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#4773052 - 05/31/18 08:34 AM Alternator Voltage under full load
inquirer Offline


Registered: 02/10/15
Posts: 121
Loc: Greece
I was testing my car's alternator with a multimeter and I wonder if the voltage under full load is normal. The car is a Mitsubishi Lancer 1.5L, with a petrol engine, and I think it has a 85 A alternator.

With the car idling @ 1000 RPMs and every electrical device turned on:
  • high beams
  • front fog lights
  • rear fog light
  • brake lights
  • reverse gear light
  • interior lights
  • blower at full speed
  • A/C turned on at lower temperature possible
  • rear window defroster
  • radio

I got a reading as low as 12.25V or 12.20V.

With the engine running at same RPMs and every device disabled the voltage was over 14V.


Normal or abnormal?

Thanks!

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#4773055 - 05/31/18 08:37 AM Re: Alternator Voltage under full load [Re: inquirer]
CT8 Offline


Registered: 10/09/14
Posts: 11017
Loc: Idaho
What is the voltage with every thing turned on and the engine running at 3000 rpm?
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#4773060 - 05/31/18 08:42 AM Re: Alternator Voltage under full load [Re: inquirer]
JHZR2 Offline



Registered: 12/14/02
Posts: 41804
Loc: New Jersey
Seems a bit low, but not abnormal from a phenomena perspective - voltage will drop with load. The issue is that you're actually drawing from the battery too at that point. Id hope to see 12.6-12.7v on max load.


I would expect to see it rise with higher RPMs. I agree with the 3000rpm question.

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#4773061 - 05/31/18 08:43 AM Re: Alternator Voltage under full load [Re: inquirer]
VeryNoisyPoet Online   content


Registered: 12/16/17
Posts: 291
Loc: Massachusetts
Are you measuring at the alternator output terminal, or at the battery terminals?
_________________________
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#4773065 - 05/31/18 08:47 AM Re: Alternator Voltage under full load [Re: inquirer]
WyrTwister Offline


Registered: 01/13/13
Posts: 1532
Loc: Texas
Be sure to check and / or clean all the connections , including all the grounds .

Any of the wiring getting hot ?
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God bless

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#4773081 - 05/31/18 08:58 AM Re: Alternator Voltage under full load [Re: inquirer]
inquirer Offline


Registered: 02/10/15
Posts: 121
Loc: Greece
I added the hazard lights to the above list. At 3000 RPMs I got 12.75V.

At idle it started at around 13 V and it started to drop steadily to lower 12 V.

All connections seem clean. I measure the voltage at the battery terminals, by the way.


Edited by inquirer (05/31/18 08:59 AM)

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#4773094 - 05/31/18 09:04 AM Re: Alternator Voltage under full load [Re: inquirer]
racin4ds Offline


Registered: 05/07/10
Posts: 984
Loc: Ocala, Florida
An alternator/charging system should always be able to maintain a MIN of 12.6VDC or discharging begins to occur. Many don't know, but a 12V charging system is discharging at anything below 12.6VDC. 12.0VDC on a battery is actually ~25% discharged.

Also don't forget your grounds! They are every bit as important as the positive connections!
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#4773106 - 05/31/18 09:14 AM Re: Alternator Voltage under full load [Re: racin4ds]
inquirer Offline


Registered: 02/10/15
Posts: 121
Loc: Greece
Originally Posted By: racin4ds
An alternator/charging system should always be able to maintain a MIN of 12.6VDC or discharging begins to occur. Many don't know, but a 12V charging system is discharging at anything below 12.6VDC. 12.0VDC on a battery is actually ~25% discharged.

Also don't forget your grounds! They are every bit as important as the positive connections!


Always? Even at idle (@700 RPM) and under full load?

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#4773116 - 05/31/18 09:26 AM Re: Alternator Voltage under full load [Re: inquirer]
mightymousetech Offline


Registered: 04/03/17
Posts: 1444
Loc: Ontario, Canada
Let the car idle with everything turned on, and it will drain the battery. This is normal.
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#4773124 - 05/31/18 09:33 AM Re: Alternator Voltage under full load [Re: mightymousetech]
inquirer Offline


Registered: 02/10/15
Posts: 121
Loc: Greece
Originally Posted By: mightymousetech
Let the car idle with everything turned on, and it will drain the battery. This is normal.


Is it or not?
https://youtu.be/LGB6ZEjGm7Q?t=2m1s
If you watch the video above with many devices turned on - if not every device - and the car idling he gets almost 14V.

So which is the expected behavior?

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#4773140 - 05/31/18 09:48 AM Re: Alternator Voltage under full load [Re: inquirer]
George7941 Offline


Registered: 05/10/05
Posts: 2158
Loc: Toronto, Canada
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.
_________________________
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1987 BMW R80 - sold
2015 Honda CBR300R

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#4773144 - 05/31/18 09:49 AM Re: Alternator Voltage under full load [Re: inquirer]
AdamZ Offline


Registered: 04/06/17
Posts: 52
Loc: Ontario, Canada
That sounds low to me. I just tried this the other day and the lowest mine went, with all turned on, was 13.3~ volts.

Does your meter have fresh batteries to ensure good accuracy? If mine are low the readings are off and I've been bitten in the [censored] by that before.


Edited by AdamZ (05/31/18 09:51 AM)
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#4773159 - 05/31/18 09:59 AM Re: Alternator Voltage under full load [Re: AdamZ]
inquirer Offline


Registered: 02/10/15
Posts: 121
Loc: Greece
Originally Posted By: AdamZ
That sounds low to me. I just tried this the other day and the lowest mine went, with all turned on, was 13.3~ volts.

Does your meter have fresh batteries to ensure good accuracy? If mine are low the readings are off and I've been bitten in the [censored] by that before.


It doesn't have fresh batteries, but I don't think that's an issue. I have the multimeter for 3 and a half years now. 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.

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#4773162 - 05/31/18 10:00 AM Re: Alternator Voltage under full load [Re: inquirer]
WobblyElvis Offline


Registered: 09/23/14
Posts: 1355
Loc: Toronto Canada
As long at it charges under load at 1500 rpm you shouldn't have a problem. It is normal to discharge the battery at idle under load. Vehicles that regularly sit at idle under high electrical load have special alternators. [Police cars}

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#4773214 - 05/31/18 10:50 AM Re: Alternator Voltage under full load [Re: George7941]
inquirer Offline


Registered: 02/10/15
Posts: 121
Loc: Greece
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.

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