How To Test Alternator By Disconnecting Battery: Step By Step Guide

• Ibro Cehic

Updated on September 23, 2022

There are several ways to test an alternator, from the most common multimeter test to a more advanced bench test. However, there is still one test that few people know, young people at least, and that’s by disconnecting a battery terminal.

So, here is how to test the alternator by disconnecting the battery. First, pull the hand brake to secure the car and start the engine. After that, disconnect the negative battery terminal while the engine is running, and if it turns off, the alternator is faulty.

The test itself is pretty straightforward. However, it’s questionable how safe and how accurate it is. So, even though you know how to perform the test, the question of whether or not you should do it remains. Well, stick around to find out and see other better methods you can try.

What Is an Alternator

The sole purpose of an alternator is to charge the car’s battery while you’re driving. It also supports electronic auxiliaries like headlights, radio, and other sensors or screens your vehicle might have.

Rotor and Stator

Essentially the alternator is composed of a belt-driven rotor (magnet) and a copper winding stator around the magnet. Move a magnet through a copper coil, and you make an electric current. Simple.

Voltage Regulator and Alternator Brushes

A more complicated component of the alternator is a voltage regulator. The name itself implies that it regulates the voltage going to the battery. If this component fails, the alternator can not only undercharge but also overcharge the battery.

The regulator uses alternator brushes to increase or reduce the voltage supplied to the rotor, which in turn raises or lowers the alternator output. The brushes are in constant physical contact with two field coils mounted directly on the rotor.

Also, the voltage regulator protects the whole electrical system against voltage spikes, which can be devastating for the car.

Alternator Diode

Since an alternator makes AC current and the vehicle’s electrical system runs on DC, the current needs to be converted. An alternator uses a rectifier, essentially a system of diodes connected in a series, to do the job.

Faulty Alternator Symptoms

We can divide faulty alternator symptoms into two groups, overcharging and undercharging. With the disconnecting battery terminal alternator test, you will only test undercharging. However, with a multimeter test that I will also explain, you can test both, so if you can, go with the multimeter test.

Alternator Undercharging Symptoms

• Headlights and interior lights are dimmer than usual. This one is self-explanatory; when you start the car, the alternator isn’t making enough current to power the engine (ignitions and fuel system), so the auxiliaries will suffer. Also, if you have LED light bulbs, they might flicker.
• The engine is harder to start, or it cranks slower. Since the alternator isn’t making enough current while driving, the vehicle uses the battery reserve to compensate. That means the battery charge will get lower and lower each time you drive the car until, eventually, it can’t turn the engine, and you cannot start. The vehicle also might stall while driving and leave you stranded.
• Check engine light and other unexplainable electronic gremlins in modern vehicles. You will probably first see a battery warning light and soon after a check engine light when the sensors start to malfunction. Also, if you have a modern car with screens and microchips for everything, it can cause a slew of different symptoms.

Alternator Overcharging Symptoms

• The headlights and interior lights are brighter than usual. When the voltage regulator isn’t doing its job, too much current gets to all the lightbulbs, and they burn out faster. So if you have to change your headlight bulbs more often than usual, this is likely why.
• Check engine light or battery warning light illuminates. Again, you will probably first see a battery warning light, and you can also encounter all kinds of issues if you have a modern vehicle.
• You may notice a buzzing noise in older cars when listening to the radio that raises and lowers with engine speed. You might also hear it only after a cold start for a short period.
• A sulfuric smell (rotten eggs) and a bulging car battery. If you notice any of the two, your alternator has been overcharging for months or making enough current to explode the battery. Well, maybe not that much, but it’s extreme, and you should take your car to a repair shop immediately.

Should You Disconnect the Battery to Test Your Alternator

The short answer is no, it will reveal almost nothing, it’s bad for the vehicle, and it’s a safety hazard.

What Does It Reveal

First of all, if you notice any undercharging symptoms, there is nothing to test; the alternator is faulty.

On the other hand, if it’s overcharging, disconnecting your battery won’t reveal anything, and the car will continue running as it would when the alternator is in good working order.

Moreover, the alternator could produce enough current to power the engine but not other electrical components. That means the engine can continue running when you disconnect the battery but still be undercharging, so again, you won’t find out much.

Potential Damage

Now, disconnecting the battery while the engine is running can produce a voltage spike. As we mentioned talking about the symptoms, a voltage spike can ruin your vehicle’s electrical system. If you have a modern car, that could turn into a repair that will cost you thousands.

Safety Hazard

As for safety, all batteries produce highly flammable hydrogen gas, although a healthy battery doesn’t make enough of it to cause a safety hazard in normal conditions.

However, when you take a battery terminal off a running car, chances are high that it will make a spark.

Now suppose you have an old battery that makes more hydrogen gas than average, or the alternator is overcharging, causing the same effect, and you could be facing a mistake that will ruin your life.

To Summarize

Disconnecting the battery terminal off a running car is a simple field test decades old, which can’t cause much damage to older cars.

Still, it will only work and reveal something valuable if the alternator isn’t charging at all or it’s undercharging to such a degree that there are numerous other symptoms you will notice beforehand.

Since multimeters are dirt cheap and easy to use, honestly, there is absolutely no sense in putting yourself and your vehicle in danger to find out almost nothing. However, if you still want to try it out, here is a quick guide. Otherwise, you can skip to the multimeter guide.

How to Test the Alternator by Disconnecting the Battery

1. Put the vehicle in neutral and secure it with the handbrake
2. Start the engine
3. While the engine is running, remove the negative battery terminal
4. If the engine shuts down, the alternator is faulty. Unfortunately, it can still be defective if the engine doesn’t shut down.

How to Test an Alternator With a Multimeter

Testing an alternator with a multimeter is superior to disconnecting the battery in every way imaginable. You can pick between battery or alternator tests, and it will reveal both undercharging and overcharging.

Moreover, there are no safety hazards, and you are not risking causing any damage to your vehicle. Sure, a multimeter isn’t free, but it won’t bankrupt you, either. In any case, it’s better to borrow one from a friend than to perform the previous test.

Step-by-Step Guide:

1. First, set your multimeter to DC voltage designated with “DCV” or a symbol. The symbol is a “V” with one full line and one dotted line below it. Also, set it to more than 15 volts; for most multimeters, that’s DCV 20.
2. After that, test the battery before starting the engine. Put the black probe on the negative terminal and the red one on the positive terminal. The multimeter should read around 12.5V to 12.8V. If it’s below that, it’s a good idea to charge your battery; otherwise, you might get an inaccurate reading.
3. Now, turn the engine on and place the probes as they were, black-negative, red-positive. If the multimeter reads between 13.5V and 14.5V, the alternator is fine. However, if it goes below 13V, it’s undercharging; if it’s over 15V, it’s overcharging.
4. Suppose you get a good reading right off the bat. In that case, it’s a good idea to turn on as many auxiliaries as possible, radio, fan, headlights, etc. The voltage should drop after that, but the alternator is faulty if it goes below or over normal values (13V or 15V).

FAQs

How Do You Tell If It’s Your Alternator or Your Battery?

If it’s your alternator, you will probably be able to start the car, but it won’t keep running, or it will turn off eventually. If it’s your battery, the car will start with jumper cables and drive until you turn it off. After that, you won’t be able to start it again.

What Causes the Alternator to Lose Voltage?

One of the causes for the alternator to lose voltage are bad or loose ground connections. Otherwise, it could be voltage drops on either of the battery terminals.

Should the Alternator Charge at Idle?

Yes, the alternator does charge at idle if everything is in good working order. It doesn’t charge as fast as at high RPMs, but it’s still charging.

At What RPM Does an Alternator Charge?

An alternator charges at all RPMs, starting with idle speed. However, it charges the battery faster at higher RPMs, and the ideal speed is between 2000 and 2500 RPMs.

Is an Alternator AC or DC?

A car alternator generates AC current, which then the alternator’s rectifier converts into DC.

What Can Damage an Alternator?

Many things can damage an alternator, such as a bad bearing, a tight or loose belt, leaking fluids (oil, fuel, and coolant), and wrong use of jumper cables.

Do Alternators Fail Suddenly?

Alternators can fail suddenly or over a longer period. A voltage regulator, diode, brushes, brass rings, terminals, and ground wires can all cause a slew of mild and severe symptoms.

Even with all the symptoms, you can often drive for a month or more. However, each of those can fail suddenly as well.

Can an Alternator Be Repaired?

Yes, an alternator can be repaired. You can replace everything from the stator and rotor to brushes, diodes, and voltage regulators.

Conclusion

In the end, I would once more advise you not to test your alternator by disconnecting the battery while the engine is running. But, if you still decide to do it, it’s pretty straightforward, so you can refer to the step-by-step guide and go ahead.

However, by testing an alternator with a multimeter, you will get an immeasurably better picture of its condition and avoid the risk of battery acid on your face.

Using one is a tad more complicated, which is why we made a step-by-step guide for it, too, and it will cost you some pocket change, but the benefits are worth it.

Here are some other articles to read:

Alternator Product Cross Reference Guide

Battery Light On But Alternator Is Charging – Why Is This Happening?

How Long Does It Take to Replace an Alternator?