Can You Jumpstart a Car With a Bad Alternator?


When you have a faulty alternator, there is no doubt that it should be replaced as soon as possible, but what if you can’t, or at least not right away, and it has caused your battery to drain so low that it needs a jumpstart, is it safe? or can you even jumpstart a battery with a bad alternator?

You actually can jumpstart a car with a bad alternator, and the car will keep running for some time, no matter how bad the alternator is, well, considering you have a good battery. This is because if the alternator is bad and not producing the voltage required, the car will be running purely off the battery.

This isn’t recommended by any means, but if you just need to make a short drive, it may be necessary. The point of jumpstarting is to get a flat battery running,  so if you’re jumpstarting, chances are, your battery has gone pretty low. 

Since the battery will not be getting assistance from the alternator, a jumpstarted battery will go flat way faster than if it was properly charged. This, of course, can be accelerated if you have a high electrical load on such as heating, phone charging, etc.

Contrary to belief, the engine will not stop as soon as you remove the jumper cables, It will keep running until the battery voltage falls so low that any electrical systems required for operation no longer function properly.

What Are The Dangers Of Jumpstarting A Car With A Bad Alternator?

There probably won’t be any damage to the donor or recipient’s car if you are jump-starting correctly and is generally safe to do, but driving a car with a bad alternator poses a risk, especially if you have a newer car.

The alternator powers everything in your car, including the lights, AC, radio, and so on. When you first start the engine, it has the additional duty of charging the battery back up to full power. As a result, most alternators are designed to produce about 14.5-volts for a 12-volt system. Many Asian vehicles can produce much higher voltages, some even reaching 15 volts.

As you can imagine, the alternator is important as almost all car components rely on it, especially if you have a newer car, and that’s why newer cars are more at risk of component failure as they do not like lower voltages, those can include:

  • Electric Water Pumps: It is responsible for pumping water from the radiator, moving it through the motor, keeping the car running smoothly, If your water pump fails and the car keeps going, it will overheat and potentially cause serious, permanent engine damage.
  • Power Steering:  The power steering in certain vehicles is electrically driven. Steering can be difficult without that boost provided by the power steering pump, which can put you at risk, as it will be harder to maneuver or steer quickly.
  • Fuel Pump: The fuel pump is responsible for pumping fuel to your engine, and is almost certainly electric even on some old cars. If the fuel pump is unable to pump fuel to the engine, the engine will stop running.

Although your car will likely shut off before any real damage can be done. This in itself can be a devastating scenario, as the car can suddenly shut off after you’ve just jumpstarted it, If this happens, or even if it doesn’t happen, it’s better not to test your luck and slowly pull over and call a tow truck.

How Far Can You Drive With a Bad Alternator?

This question doesn’t have a clear answer. There are far too many factors that contribute to how long you can drive with a bad alternator, some of them being:

  • State of the alternator
  • State of the battery
  • Electrical Load (AC, Headlights, etc)
  • Car Age
  • Speed

An extremely rough estimation would be around 20 minutes or 13-15 miles, which can be extended if you drive slowly and keep your RPM low. If your battery is low, or you’ve just jumpstarted, it would be surprising if your car can last longer than 10 minutes or 7-10 miles.

Related: How Long Can a Car Battery Sit Unused?

How To Jumpstart a Car With a Bad Alternator 

The jumpstarting procedure with a bad alternator isn’t different from the normal procedure, but it does have a few quirks.

If your wondering how long you should leave cables connected to charge the battery with a bad alternator, then 3+ minutes should be enough to start the car.

Related:  5 Reasons Why Jumper Cables Melted

But keep in mind that if you are going to be driving, you would need to charge it for at least 15 minutes for precaution, as the battery will drain very fast with a bad alternator until it can no longer provide enough voltage to operate the car, which can be dangerous.

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0. Before Jumpstarting:

  • Make sure that both cars are separated and don’t touch each other
  • Take the keys out of the ignition for both vehicles
  • Find the positive(RED) and the negative(BLACK) wires on the battery.
  • Do NOT let clamps touch at anytime
  • Do the steps below in order

1. Attach Clamps:

  • Attach the RED clamp to the dead battery
  • Attach the other RED clamp to the good battery
  • Attach the BLACK clamp to the good battery
  • Ground the other BLACK clamp to bare metal (stud, nut, bolt head, or unpainted frame)

2. Start Engine:

  • Start the engine with the good battery
  • Run for a minimum of 3 minutes at 2000rpm
  • If your planning to drive on a car with a bad alternator, run it for a minimum of 15 minutes at 2000rpm
  • If it is colder, it may be required to run for longer
  • Try cranking the car with the bad alternator, if it doesn’t, try another 3 minutes.

3. Remove Clamps:

  • Remove BLACK clamp on the dead battery
  • Remove RED clamp on the dead battery
  • Remove RED clamp on the good battery
  • Remove BLACK clamp on the good battery 

4. Get The Alternator Fixed:

  • Don’t wait, get your alternator fixed as soon as possible. It can be a costly repair, but it’s something that can be relatively easily done by yourself or by a trusted mechanic. This will save you from a headache later on and prevent any bad consequences to both you, and your car.

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