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A bad alternator can drain a battery overnight, but a gradual decline in battery strength is often noticed. A bad diode inside the alternator causes this. This event is called a parasitic draw and occurs when the alternator draws power and drains the battery when the car is off.

Will It Kill My Car Battery?

Batteries are not meant to be drained fully. If they are, permanent damage can happen to the plates inside.

An alternator should not kill it if you don’t allow your battery to drain fully. There will be plenty of signs to warn you that this is about to happen unless you take action.

Unfortunately, it’s not always possible to know how close your battery is to being fully discharged unless you take a reading with a multimeter. This will give you a reading in volts.

When the car is turned off, you’re looking for a minimum of 12.5 volts. If the voltage goes below 10.5 volts when the vehicle runs, the battery has charging problems and may not be salvageable. Below 5 volts, and your battery is done.

Can a Bad Alternator Kill a New Battery?

Yes, and it happens a lot.

The three faults in the alternator that make your new battery go bad are as follows:

1. The alternator is charging the battery with too many volts. This cooks the battery cells and damages them beyond repair.

2. A ripple current causes alternating current – not the direct current needed- to get to the battery. Not great for any part of your car’s electrical system.

3. The alternator only charges the new battery sporadically and puts much more stress on it than it was designed for.

Will a Bad Alternator Diode Drain a Battery? – Parasitic Draw

The diode is meant to convert the current from AC to DC and send it to the battery. When a diode goes wrong, the battery can leak energy back the other way towards the alternator.

This is also called a parasitic draw. The alternator becomes the parasite that sucks the life from the battery. Depending on how bad the diode has gone and how healthy your battery is in the first place dictates when the battery will drain away completely.

How Long Does it Take for a Bad Alternator to Drain a Battery?

A defective alternator won’t charge the car at all when you’re driving. If you are using headlights, air-con, and the radio, these will now be powered by the battery.

This is not what the battery was invented for.

The battery is supposed to start the car and hand over all power-producing requirements to the alternator. 

A bad alternator can result in dim lights

You should expect no more than 30 mins driving time with a bad alternator. However, if you are using air-con and other high power requirement features, your battery may drain even quicker depending on how healthy it is and what charge it was holding before the alternator went bad.

You may notice the alternator getting hot as the diodes are failing inside it.

Can a Bad Alternator Drain a Battery Overnight?

Yes, even with the engine off. When your car sits at night, if one of the diodes is faulty, the charge will leak out of your battery and towards the alternator.

If the alternator isn’t fixed, you may find that the battery has become so damaged that it also needs replacing. 

How Long Will a Battery Last With a Bad Alternator?

Not very long, but it does depend on

How robust was the battery/is, and what is wrong with the alternator

Is the battery new or coming to the end of its four-year life?

Read our latest article: The 5 Reasons Your Car Battery Needs Replacing So Often

Is the alternator sporadic or failed completely? Is the diode defective and draining the battery at night? 

The alternator, if faulty, will end up killing your battery, so it needs to be fixed or replaced.

6 Signs of a Bad Alternator

1. Car warning light comes on.

It may be that the battery light never goes off when you start the car or comes on when you’re driving. Either way, your battery isn’t being or hasn’t been charged.

A dash battery warning light may be due to the alternator
A dash battery warning light may be due to the alternator

2. Headlights and internal lights are dim.

It’s not your battery’s job to provide power while driving. The alternator is supposed to take over as soon as the engine fires. If you see dim lights inside the car or the headlights, this is a sign that your battery has to provide the power because your alternator isn’t able to.

3. Weird noises from under the hood.

The alternator has a shaft that spins rapidly when the car is running. Also inside the casing are electromagnetic copper coils, brushes, and rectifiers. If one of these parts works loose, it’ll create a metallic grinding noise. The alternator can overheat if the air outlets on the casing are blocked too.

4. Car stalls or has difficulty starting at all.

Even if the car starts, if the alternator isn’t turning, there’ll be no power to run any of the components needed to keep it running.

Is the battery new or coming to the end of its five-year life?

5. Aircon, electric windows, wipers, or radio are not working.

The battery isn’t designed to provide the power for these energy-hungry extras. The battery will drain very quickly if the alternator has gone bad.

A bad alternator causing poor wiper performance
A bad alternator can cause slow wipers

It might only take ten minutes into a journey before you realize your blower isn’t working or the radio seems quieter than usual. 

6. Burning smell.

The alternator produces an enormous amount of power. A by-product of power is heating. The alternator belt spins at high speeds and, if loose, can rub as it turns.

There’s a lot of copper wiring inside the alternator that can spark if it comes in contact with the shaft.

If you smell burning, it could be that your alternator is overheating.

How Long Does an Alternator Last?

100,000 – 150,000 miles depending on the quality of the battery fitted. Also, how often it has had to provide energy while the car is being driven. An alternator on a vehicle in a cool climate-driven during the day should last longer than a car in a hot climate-driven at night that has had its air-con on and headlights on much more.

More details here : What Causes A Brand New Alternator To Go Bad? Troubleshooter and Fixes

How Much Does An Alternator Cost To Replace?

$300 – $800, which includes fitting. This depends on whether you are fitting branded or aftermarket parts and the model of the car. Taxes need to be added to this price.

In conclusion, always have your alternator checked before replacing a car battery. It can be easy to believe that the battery is at fault, go and buy a replacement only to find that the new battery drains too. You’ll have wasted $50+ for no reason.

In conclusion

Bad alternator diodes can allow charge to pass from the battery to the alternator and drain the battery. This usually happens when a car is left to sit for long periods and will eventually take all the power from the battery.

Please read our latest alternator articles and save yourself expense and time.

Do You Really Need an Expensive Alternator At All

Alternator Smoking Dangers [TROUBLESHOOTER GUIDE]

Bad Alternator Cause A Car To Run Hot and Overheat? [ANSWERED]

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