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Voltage regulator/charging problem

Looking for second opinions or other leads to investigate for a charging problem.

On the return leg of a cross-country trip, the battery in my 2007 Kia Rondo died. I was able to limp to a parts store, where they checked the battery, found it marginal, and replaced it. The battery was three years old. I got back on the road and drove for an hour before the same problem cropped up again. Limped the car off the highway and got it to a service station. After a jump start, the mechanic there put a voltmeter across the battery and checked voltage at idle, under acceleration, and under load (headlights, blower motor, wipers, etc). The voltage started around 13.6 V and dropped rapidly to about 6 V. He diagnosed it as a bad alternator - specifically the voltage regulator within the alternator.

Since I was eager to get home, I found a rebuilt alternator at Carquest and got it for him while he started getting the old one out. He was a little skeptical about a rebuilt alternator, but I couldn’t get a new one quickly enough for my needs.

He put in the new alternator, charged the battery, and re-started the car. Voltage across the battery terminals started at 14.2 V, then dropped gradually to 13.9, 13.6, 12.6, and so on. After a few minutes, it would only get up to 14.0 under hard acceleration. The mechanic declared that the voltage regulator in the remanufactured alternator was also bad, and that this is a common problem with reman alternators.

Was something else wrong to begin with? Is something else frying the regulator?

(I don’t know this mechanic, but I got the chance to talk with a lot of his customers while i hung around his shop waiting on the repair. Numerous loyal return customers who spoke highly of him, including one who said she was his very first customer 35 years ago. He seemed to me honest and knowledgeable.)

I have to agree with your mechanic and would never purchase a rebuilt alternator. As well as the factory wants to rebuild it, there is not sufficient time for extensive testing. Voltage regulators may work just fine for a few minutes or even hours but gradually fail as winding and other parts heat up.

As long as you have 12V to start the engine you should be fine, but have to keep a close eye on it and maybe pack a jumper cable on your next trip. Get yourself something like this for $10-15 to monitor voltage in real time. Plugs into your accessory outlet.

Thanks for that validation and voltmeter suggestion.

Before undertaking the labor of removing the alternator again, I want to make sure he has run down other leads.

I’m going to ask him to check grounding issues between battery and chassis, alternator and chassis.

Is there a critical voltage that the alternator needs to get from the battery? Any signal that the computer should send to the alternator? Would checking the OBD reveal anything?

There is no voltage regulator in the alternator for your vehicle.

The voltage is regulated by the Powertrain Control Module. (computer)


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That is good to know. I wish I had known before the first alternator replacement. More to the point, I wish the mechanic had known!

true, but not the whole story.
ALL alternators have built-in voltage regulators, including the PO’s car, even if the ECU regulates the final voltage to the electric system. This is to prevent an accidental flux in voltage and amperage. The ECU controls only voltage but not amperage. This can happen if the positive battery cable is removed while the engine is running and potentially causing an influx of over 100 volts. That’s not what the ECU would like to experience even fused.


The battery does not supply voltage to the alternator. It’s the other way around. Rectifiers (diodes) in the alternator prevent current to backflow from the battery to the alternator.

Okay, that all makes sense. Thanks all.

Should I have the mechanic check the ECU?

I don’t think there is nothing wrong with telling the mechanic that you went to a forum to “educate” yourself and this and that is what you heard.

I would be reluctant to direct him/her to perform work that no one here is certain to solve your problem; we can’t touch your car from here. My suggestion is to let him test the alternator and if bad to replace it. If good, go on and test other sources, including ECU, but I highly doubt it is the ECU.

This said, I would not replace the alternator unless it charges less than 12 - 12.5 volts. That’s my opinion.

At normal temperatures, the voltage would be at least 12.6 and as high as 14.5 volts. Less than 12.5 means the battery is not being charged at all.

I think you should take this to your local mechanic and explain what the problem is. Let his test the system and if he finds that the alternator is bad, you can take it back to any “Carquest” with the receipt and get you money back.


If the batter is charged at 12 or 12.5V (12.5 or 12.6V don’t make any difference whatsoever, it’s a myth) and maintains the charge it will start an engine very nicely. You don’t need more than 12V to run a car’s electrical system. It’s the amperage that makes a difference at engine start.

"12.5 or 12.5V don’t make any difference whatsoever"
very true.

But a good battery has a no load (or light load) voltage of pretty close to 12.6 volts. 12.0 indicates a problem. And when it is being charged, it goes up from there, how high depends on the charging current.

I don’t know about anyone else, but a battery that shows 12 VDC is by all practicable purposes a dead battery.


Yah, it’s on the Internet, thus it must be true.

Read the article again. The author makes a generic statement in the beginning, claiming that 12V battery is literally dead. In later sentences he lists a number of symptoms of a dying or dead battery that have nothing to do with the 12V claim. That’s a typical method of misleading people of cause and affect. However unintentionally.

A battery is not dead as long as it starts your engine and no one can tell for sure if it will fail to do so in a day, a week or a year.

When a customer comes into your shop with the symptoms described in the article and you find a failed battery you really don’t know if it was running on 12V or below for a day, a week or a year. You assume it just failed because it is dead.

My old 1999 Dodge Durango 5.9L has a “lazy” alternator and is unable to charge above 12.5-12.8V. The battery typically shows around 11.8V when I get to drive the car. It has started the car faithfully for almost 2 years now. (I do live monitor battery voltage and have jumper cables in the back).

This said, I do understand why professional auto mechanics prefer to replace a battery that may or may not fail soon. They will likely get blamed for not doing anything in advance to avoid this.

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That is a good example of a PCM controlled charging system. The alternator has no voltage regulator, the PCM directly controls the alternator field winding to achieve the desired voltage. A charging rate lower than 13.1 volts is very unusual, the ambient temperature would have to be greater than 120F, I believe your volt/ohm meter is inaccurate.

Nah, long time ago, I pulled it out and bench tested it. It’s the original alternator, just getting real old. I am just seeing who lives longer, me or the alternator :grin:

It is much better to test an alternator as installed in the vehicle, you can analyze the field control. Charging systems generally operate at 13.0 volts and above.

It dropped to 6V at the battery terminals?
Where did the mechanic connect his voltmeter?

our 2015 civic has no gauges to speak of. no temp gauge, no alternator/battery gauge. what happens to alert the driver if the alternator decides to stop working? my kids 07 ford focus has a battery light that comes on when the voltage is low? so, when the alt dies as you drive, there is no warning light initially. you drive until the battery voltage drops low enough to stall the motor and than you get a light.

Are you sure you don’t have a battery light on the dash? From my Insight owner’s Manual:


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