Any fuse with a melted wire is a sign that it is blown, causing your new alternator and battery not to charge.
How to fix
If a blown fuse is the reason your car alternator is not charging battery but the alternator is okay, you need a replacement fuse.
Check to make sure the replacement fuse uses the same voltage and is similar to the blown fuse. The replacement fuse should fit the position of the former.
After replacement, restore the fuse box and scan to test the electronic system before starting the car.
Malfunctioning serpentine belt
The serpentine belt is a single, continuous belt that drives multiple components in a car engine, including the alternator, power steering pump, water pump, air conditioning compressor, air pump, and more.
An idler pulley and belt tensioner guide this belt. It is also responsible for converting mechanical energy to electrical energy.
The serpentine belt is made of rubber, so it wears with time due to exposure to heat, and wear and tear. A worn serpentine belt will no longer grip firmly on the alternator pulley, causing a slip.
This slip prevents the alternator from being fast. As such, adequate power is not generated to keep up with other electrical systems.
You can tell if the serpentine belt is the reason your car alternator will not charge the battery even though it is new.
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Typically, you get a screeching sound from the engine pulleys, which indicates a failing serpentine belt.
Visually inspect the serpentine belt for slippage. If you notice a slip, the alternator is unable to spin with the engine, causing the battery charging problem despite a working alternator.
How to fix
The solution is to get a replacement serpentine belt. Do the following to replace the belt yourself:
- Turn off the engine and wear your PPE, including gloves and goggles.
- Open the engine bonnet or hood and remove any plastic coverings blocking access to the engine.
- Locate the belt. Get a wrench to loosen the belt tensioner holding the belt firmly.
- Place the replacement serpentine belt, making sure the specifications are similar to the old one.
Ensure that the pulleys align with the belt grooves.
Damaged voltage regulator
The voltage regulator ensures adequate electrical power flow through the electrical components of a vehicle. It is responsible for maintaining a predetermined charging voltage for the battery of a car.
If the voltage regulator fails, the battery will not charge. The electrical system will find it difficult to function or work at all, allowing too much power to the battery, which damages wires or even destroys the battery.
A failing voltage regulator will also keep the alternator from supplying enough power to the battery, causing the battery to undercharge or not charge at all.
You can diagnose the voltage regulator if you suspect it to be the reason for a good alternator not charging the battery. You need an automotive multimeter to test the voltage regulator connected to the alternator. Do the following:
- Put the car in neutral and open the bonnet.
- Set the multimeter to voltage.
- Remove the plastic bag on the terminals (if any).
- The multimeter has a red and black cord with clamps. Attach the red clamp to the positive terminal of the battery, and the black clamp to the negative terminal of the battery.
The multimeter should read about 12.5 to 12.8 volts when the car is off. When the car is turned on, the multimeter should read around 13.8 volts.
Get someone to rev the engine while you read the multimeter. Your assistant should increase the vehicle RPM up to 1,500 to 2,000. If the multimeter reads more than 4.5 volts, the voltage regulator is bad.
How to fix
If the voltage regulator is the culprit, adjust the voltage output on the regulator or get a replacement voltage regulator.
Check the owner’s manual to determine if your vehicle has an adjustable voltage regulator. Find the adjusting screw on the alternator and modify the charging voltage output.
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Meanwhile, replacing the regulator involves removing it from the alternator. Follow the automaker’s manual to correctly replace the damaged voltage regulator.