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Thread: what to do if alternator wont charge at 14.5v?

  1. #1
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    what to do if alternator wont charge at 14.5v?

    hello forum. in my car (1992 audi 80) , the alternator, has a brand new voltage regulator, rated at 14.5v. i also have two batteries installed, isoalted with relay. the problem is, that the alternator, does not charge them at 14.5v. even with no load, (And only one battery) the voltage is max 13.9 ...., with the load, its aroudn 13.2 or so... i know the best option is just to replace the alternator, but for various reasons, i cant do it yet.
    so my question is this: Do i need to worry?? i know that batteries want to be charged near 14.5v...
    what can i do to increase the voltage? any internal mod in the alternator?? or could i use a high amp switching dc-dc converter?? thanks!

  2. #2
    FLAC SNOtwistR's Avatar
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    What is the condition of the battery? have you done a load test on the battery to see if it's that? Is that an internal regulator? If it is then maybe the other parts in the altenaor are worn out like brushes or stator. Just a thought and a couple of questions. SNO

  3. #3
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    Quote Originally Posted by SNOtwistR View Post
    What is the condition of the battery?
    The trunk-mounted battery is brand new (1 week old)(this is the one that even when disconnected, the output is the same) . the main battery: i haven't run a test, cause where i leave, i don't even know if something like that exists... BUT, it has only run down just once, and not anything hard... the car starts normally enough, so i guess the battery always has allot of charge in it...

    Quote Originally Posted by SNOtwistR View Post
    Is that an internal regulator?
    yiap. and now that you mention it, the alternator is indeed 20years old, so there is a good Chance something is worn out... with my old internal Vreg, outpout was 13.5. with the new, its 13.9 . the question is, what can be worn out, and if i change it , it might fix the problem???

    btw the alt is this : http://www.lulusoso.com/upload/20120...ed_On_Audi.jpg
    with a Vreg taht looks like this : http://thumbs1.ebaystatic.com/d/l225...62T5smTziA.jpg

  4. #4
    Constant Bitrate Rickk's Avatar
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    Regulators are supposed to be temperature compensated. The output voltage can vary by a volt or so depending on ambient temperature.

    It takes more volts to charge a cold battery than a warm battery.

    Without temperature compensation it would under-charge in the winter and over-charge in the summer.

    So, there may be absolutely nothing wrong with your regulator.

    You can most accurately test the batter charge condition with a temperature-compensated battery hydrometer. Anything else is sort of a guess. I just looked on amazon and saw one for 10 bucks.

    Rick
    Last edited by Rickk; 05-06-2014 at 06:41 PM.

  5. #5
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    Can you check the voltage to the regulator?

    Also where are you checking your voltage?

    What size wire do you have running to your rear battery?

    To test your battery you need to see the voltage drop under load. With a standard battery tester there is a ceramic heater built into the unit and it will get hot when you flip a switch but the voltage on the battery should drop a little and hold. If it drops dramatically or drops off noticeably while you are looking at it then the battery is bad.

    The alternator basically has three systems.

    Brush and coils. The moving part of the Alternator. This makes AC and you will see it may go from a little over +15 to a little under -15 volts if you could actually monitor its output. If these go bad you will lose voltage but generally if these go bad it is obvious and it either has a bearing failure where you lose everything or you get no voltage.

    Diode Rectifier. This converts the AC to DC. If you look at the voltage coming out it is more of a saw tooth than a real smooth DC. I have seen where some use 2 Diodes and others use 4. The use of diodes will drop the voltage output by the alternator by .7 for a standard diode or 1.4 for 2. (2 Diode setup would run voltage through 1 Diode for a .7 drop or 4 Diode setup runs it through 2 Diodes or 1.4). There are a different type of Diode that is only a .4 drop but I am not 100% certain which type is used in most Automotive applications. If one of these diodes goes bad then the saw tooth I mentioned has a missing tooth every other tooth. This of course would half the alternators normal output and may show up as a hum or buzz in your radio. It may not show up at all other than your alternator just doesn't seem to output the right power any more. Generally if you lose a diode you are sort of charging the battery when your lights are off but you drain the battery with headlights on.

    The last system is the Voltage regulator that will take that saw tooth and drop the high points of the saw tooth down to within range. Depending on the design of the voltage regulator you should see some smoothing of the saw tooth but it is more to limit the max voltage than for any smoothing.

    A related item that is absolutely necessary is the Battery. Not only does it hold storage to start your vehicle and help with momentary power requirements that the alternator is not designed for but it also smooths out the saw tooth mentioned earlier. NEVER run a vehicle without a battery because the noise coming from the alternator can damage components quickly. Older vehicles (prior to fuel injection) are not as susceptible to voltage spikes or drops that occur without a battery.

  6. #6
    Variable Bitrate
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    rick and red. i siriuosly doubt my alternator has any way of doing temperature compensation... remember, it 20 years old. although i agree that a colder battery should charge different than a hot.
    The wires are pretty Big. alt->2gauge->relay isolator (this is where i mesure the voltage) ->2g to one battery /4g to the other battery (which ofc has 0.1 or 0.2 volt less).

    so Red you are saying that if something was bad with the alternator, it would be totally obvious? well now that you mention it, the "OEM" voltage regulator, was 13.9V.... so maybe the whole alt was designed for less voltage, (i think that back then, the batteries did not want 14.5 volt as today).

    so what should it do?? is the dc-dc converter idea any good? or are there, battery chargers of that kind??

  7. #7
    Constant Bitrate Rickk's Avatar
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    Settra, believe it or not, automotive voltage regulators have had at least crude temperature compensation built into them just about forever. If they didn't have it, they wouldn't have worked out at all.

    As a for instance, check this link out:

    http://matchlessclueless.com/electri...age-regulator/


    This thread discusses some observations that people had on a cold morning...

    http://www.bobistheoilguy.com/forums...Number=2882395

    Here is a graph of preferred charging voltage versus temperature on a lead-acid battery....

    http://www.solar-electric.com/media/...ge-voltage.gif

    I spend 4 years of my life not all that far back designing a 28 volt/1000 amp alternator for use in an MRAP truck. We tested things for months - learned a lot. One thing I learned is that if you put 14.5 volts on a "12 volt" battery at normal temperatures for any length of time, it might explode. We put 100 amp circuit breakers in series with the batteries in case something went wrong. We used to trip the breakers a lot while we were working on charging algorithms. We wore out a few sets of really big truck batteries due to the abuse of over-charging them. At 14.5 volts, at normal to hot temperatures, your alternator will be maxed out dumping everything that it can into the battery. If the battery is fully charged, it will still pump all it can into the battery, making the water boil off and also warping the plates from overheating.


    Rick



    Edit:

    I thought I would throw this picture up. We put one of our 30 KW alternators in a 26,000# truck and I personally got to drive it 2800 miles (in 5 days) to Twentynine Palms Marine base (right near the Mojave Desert) for a demo. It was 110F during the day and we had to run the system for 8 hours straight each day (at full rated power) for a week.... closest thing to Afghanistan that we have in the USA.

    Name:  desert.jpg
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    Last edited by Rickk; 05-07-2014 at 04:15 PM.

  8. #8
    FLAC PhilG's Avatar
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    Quote Originally Posted by settra View Post
    hello forum. in my car (1992 audi 80) , the alternator, has a brand new voltage regulator, rated at 14.5v. i also have two batteries installed, isoalted with relay. the problem is, that the alternator, does not charge them at 14.5v. even with no load, (And only one battery) the voltage is max 13.9 ...., with the load, its aroudn 13.2 or so... i know the best option is just to replace the alternator, but for various reasons, i cant do it yet.
    so my question is this: Do i need to worry?? i know that batteries want to be charged near 14.5v...
    what can i do to increase the voltage? any internal mod in the alternator?? or could i use a high amp switching dc-dc converter?? thanks!
    Have a look here: http://www.smartgauge.co.uk/alt_mod.html

    I haven't tried it but....who knows.
    My 2007 Ford F350 Work Log located HERE

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