It can always be a worry when you have an unexpected light show up on your dashboard. Your worries are more than doubled when you also don’t understand what that light indicates.
So, in this article, not only will we talk you through what a battery light is, but also what the top 8 most common reasons why it may show up whilst you’re driving.
Your battery light DOES NOT always indicate whether or not your battery is faulty. It is a simple indicator to tell you whether or not your vehicle’s alternator is charging your battery.
As your alternator is driven by the auxiliary drive belt (serpentine belt), it generates an electrical charge designed to run the electrics on your engine whilst you drive and recharge your battery. Your battery needs recharging as it loses a percentage of its charge every time you start your vehicle.
Therefore, if your vehicle detects that its battery is no longer being charged, it will illuminate your battery light as you’re driving.
Causes of Battery Light On While Driving
- Broken Auxiliary Drive belt (Serpentine Belt)
- Faulty Alternator
- Broken Alternator to Dash Cluster Wire
- Faulty dash cluster
- Faulty Car Battery
- Corroded battery terminals
- Broken/ Corroded Engine Earth Strap
- Worn Alternator Pulley
There are actually quite many things that can cause your car battery light to come on.
Here is a more detailed list of the 8 most common causes of why your car battery warning light is on.
Broken Auxiliary Drive belt (Serpentine Belt)
This is by far the most common reason for your battery light to illuminate whilst your driving. If your auxiliary drive belt has broken, then it’s no longer able to spin your alternator, meaning there is no electrical charge being generated in which to charge your battery. Your vehicle is smart enough to recognize this as a fault and tell you through the battery light.
If your belt has snapped, you will usually also notice a couple of other symptoms to indicate this, such as loss of power steering and loss of air conditioning.
The process for diagnosing this fault couldn’t be simpler, carry out a visual inspection on the engine and it should be immediately obvious if your belt has snapped. As long as your vehicle hasn’t been driven for a prolonged amount of time without a belt, you should be able to get away with just replacing it and driving as normal.
Top Tip: Make sure you remove ALL debris from the old belt that may have gotten lodged in various parts of your engine bay as this could cause you problems down the road if it gets caught in pulleys or cooling fans.
You can find more information about how to diagnose it here: Serpentine belt symptoms
So, if your serpentine belt seems to be fine, the next component we look to is your vehicle’s alternator. Your alternator is a fairly complex unit with multiple internal components that could go wrong. From faulty diodes to broken windings, they will all cause your alternator to stop charging at the current rate that your vehicle requires.
This is quite a technical diagnosing process, however, if you’re not comfortable working with electrics on a running engine then take your vehicle to a local garage. They will often have a specialized tester that will be able to easily diagnose your alternator for you at a small fee.
You can find more information about how to diagnose and check your alternator here: Alternator symptoms
Broken Alternator to Dash Cluster Wire
This is a fault that can be a bit more tricky to determine, but I have seen it happen many times. The wire that feeds your dash cluster is often only a small 15 amp cable, making it prone to wear and corrosion. This wire will often be connected to the alternator via a standard block connector and will then run either directly to your dash cluster, or it will run through an ECU.
The easiest way to determine this fault is the process of elimination. Have your battery and alternator tested to make sure that they’re both operating as they should be. If no fault is found with these, then you will need to check this wire next. Carry out a visual inspection on the block connector fitted on the alternator’s back and make sure a good connection is visible. Next, carry out a visual inspection on the actual wire to check for corrosion or breaks in the cable. If any are found, repair and run your vehicle to make sure the battery light extinguishes after starting.
Faulty dash cluster
Although this fault lies towards the more uncommon end of the list, it still happens frequently enough to make it onto here.
Usually, with a faulty dash cluster, you will also notice other faults such as faulty odometer readings or other illumination lights when they shouldn’t be and vice versa.
In order to determine if your dash cluster is faulty or not, you will usually have to have it bench tested by a specialist.
Another common problem that would be indicated by your battery light illuminating whilst you’re driving is that your battery may be faulty. A shorted cell in your battery could prevent it from accepting the electrical feed from your alternator and, therefore, will not be getting charged. This fault may also be shown by your vehicle being slow to start, especially on colder mornings.
In order to diagnose this fault, you will need to take your battery to get tested. A simple multimeter will not always indicate a shorted cell within a battery. Most garages will have an appropriate battery tester however and will carry out the test for a small fee.
Corroded Battery Terminals
Corroded/ loose battery terminals can cause a whole range of peculiar faults on your vehicle. Whilst it’s unlikely to, it can sometimes be the case that it will illuminate your battery light whilst your driving.
Carry out a visual check on the battery terminals to check for corrosion and to ensure the connections are tight. Top Tip: Spray a small amount of clear grease onto your battery terminals to help prevent the build-up of corrosion.
Broken/ Corroded Engine Earth Strap
Similar to the loose/ corroded battery connections, a broken or corroded earth strap can also cause a whole range of strange faults on your vehicle. It’s a very uncommon but not unheard of fault that’s actually quite easy to check for. You can often find one between the body and the engine and one between the car battery negative terminal to the body.
Simply carry out a visual inspection of your vehicle’s earthing straps and check for any breaks or tears. At either end of the straps, it’s important that you check for a build-up of corrosion also as this could cause a poor earth connection. Top Tip: Remove fixing bolts and clean up the contact points with a fine piece of emery paper to ensure a good connection. Then refit the strap and fully torque the fixing bolts.
Worn Alternator Pulley
This is a fault that is uncommon on most cars but quite a common occurrence on the German car models. Some of these alternators feature a specially designed pulley that is designed to only spin one way. Whilst this design is great for preventing damage to the alternator; it can be an awkward fault when it goes wrong.
To diagnose, simply remove your auxiliary drive belt and try manually turning your alternator pulley hand. It should spin freely in the direction of rotation. If not, then the pulley will need replacing; this can often be done without replacing the whole alternator assembly.
So, if your battery light illuminates whilst you’re driving, just be aware that your vehicle won’t run for very long – depending on battery health and what accessories you have switched on.
Pull over as soon as it’s safe to do so and have your vehicle towed home or to a garage because by continuing to drive you run the risk of causing further damage to your engine.
It’s important to have your battery regularly tested as it can lower the chance you get caught out with a fault such as this.
We hope this list can be of some great help for you, so please feel free to refer back to it when you need to.
Share this Post
Founder, owner & main author of Mechanic Base. I have been repairing cars for more than 10 years, specialized in advanced diagnostics & troubleshooting. I have also been a drifting driver and mechanic for over 7 years.