Minimum acceptable alternator voltage?

[Avery4]

Hello everyone, I am wondering what the lowest voltage an alternator should produce under full load at idle is. I would think it should always be able to produce at least battery voltage (12.6 volts or so) even under a full load (all the stock electrical accessories on) with the engine idling to make sure the car doesn't drain the battery under a worst case scenario. However, a lot of the alternators I tested can not.

For example, I replaced my 05 Civic's alternator years ago because the battery voltage was dropping down to around 12.1 volts under full load at idle. Seemed like a bad alternator to me since it couldn't keep up at idle and the battery would have eventually went dead if I got stuck in traffic on a rainy day with the headlights, AC, rear defroster, radio, and wipers on, not a totally unrealistic scenario. However, the new alternator wasn't much better. Under the same conditions, it was only able to produce 12.2 volts. I showed an employee at the store I bough that alternator and he said it shouldn't be dropping below 12.8 volts or it will drain the battery and he told me to take it off and they will replace it. It tested good, but he replaced it anyways and I got a new alternator that time instead of a reman. Same thing. No significant voltage drop on any grounds or the alternator power cable, I upgraded them all. I ended up getting a high output alternator on eBay, the voltage now only drops down to 13.4 volts under those same conditions, which seems good to me since under a worst case scenario it is still charging the battery rather than draining it.

Another example is my friend's Cadillac. I was at his house yesterday doing some other work on it, so I checked the alternator voltage just to make sure it is good. With just the AC, high beams, and rear defroster on, the voltage goes down to 12.3 volts and keeps dropping. That also seems low to me since the battery would eventually die, but who knows if a new alternator would be any better.

So the question is how low is too low for alternator voltage under load? Apparently battery voltage isn't the answer since I have seen so many alternators go under that without any obvious problems or complaints, so apparently it is "acceptable" for the battery to drain from idling with a heavily loaded electrical system?

 

[ripcord]

I'm not a Honda guy, but I think you've got a load detection circuit in the junction box that commands the alternator field voltage. If that is not working right, the alternator will not put out the right amount of current.

 

[ripcord]

I googled around and found this page: http://www.gonzostoolbox.com/knowledgefolder/hondaeld.html

To answer your question about voltage, you should never see less than 13.2v and up to 15v depending on the make and model and the state of charge of your battery.

 

[Avery4]

I'm not a Honda guy, but I think you've got a load detection circuit in the junction box that commands the alternator field voltage. If that is not working right, the alternator will not put out the right amount of current.

You are correct, good point! Those go bad quite a bit on these cars. A failure won't cause low voltage under load at idle though, the ECU will only activate low charge mode when moving or when idling in drive. Usually a failure just sets a P1298 code and disables low charge mode. These alternators are internally regulated and default to high charge mode (around 14-14.5 volts) unless the ECU requests low charge mode (around 12.6 volts).

 

[Avery4]

I googled around and found this page: http://www.gonzostoolbox.com/knowledgefolder/hondaeld.html

To answer your question about voltage, you should never see less than 13.2v and up to 15v depending on the make and model and the state of charge of your battery.

Thank you! So if an alternator can't produce 13.2 volts under load at idle you would condemn it after eliminating all other possibilities?

 

[MolaKule]

With three vehicles of various brands and years of make here is my charging checking method (assuming a good battery):

Engine idling,
Headlights on Bright,
AC on with blower at max,
turn signal on,
Interior lights on,
CD player or radio going medium blast.

All three vehicles average a minimum of 13.8 volts under these conditions.

 

[Avery4]

With three vehicles of various brands and years of make here is my charging checking method (assuming a good battery):

Engine idling,
Headlights on Bright,
AC on with blower at max,
turn signal on,
Interior lights on,
CD player or radio going medium blast.

All three vehicles average a minimum of 13.8 volts under these conditions.

You don't turn on the rear defroster? It draws a lot of power too, possibly more than any other accessory depending on the vehicle

 

[MolaKule]

Yes, that is one I forgot to list and it should be turned on as well.

Two of my vehicles do have them and they are both Nissans, one is the Frontier and the other a Pathfinder.

My older 1999 S-10 did not have that as an option. I guess they figured back then human energy was all that was needed to remove the ice and fog.

 

[tcp71]

A lot of fords have "smart" charging systems. They reduce the output voltage when the battery is at a significant charge so your testing doesn't work. I would assume Cadillac has a similar system. It buys them EPA credits.

Further reading:

Ford Smart Charging system - Ford Flex Forum

 

[Avery4]

A lot of fords have "smart" charging systems. They reduce the output voltage when the battery is at a significant charge so your testing doesn't work. I would assume Cadillac has a similar system. It buys them EPA credits.

Further reading:

Ford Smart Charging system - Ford Flex Forum

It still shouldn't be dropping below battery voltage and draining the battery at idle under load though, right?

 

[ripcord]

That's correct

 

[Avery4]

That's correct

Thank you. Would you condemn an alternator if it can't keep up under load at idle after eliminating all other possibilities?

 

[JHZR2]

It is very common for alternators to not be able to keep up, especially if they are small, and they are only at idle.

My old Euro cars are a prime example; at idle, they will put out between 12.6 and 13.2V typically, depending upon load, how long since last excited, and battery SOC. At speed (>1200 RPM), theyll generally put out between 13.7 and 13.1V. Alternators dont necessarily put out their rated levels at idle, and how bad the fall-off is, determines if the battery has to keep up too.

Even if the voltage regulator bumps the output to a higher voltage, if the alternator cant keep up, it is going to create challenges.

In some alternator designs, a diode can be installed between the voltage regulator and case to provide a bit of artificial voltage drop, which causes the regulator to artifically bump the output voltage. This helps somewhat, but the alternator will still be limited to what it can put out.

The key thing though is to ensure that there are no fouled connections, which cause issues with charging or sensing.

I recall from working on a family member's 2002 accord, that the battery/alternator couldn't keep up under full load, and it would drop to mid-12V. Now, some of this may have been due to a chronically undercharged battery, but its certainly common that the voltages went so low.

Thank you. Would you condemn an alternator if it can't keep up under load at idle after eliminating all other possibilities?

Id argue no. Some designs just arent sufficient. You could pursue a larger alternator (though youll need to be considerate of cable sizing so you dont create a fire hazard), as long as you can handle the additional load it puts on your engine. It may or may not help... if something else is also a culprit...

From here: https://www.quora.com/What-will-happen-if-AC-alternator-is-run-at-higher-RPM-than-the-rated-RPM

The alternator's output current is regulated by the regulator, which tries to keep the votage at some level. If the alternator is puting out as much as it can at this speed then the voltage will drop.

 

[Avery4]

It is very common for alternators to not be able to keep up, especially if they are small, and they are only at idle.

My old Euro cars are a prime example; at idle, they will put out between 12.6 and 13.2V typically, depending upon load, how long since last excited, and battery SOC. At speed (>1200 RPM), theyll generally put out between 13.7 and 13.1V. Alternators dont necessarily put out their rated levels at idle, and how bad the fall-off is, determines if the battery has to keep up too.

Even if the voltage regulator bumps the output to a higher voltage, if the alternator cant keep up, it is going to create challenges.

In some alternator designs, a diode can be installed between the voltage regulator and case to provide a bit of artificial voltage drop, which causes the regulator to artifically bump the output voltage. This helps somewhat, but the alternator will still be limited to what it can put out.

The key thing though is to ensure that there are no fouled connections, which cause issues with charging or sensing.

I recall from working on a family member's 2002 accord, that the battery/alternator couldn't keep up under full load, and it would drop to mid-12V. Now, some of this may have been due to a chronically undercharged battery, but its certainly common that the voltages went so low.



Id argue no. Some designs just arent sufficient. You could pursue a larger alternator (though youll need to be considerate of cable sizing so you dont create a fire hazard), as long as you can handle the additional load it puts on your engine. It may or may not help... if something else is also a culprit...

From here: https://www.quora.com/What-will-happen-if-AC-alternator-is-run-at-higher-RPM-than-the-rated-RPM

The alternator's output current is regulated by the regulator, which tries to keep the votage at some level. If the alternator is puting out as much as it can at this speed then the voltage will drop.

Great information, thank you! Seems like tricking the voltage regulator into seeing a lower voltage could create other problems like excessively high voltage under low loads, but I never did that so I don’t know. Also seems pretty pointless since it won’t increase the alternator’s amperage capacity anyways. If the ECU or regulator is already commanding 100% as it should be with that low of a voltage the alternator should be producing all the power it can anyways.

Before I installed the high output alternator I installed a smaller pulley on the stock alternator to spin it about 30% faster, hoping it would work better at idle. It did help, but not as much as I was hoping for. I was hoping for at least ~12.8V at idle so my battery wasn’t being drained, but I ended up getting around 12.5V. Oh well, still better than before and the experiment only cost me $15 and the time it took to change the pulley. I wish I could have checked the amperage before and after the pulley mod, but I didn’t have an amp clamp back then.

The battery’s SOC makes a big difference. If the battery is slightly run down and drawing 10 amps, that may be 20% or more of what a small alternator can produce at idle and will definitely be seen on the voltage test.

Upgrading the alternator's power cable, battery cables, and engine grounds is a good idea, from what I have seen the factory alternator wiring is usually barely adequate for the stock alternator as is. As for extra load on the engine, a high output alternator will not necessarily draw more power from the engine. The draw will be pretty similar under normal loads and possibly lower if the alternator is more efficient. However, a high output alternator is capable of loading the engine down more than a less powerful stock alternator under heavy loads if it is capable of keeping up while the stock alternator isn't. It takes power to generate electricity.

It seems to me that the charging system should be designed so it can keep up under a worst case scenario (all factory installed accessories on with the engine idling), but inadequate alternators are certainly not the only way manufacturers cheap out when designing vehicles.

 

[Avery4]

Great information, thank you! Seems like tricking the voltage regulator into seeing a lower voltage could create other problems like excessively high voltage under low loads, but I never did that so I don’t know. Also seems pretty pointless since it won’t increase the alternator’s amperage capacity anyways. If the ECU or regulator is already commanding 100% as it should be with that low of a voltage the alternator should be producing all the power it can anyways.

Before I installed the high output alternator I installed a smaller pulley on the stock alternator to spin it about 30% faster, hoping it would work better at idle. It did help, but not as much as I was hoping for. I was hoping for at least ~12.8V at idle so my battery wasn’t being drained, but I ended up getting around 12.5V. Oh well, still better than before and the experiment only cost me $15 and the time it took to change the pulley. I wish I could have checked the amperage before and after the pulley mod, but I didn’t have an amp clamp back then.

The battery’s SOC makes a big difference. If the battery is slightly run down and drawing 10 amps, that may be 20% or more of what a small alternator can produce at idle and will definitely be seen on the voltage test.

Upgrading the alternator's power cable, battery cables, and engine grounds is a good idea, from what I have seen the factory alternator wiring is usually barely adequate for the stock alternator as is. As for extra load on the engine, a high output alternator will not necessarily draw more power from the engine. The draw will be pretty similar under normal loads and possibly lower if the alternator is more efficient. However, a high output alternator is capable of loading the engine down more than a less powerful stock alternator under heavy loads if it is capable of keeping up while the stock alternator isn't. It takes power to generate electricity.

It seems to me that the charging system should be designed so it can keep up under a worst case scenario (all factory installed accessories on with the engine idling), but inadequate alternators are certainly not the only way manufacturers cheap out when designing vehicles.

 

[meep]

Concur with jhrz. I’ve only had one vehicle which seemed to keep up with most loads at idle - a Chrysler minivan with front-rear AC. BUT- it was programmed to raise idle speed when the voltage was too low, as they knew the alternator alone at regular idle wouldn’t be able to meet all demands.

 

[Avery4]

Concur with jhrz. I’ve only had one vehicle which seemed to keep up with most loads at idle - a Chrysler minivan with front-rear AC. BUT- it was programmed to raise idle speed when the voltage was too low, as they knew the alternator alone at regular idle wouldn’t be able to meet all demands.

I have seen vehicles that will raise idle speed under load to keep the voltage up. The only problem with that is it can't raise idle speed much without frying the transmission when it is in drive and being held back with the brakes, so that's not going to help much when sitting in traffic. With a manual transmission or when idling in park or neutral that strategy would work fine though

 

[ripcord]

Not all cars are starving for current at idle. I worked on a early 2000's GM a few weeks ago with a no spark condition which was due to a bad ground. After I got it running, I threw an amp clamp on to check to see if it was charging OK as the battery was pretty flat. It was pushing 90 amps idling in park.

 

[Avery4]

Not all cars are starving for current at idle. I worked on a early 2000's GM a few weeks ago with a no spark condition which was due to a bad ground. After I got it running, I threw an amp clamp on to check to see if it was charging OK as the battery was pretty flat. It was pushing 90 amps idling in park.

That's way more than most stock alternators will produce at idle and more than a lot of alternator's max rating.

 

[ripcord]

There are quite a few modern cars that have alternators rated for more than 100 peak amps. I looked it up and that particular comes with a 105 amp rated alternator. Still, I was surprised it would push that many amps at idle. It's actually not great for the alternator to run that way as it's likely to overheat putting out that much current with the fan running slowly. That's one reason it's better charge a dead battery with a charger and not rely on the alternator.