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12 May 2025 (updated) Bad Battery or Bad Alternator? How to Tell the Difference and What to Do
When your car refuses to start or electrical components start to malfunction, the culprit is often either a failing battery or faulty alternator. Since these components work together to power your vehicle's electrical system, it can sometimes be difficult to work out what's causing the problem.
Understanding the difference between battery and alternator issues can save you time, money, and the frustration of useless repairs.
Battery vs alternator: what's the difference?
Before diagnosing the fault, it's important to understand how these two components work together but serve different functions in your vehicle.
What does a car battery do?
The car battery is the central power source of your vehicle's electrical system. It provides the initial surge of power needed to start the engine and supplies electricity to various devices, such as the lights and radio, when the engine is not running. The battery stores electrical energy in the form of chemical energy and releases it as electricity to power the vehicle's lights, accessories, and ignition system when needed.
A typical car battery is designed to last 3 to 5 years, though this may vary based on your habits, the climate conditions, and the vehicle model. When functioning properly, a fully charged battery should maintain a voltage of approximately 12.6 volts when the engine is off.
What is the purpose of an alternator?
The alternator for car acts as a generator, converting mechanical energy from the engine into electrical energy. Its primary function is to recharge the battery while the engine is running and power the vehicle's electrical systems simultaneously.
When your engine is running, the alternator should produce between 13.5 and 14.5 volts. This higher voltage is necessary to charge the battery. The alternator is typically driven by a belt connected to the engine, and most alternators last roughly 7 to 10 years before requiring replacement.
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Signs of a bad battery vs bad alternator
Knowing the specific symptoms of each component can help you pinpoint the problem more accurately.
Symptoms of a failing car battery
A battery that's reaching the end of its life will typically show the following warning signs:
- Slow engine cranking – the engine turns over sluggishly when starting
- Dim headlights and interior lights when the engine is off
- Electrical accessories work poorly or only intermittently when the engine isn't running
- Corrosion on the battery terminals – you may see a build up of white or greenish powder
- Swollen or bulging battery case – indicates internal damage
- Sulphuric or rotten egg smell – suggests that there is a battery acid leak
- Battery warning light illuminated
- Vehicle starts after jump-starting but won't start again later
Symptoms of a bad alternator
When your alternator is failing, you'll likely notice these tell-tale signs:
- Battery warning light illuminated on the dashboard
- Dim or flickering headlights while driving, especially at idle speeds
- Electrical components malfunction while the engine is running
- Strange noises from the engine bay (whining, grinding, or squealing sounds)
- Vehicle stalls shortly after jump-starting
- Burning smell from the engine compartment
- Headlights brighten when revving the engine
- Multiple electrical failures occur simultaneously
How to tell if it's the battery or the alternator
When faced with electrical issues, these troubleshooting steps can help determine whether the battery or alternator is at fault.
Battery or alternator test: how to troubleshoot
1. The jump-start test:
If your car starts with a jump starter but dies shortly afterwards, it could be because the alternator is failing to charge the battery. If it starts and continues running after a jump start, but the engine won't start again later, the battery is probably the issue.
2. The headlight test:
Start your car and turn on the headlights. If they're dim, rev the engine. If the headlights brighten significantly, your alternator may not be producing enough power at idle.
3. The multimeter test:
Use a multimeter to check the battery voltage:
- With the engine switched off, the battery voltage should read between 12.5 and 12.7 volts
- While the engine is running, the voltage should be between 13.5 and 14.5 volts
- Readings below these ranges indicate problems with the car battery
4. The accessory test:
Turn on multiple electrical accessories (radio, lights, wipers, etc.) with the engine running. If the car stalls or the accessories perform poorly, the alternator may not be generating enough power.
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How to prevent battery and alternator problems
Taking proactive measures can extend the life of both components and prevent unexpected breakdowns:
- Regular battery maintenance: keep terminals clean and remove corrosion using a wire brush and baking soda solution
- Secure the battery well: ensure the battery is tightly mounted to the vehicle to prevent damage related to vibrations.
- Check the alternator belt: regularly inspect the drive belt for wear, cracks, and incorrect tension.
- Limit short trips: short journeys may not give the alternator enough time to recharge the battery properly.
- Reduce the electrical load: avoid using multiple high-consuming accessories at once or when not required.
- Protect the unit from extreme temperatures: where possible, park inside a garage during very hot or cold weather.
- Regular voltage checks: test the battery and alternator voltage every 6 to 12 months.
FAQ
How long can you drive with a failing battery or alternator?
With a failing battery but a working alternator, you can typically continue driving as the alternator will power the vehicle's systems. However, starting the car may become increasingly difficult and eventually impossible.
With a failing alternator, you might have 30 to 60 minutes of driving time before the battery depletes completely, depending on the battery's condition and electrical needs. Minimising electrical consumption (turning off the radio or AC, etc.) can extend this time.
What happens if the alternator dies while driving?
If the alternator fails completely while driving, your vehicle will run solely on battery power until it depletes. You'll likely notice dimming lights and failing electrical systems before the engine eventually stalls. The battery warning light usually flashes up on the dashboard. If this occurs, turn off non-essential electrical devices and drive directly to a safe location or garage.
Can you drive with a bad battery but a working alternator?
Yes, you can drive with a bad battery if the alternator is functioning properly, but starting the vehicle may be difficult or require a jump start. Once running, the alternator will supply power to the electrical systems. However, this puts additional strain on the alternator and is not recommended for extended periods. Replace the failing battery as soon as you can to avoid potential damage to the alternator and other electrical components.
Understanding the differences between battery and alternator problems can help you diagnose issues quickly and take the right action. If you're unsure after performing basic tests, consult a professional mechanic who can perform more comprehensive diagnostics and recommend the most cost-effective solution.
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