Alternator Voltage under full load

[Carbuff]

I've owned cars that had OEM alternators that 'ran off the battery' at idle, when electrical loads were present. Needless to say, the voltage regulators didn't last long...they would run hot and the potting material in the VR would drip. A different brand alternator fixed that.

I've also had drive belts that were loose but not squealing or making any noise. This resulted in low charging amperage at low RPMs.

 

[VeryNoisyPoet]

Originally Posted By: WyrTwister
I thought a clamp on meter only worked on AC ? But I have never tryed one on DC .


Many digital ones can measure DC, but need to be carefully zeroed in the same position where you will be measuring or will give wildly inaccurate readings.

 

[VeryNoisyPoet]

Originally Posted By: racin4ds
A PROPERLY designed charging system will NOT discharge even at idle with all loads on. Sure, many are used to seeing this behavior but it certainly is NOT correct...


I agree. It should at least hold the battery at 12.5 or more. When my alternator was failing last year, it would sag to 12.3 under full load. The battery would never fully recharge, which eventually damaged it.

A reman alternator and cleaned up wires and it now stays above 13.2 even with everything switched on at idle and a measured draw from the alternator of 70 amps.

 

[Dave9]

It's fine. Why would it surprise you that if you try to cause a problem, sometimes you will succeed?

It's real simple, either the alternator keeps your battery charged in your use or not. If you insist on putting the max load on it that is possible, sure, you cause yourself to need to buy a higher output alternator.

There are many ways one can do this. High output stereo, silly aftermarket light bars, etc.

The stock alternator is sized for good cost:benefit and the average loads expected, not someone trying to overload their vehicle for some ??? reason.

 

[inquirer]

Originally Posted By: Dave9
It's fine. Why would it surprise you that if you try to cause a problem, sometimes you will succeed?

It's real simple, either the alternator keeps your battery charged in your use or not. If you insist on putting the max load on it that is possible, sure, you cause yourself to need to buy a higher output alternator.

There are many ways one can do this. High output stereo, silly aftermarket light bars, etc.

The stock alternator is sized for good cost:benefit and the average loads expected, not someone trying to overload their vehicle for some ??? reason.


You can find many sources online (I included one Youtube video) that use stress testing as an indication of a problem. If the voltage under load is under 13V, then this may be an indication of a problem according to them.

There are real life scenarios though where heavy loads are not uncommon. Imagine that you drive while there is relatively thick fog and cold weather. You are going to use the A/C for keeping the windshield and the side windows clear. You are also going to use the rear window defroster as well as the front fog lights and is some cases the rear fog light too. In some areas that's not an uncommon scenario. It's everyday life for several months every year. Maybe that's not 100% load, but it is very close to it.

And I'm not talking about aftermarket stuff. I'm talking about equipment that was installed on the car by the manufacturer. I would expect that would be fully covered by the OEM alternator.

 

[PeterPolyol]

It could be considered a problem if the alternator was of greater capacity than 85A

At idle, with that alternator and with maximum electrical load, 12.25V would be a normal expectation for me. When you cut all the loads,

Originally Posted By: inquirer
[ In idle with everything turned off the reading was 14.3V, so I think the multimeter is doing fine. If it was off, it would be off in all measurements.

Yeah, that's fine. I wouldn't expect too much from your 85Amp alt, with full load at idle. I bet it's not winter in Greece right now, so consider the temp/voltage compensation also.

 

[Panos]

You forgot to also turn on the wipers at full speed

 

[inquirer]

Originally Posted By: PeterPolyol
It could be considered a problem if the alternator was of greater capacity than 85A

At idle, with that alternator and with maximum electrical load, 12.25V would be a normal expectation for me. When you cut all the loads,

Originally Posted By: inquirer
[ In idle with everything turned off the reading was 14.3V, so I think the multimeter is doing fine. If it was off, it would be off in all measurements.

Yeah, that's fine. I wouldn't expect too much from your 85Amp alt, with full load at idle. I bet it's not winter in Greece right now, so consider the temp/voltage compensation also.


No. It's actually 86 ºF here. Does temperature alter the measurement or has it got any effect on the alternator? I only knew that you had to take temerature into account when calculating the charging state of the battery.

 

[mightymousetech]

Originally Posted By: inquirer
Originally Posted By: George7941
Both. It depends on the strength of the alternator and the total current drawn by the devices on the car, which will vary from car to car.


I would expect that the engineer who is responsible for choosing the alternator would have calculated the car's needs and would have chosen one adequate of providing the minimum required voltage in order not to let the battery drain under any circumstances.

If both things happen how should I know if the alternator was always weak or if for some reason is malfunctioning?

I should add that I changed my last battery, a Bosch S5, in 23 months after installation and that makes me more suspicious of an issue.


Why is that? Make the alternator bigger than it has to be is wasting weight/fuel/money.

BMW designs their charging systems to intentionally run the battery down, and tries to only charge during deceleration. Saves fuel, lowers emissions and more power. Win/win/win. Makes it a little more challenging to test the system though.

 

[inquirer]

Originally Posted By: mightymousetech
Originally Posted By: inquirer
Originally Posted By: George7941
Both. It depends on the strength of the alternator and the total current drawn by the devices on the car, which will vary from car to car.


I would expect that the engineer who is responsible for choosing the alternator would have calculated the car's needs and would have chosen one adequate of providing the minimum required voltage in order not to let the battery drain under any circumstances.

If both things happen how should I know if the alternator was always weak or if for some reason is malfunctioning?

I should add that I changed my last battery, a Bosch S5, in 23 months after installation and that makes me more suspicious of an issue.


Why is that? Make the alternator bigger than it has to be is wasting weight/fuel/money.

BMW designs their charging systems to intentionally run the battery down, and tries to only charge during deceleration. Saves fuel, lowers emissions and more power. Win/win/win. Makes it a little more challenging to test the system though.


I'm not a battery expert, but wouldn't repetitive discharging and charging cycles deteriorate charging performance? Probably, if the battery is fully charged there is no reason to overcharge it, which is also bad, but is there a reason for letting it drain while the alternator is powered by the engine?

 

[VeryNoisyPoet]

Originally Posted By: inquirer
I'm not a battery expert, but wouldn't repetitive discharging and charging cycles deteriorate charging performance? Probably, if the battery is fully charged there is no reason to overcharge it, which is also bad, but is there a reason for letting it drain while the alternator is powered by the engine?


Lead acid batteries like being kept fully charged, as long as they are not overcharged / boiled. A few percent discharge will have the battery last thousands of cycles, but all discharge cycles will cause some wear. Deeper discharge will accelerate wear, especially in a car battery designed for high current starting instead of cycling. Left at the perfect float voltage, a lead acid battery can last a very long time. Chronic undercharging from a weak alternator can eventually damage the battery, especially if the car sees short, slow trips in conditions with heavy electrical loads.

As a test, start measuring your battery voltage just after you shut off the car, and first thing in the morning before you start the car. Just after shutdown, the battery should show a surface charge around 12.8 to 13.2 volts. After sitting overnight, it should be 12.5 volts or higher.

 

[mightymousetech]

Originally Posted By: inquirer
Originally Posted By: mightymousetech
Originally Posted By: inquirer
Originally Posted By: George7941
Both. It depends on the strength of the alternator and the total current drawn by the devices on the car, which will vary from car to car.


I would expect that the engineer who is responsible for choosing the alternator would have calculated the car's needs and would have chosen one adequate of providing the minimum required voltage in order not to let the battery drain under any circumstances.

If both things happen how should I know if the alternator was always weak or if for some reason is malfunctioning?

I should add that I changed my last battery, a Bosch S5, in 23 months after installation and that makes me more suspicious of an issue.


Why is that? Make the alternator bigger than it has to be is wasting weight/fuel/money.

BMW designs their charging systems to intentionally run the battery down, and tries to only charge during deceleration. Saves fuel, lowers emissions and more power. Win/win/win. Makes it a little more challenging to test the system though.


I'm not a battery expert, but wouldn't repetitive discharging and charging cycles deteriorate charging performance? Probably, if the battery is fully charged there is no reason to overcharge it, which is also bad, but is there a reason for letting it drain while the alternator is powered by the engine?


The battery in my 06 330 was still original when I sold it last year. How many cars have 11 year old batteries? Can't be too bad for it.

 

[WyrTwister]

The battery only has to last until the warranty expires .

 

[PeterPolyol]

Originally Posted By: inquirer
Originally Posted By: PeterPolyol
It could be considered a problem if the alternator was of greater capacity than 85A

At idle, with that alternator and with maximum electrical load, 12.25V would be a normal expectation for me. When you cut all the loads,

Originally Posted By: inquirer
[ In idle with everything turned off the reading was 14.3V, so I think the multimeter is doing fine. If it was off, it would be off in all measurements.

Yeah, that's fine. I wouldn't expect too much from your 85Amp alt, with full load at idle. I bet it's not winter in Greece right now, so consider the temp/voltage compensation also.


No. It's actually 86 ºF here. Does temperature alter the measurement or has it got any effect on the alternator? I only knew that you had to take temerature into account when calculating the charging state of the battery.


I'd just mentioned winter because on some temperature regulated charging systems, voltage can be regulated as high as 15V in cold and high-12's in hot. Lancer is not temp compensated AFAIK so just trivia

 

[inquirer]

I got the car to an auto electrician today, just to take an opinion.
At idle he measured the same voltage pretty much. 12.1V - 12.2V.

But @2000 RPMs he got a measurement of 13.1V or something like this. I had measured 12.75 V yesterday @3000 RPMs. Maybe the difference is because he took measurements with the engine hot.

He said as the voltage crosses 13V @2000 RPMs the alternator works within specification.

(All values I mentioned above are at full load with all accessories turned on.)

 

[eljefino]

A 85 Amp alt is rated at cruise RPM. You might get 35 amps at idle.

This is why cop cars have heavy duty batteries even when they're almost never shut off.

My mercedes owners manual says if you short trip it using all the accessories, it won't keep up. Johnny Carson had this problem in his Delorean.

 

[meep]

newer vehicles "hide" this by bumping idle speed if the system v is low due to high demands. Seems like the gen IV chrysler minivans (~20010 had this, I don't know about gen III.

-M