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Thread: Battery voltage specs at idle?

  1. #1
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    Default Battery voltage specs at idle?

    I saw on one of the forums something about measuring voltage through the cigarette light/power outlet. I just cannot find it.

    Does anyone know what the voltage is supposed to be in the following situations? Perhaps there are other situations as well.

    Ignition off
    Ignition on-not running
    Ignition on- running at idle with low beams on
    Ignition on- running at idle with low beams on, seat heater on
    Ignition on- running with at idle low beams on, seat heater on, fan on level 2 or 3
    Ignition on- running with at idle low beams on, seat heater on, fan on level 2 or 3 and rear window defroster on.

    I have noticed my idle (in traffic) when I turn on the rear defroster the idle speed drops, ever so slightly, and the dash lights dim slightly. Not sure of the significance and how to really test. BTW, my 22 month old battery is the Autozone DURALAST 49-DL recommended here.
    Thanks,

    1995 525i Auto, M50TU 2.5L, EAT chip, 1/95 build, USA, 205/65/15 tires, ASC+T, HID, lumbar, EC Mirror, BMW Alpine 5 radio with BMW-Pioneer CD Changer, abt 236k miles, Oxford Green/Parchment

  2. #2
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    Default

    1) i wouldn't try to measure your battery voltage through your cigarette lighter.
    2) advance/autozone, other places will hook it up to a charging system tester for free, and they'll probably do a more comprehensive job than you can do with a volt ohm meter. just let them know the conditions where your headlights dim, and they won't ask you to do something irrelevant to your situation.
    3) the tester won't predict a chicken-egg situation (ie, if your alternator has been straining your battery for 6 months, and your battery goes out shortly after replacing your alternator, or vice versa.
    "..Torchinski v. Peterson that it is legal to carry a concealed weapon, so long the weapon is totally slick like a huge ass machine gun that you carry under a trench coat, like in the Matrix."


  3. #3
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    The voltage testing is not really relevant at idle but at 1500RPM+ At idle it is normal for things to dim if your using heavy electricity. If I am in a prolong idle situation I turn everything (not radio) down or off to save power except regular headlights and heater on 2. Turn the heavy hitters back on once we start moving again.

    Quote Originally Posted by Russell View Post
    I saw on one of the forums something about measuring voltage through the cigarette light/power outlet. I just cannot find it.

    Does anyone know what the voltage is supposed to be in the following situations? Perhaps there are other situations as well.

    Ignition off
    Ignition on-not running
    Ignition on- running at idle with low beams on
    Ignition on- running at idle with low beams on, seat heater on
    Ignition on- running with at idle low beams on, seat heater on, fan on level 2 or 3
    Ignition on- running with at idle low beams on, seat heater on, fan on level 2 or 3 and rear window defroster on.

    I have noticed my idle (in traffic) when I turn on the rear defroster the idle speed drops, ever so slightly, and the dash lights dim slightly. Not sure of the significance and how to really test. BTW, my 22 month old battery is the Autozone DURALAST 49-DL recommended here.
    95 E34 530I V2.37
    ===========
    Those who make peaceful revolution impossible will make violent revolution inevitable.

    John F. Kennedy

  4. #4
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    This info was once posted by Patrick C750. Very good how to do:

    I tried OBC test #9 before but still prefer to measure the voltage the old sure way because my OBC for some reason measures a voltage 0.2V lower than a calibrated DMM. (=digital multi meter)

    Find a digital multimeter and stick the (one) probe into the cigarette lighter socket in the center console. Or at the plus pole under the hood and the other one at very good ground contact.

    The following are the detail figures I use for my own evaluation:
    With key out of ignition switch, should see 12.4V or above. 12.0 V is getting low.
    With key inserted and turned to position 2 (don't start car), should see 12.0V or more. 11.6V would be too low.
    Crank car, catch the lowest voltage reading during that couple of seconds. Should see 10.0V or above. 9.5V would be marginal.
    The above just consider the health of battery itself.
    Car running at 1500 RPM or more, should see 13.7 to 14.3V, 13.6V or below means the charging system is weak.
    Patrick C 88 750 130K

  5. #5
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    Everyone-thanks.
    Shogun this was the post I remembered.

    FYI, I rigged up a car cell phone charger plug (took out the voltage reducing circuit board) and clipped it to my digital mulit-meter probes. I will watch it on the way to work tommorow. From the readings on your post. I think my system is ok. However, the alternator/regulator is original.
    Last edited by Russell; 01-14-2009 at 10:02 PM.
    Thanks,

    1995 525i Auto, M50TU 2.5L, EAT chip, 1/95 build, USA, 205/65/15 tires, ASC+T, HID, lumbar, EC Mirror, BMW Alpine 5 radio with BMW-Pioneer CD Changer, abt 236k miles, Oxford Green/Parchment

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  7. #7
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    Used a DMM to measure voltage this morning. Keep in mind the temperature in my unheated garage was about 23 degrees f and the outside temperature was about 11 degrees f. The battery is a 22 month old Duralast 49DL

    Voltage with key off 12.01 should have been 12.4 or above.
    key inserted at position 2 voltage was 11.5, should have been 12.0 or above.
    cranked car, voltage was about 9.8, should have been 10.0 or above
    car running at 1500 rpm, voltage was 14.1, should be 13.7 to 14.3

    If I turn everything on, such as low beam lights, seat heater and rear defroster and fan at three. Voltage at idle was about 12.8

    My battery may be undercharged a bit since I use the car for commuting and it is dark when I leave the house and getting dark on way home so lights, heaters and fans running most of the time.

    Any opinions on if i even have a problem? TIA
    Last edited by Russell; 01-15-2009 at 10:33 AM.
    Thanks,

    1995 525i Auto, M50TU 2.5L, EAT chip, 1/95 build, USA, 205/65/15 tires, ASC+T, HID, lumbar, EC Mirror, BMW Alpine 5 radio with BMW-Pioneer CD Changer, abt 236k miles, Oxford Green/Parchment

  8. #8
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    Default Russell you don't have a problem. At

    that ambient and at idle with the loads your putting on it I'd say the system is fine. Charging system voltages checked at 1500 rpm, no loads on , and the ambient at the voltage regulator minimum 30C/86F so i would say that you're system is working as good as you can expect under the conditions.


  9. #9
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    Default You are the man!

    Thanks, I was sort of reaching that conclusion. Winter is hard on a vehicle.

    Quote Originally Posted by Bill R. View Post
    that ambient and at idle with the loads your putting on it I'd say the system is fine. Charging system voltages checked at 1500 rpm, no loads on , and the ambient at the voltage regulator minimum 30C/86F so i would say that you're system is working as good as you can expect under the conditions.
    Thanks,

    1995 525i Auto, M50TU 2.5L, EAT chip, 1/95 build, USA, 205/65/15 tires, ASC+T, HID, lumbar, EC Mirror, BMW Alpine 5 radio with BMW-Pioneer CD Changer, abt 236k miles, Oxford Green/Parchment

  10. #10
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    Claude was kind enough to measure his voltage readings for me when I was pursuing similar issues.

    Quote Originally Posted by Claude View Post
    ... test results on my car, voltages taken with a digital voltmeter directly on the 12 volts terminal under the hood:

    - No key no nothing : 12.80 volts
    - engine idle everythings fan at usual position (2) else OFF : 13.98 V
    - engine idle rear window defogger ON : 13.95 V
    - engine idle rear window defogger OFF fan at max speed (4) : 13.95 V
    - engine idle rear window defogger ON & fan at max speed (4) : 13.92 V

    My battery is 6 years old but in good shape. So as you may see the voltage is almost stable with or without those a large load like the defogger.

    Based of your tests results i would suspect the quality of your battery; how does it react (voltage drop) when starting the car, for exemple if you try to start it with all the heavy loads ON (defogger, headlights, wipers fan...)

    The 1HKR control module controls the SWORD which controls the Fan, the fan is fused 30 amp (under the hood fuses box). The 1HKR control module controls a relay which controls the rear window defogger, defogger also fused 30 amp, both the relay and the fuse are located under the rear seat.

    Hope this may help.
    full thread is here

    http://www.bimmernut.com/forum/showt...12582#poststop

    though i would defer to BillR and Regal, myself I won't be happy until I get similar readings. When it is warm enough I will get on this again.

    http://www.bimmernut.com/forum/showthread.php?t=40334

    http://twrite.org/shogunnew/fixes/el...cable_between_

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