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Alternator Voltage under full load #4773052
05/31/18 07:34 AM
05/31/18 07:34 AM
Joined: Feb 2015
Posts: 121
Greece
inquirer Offline OP
inquirer  Offline OP
Joined: Feb 2015
Posts: 121
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!

Re: Alternator Voltage under full load [Re: inquirer] #4773055
05/31/18 07:37 AM
05/31/18 07:37 AM
Joined: Oct 2014
Posts: 12,309
Idaho
CT8 Offline
CT8  Offline
Joined: Oct 2014
Posts: 12,309
Idaho
What is the voltage with every thing turned on and the engine running at 3000 rpm?


2015 Ford F150 2.7
2018 Ford F350 6.2
Re: Alternator Voltage under full load [Re: inquirer] #4773060
05/31/18 07:42 AM
05/31/18 07:42 AM
Joined: Dec 2002
Posts: 42,214
New Jersey
JHZR2 Offline
JHZR2  Offline
Joined: Dec 2002
Posts: 42,214
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.

Re: Alternator Voltage under full load [Re: inquirer] #4773061
05/31/18 07:43 AM
05/31/18 07:43 AM
Joined: Dec 2017
Posts: 417
Massachusetts
VeryNoisyPoet Offline
VeryNoisyPoet  Offline
Joined: Dec 2017
Posts: 417
Massachusetts
Are you measuring at the alternator output terminal, or at the battery terminals?


1998 V70 GLT / 195k miles [10w-30 Valvoline Syn w/ Maxlife Tech + Mann W917 filter]

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Re: Alternator Voltage under full load [Re: inquirer] #4773065
05/31/18 07:47 AM
05/31/18 07:47 AM
Joined: Jan 2013
Posts: 1,995
Texas
WyrTwister Offline
WyrTwister  Offline
Joined: Jan 2013
Posts: 1,995
Texas
Be sure to check and / or clean all the connections , including all the grounds .

Any of the wiring getting hot ?


Wyr
God bless
Re: Alternator Voltage under full load [Re: inquirer] #4773081
05/31/18 07:58 AM
05/31/18 07:58 AM
Joined: Feb 2015
Posts: 121
Greece
inquirer Offline OP
inquirer  Offline OP
Joined: Feb 2015
Posts: 121
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.

Last edited by inquirer; 05/31/18 07:59 AM.
Re: Alternator Voltage under full load [Re: inquirer] #4773094
05/31/18 08:04 AM
05/31/18 08:04 AM
Joined: May 2010
Posts: 1,112
Ocala, Florida
racin4ds Offline
racin4ds  Offline
Joined: May 2010
Posts: 1,112
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!


Just say NO to thin oils and M1!
05 F350 Powerstroke
05 Hyundai Tucson
08 Legacy GT
Re: Alternator Voltage under full load [Re: racin4ds] #4773106
05/31/18 08:14 AM
05/31/18 08:14 AM
Joined: Feb 2015
Posts: 121
Greece
inquirer Offline OP
inquirer  Offline OP
Joined: Feb 2015
Posts: 121
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?

Re: Alternator Voltage under full load [Re: inquirer] #4773116
05/31/18 08:26 AM
05/31/18 08:26 AM
Joined: Apr 2017
Posts: 1,481
Ontario, Canada
mightymousetech Offline
mightymousetech  Offline
Joined: Apr 2017
Posts: 1,481
Ontario, Canada
Let the car idle with everything turned on, and it will drain the battery. This is normal.


Mighty Mouse Tech
BMW Tech, Former Acura Tech
2010 Civic Si Castrol 0W40, Redline MTL
2013 BMW 135i M Sport Edition Castrol 0W40, Redline MTL
Re: Alternator Voltage under full load [Re: mightymousetech] #4773124
05/31/18 08:33 AM
05/31/18 08:33 AM
Joined: Feb 2015
Posts: 121
Greece
inquirer Offline OP
inquirer  Offline OP
Joined: Feb 2015
Posts: 121
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?

Re: Alternator Voltage under full load [Re: inquirer] #4773140
05/31/18 08:48 AM
05/31/18 08:48 AM
Joined: May 2005
Posts: 2,239
Toronto, Canada
George7941 Offline
George7941  Offline
Joined: May 2005
Posts: 2,239
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.


2006 GMC Sierra 4.3l, NV3500,G80 , Mobil 1 5W30, Dexcool.
1987 BMW R80 - sold
2015 Honda CBR300R

Staying out of the right lane a lot.
Re: Alternator Voltage under full load [Re: inquirer] #4773144
05/31/18 08:49 AM
05/31/18 08:49 AM
Joined: Apr 2017
Posts: 60
Ontario, Canada
AdamZ Offline
AdamZ  Offline
Joined: Apr 2017
Posts: 60
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.

Last edited by AdamZ; 05/31/18 08:51 AM.

94 Nissan Pathfinder 493k km R.I.P.
95 Nissan Pathfinder 200k km
02 Nissan Xterra SC 170k km
04 VW Golf 2.0 245k km
Re: Alternator Voltage under full load [Re: AdamZ] #4773159
05/31/18 08:59 AM
05/31/18 08:59 AM
Joined: Feb 2015
Posts: 121
Greece
inquirer Offline OP
inquirer  Offline OP
Joined: Feb 2015
Posts: 121
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.

Re: Alternator Voltage under full load [Re: inquirer] #4773162
05/31/18 09:00 AM
05/31/18 09:00 AM
Joined: Sep 2014
Posts: 1,485
Toronto Canada
WobblyElvis Offline
WobblyElvis  Offline
Joined: Sep 2014
Posts: 1,485
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}

Re: Alternator Voltage under full load [Re: George7941] #4773214
05/31/18 09:50 AM
05/31/18 09:50 AM
Joined: Feb 2015
Posts: 121
Greece
inquirer Offline OP
inquirer  Offline OP
Joined: Feb 2015
Posts: 121
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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