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Thread: Alternator is only putting out 13.78 volts

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    Default Alternator is only putting out 13.78 volts

    I have a 1990 XJ with a 1996 engine and accessories. Ever since I got it, the voltage gauge always has read a little less than 14v. Today I finally put a DMM on it, at idle with no electronics on I get 13.78v. If I remember correctly, an alternator should put out around 14.5v, correct? With the headlights and A/C on it drops to 13.5 volts.

    That said, I've never had any charging problems, and I don't have any aftermarket power sucking parts. (yet ) Should I worry about this? Is my alternator going out?

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    Registered gonesurfing's Avatar
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    13.7 Pretty close to 14 for me.

    I'm pretty sure 13-14V is normal range.

    With the engine off, battery should have 12V
    With engine on, battery should read 14-15V

    With engine on, use a screwdriver to press the ground tab on the alternator, and then check battery. It should be at least 15V.
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    for a stock jeep alternator that sounds about right, i wouldnt worry about it.
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    Default Ditto...

    -=- originally posted by 1989GApioneer -=-
    for a stock jeep alternator that sounds about right, i wouldnt worry about it.
    What he said.
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    Stock alternator numbers are right on.

    A battery sitting, off of charge source, at a full state of charge should read 13.6 or better. A battery holding 12v at storage/sitting is discharged.

    Don't ask for the math, it is ugly but true.

    If your alternator is pushing those numbers, you are fine. If your battery is holding those numbers then it is time for a change.

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    JU's Eric Cartman ZmOz's Avatar
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    -=- originally posted by Ivan Aby -=-
    A battery sitting, off of charge source, at a full state of charge should read 13.6 or better.
    Nope. If your battery reads 13+ volts then something is wrong. A fully charged automotive battery is around 12.6 to 12.8 volts.

    As for your alternator numbers, they sound a bit low to me. Mine is about 14.2v with nothing on. You might want to upgrade your alternator/battery cables.


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    http://www.landiss.com/battery.htm

    Yes, a battery has 2.1v per cell, but a healthy battery will hold much more for much longer.

    Discharge is a state of decline and a chemistry below 13v is a chemistry that is sulfating. Sulfation is a chemistry of degradation, attacking the lead in the battery and breaking off the animony from the plates, thus eliminating the ability to start by killing CCA.

    There are only two things that kill batteries over a year old. The first is sulfation (cronic discharge below 13v). The second is constant charging.

    Constant charging uses all active materials in the battery so that there is no chemistry left.

    All other failures usually shake out in the first year (shorts, dead cells, strap issues etc).

    Your alternator is fine. Have your battery tested with a conductance tester (Midtronics brand) at a dealer or local parts house.

    The Midtronics technology will give you a 90 day window of battery failure.

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    Registered XJay96's Avatar
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    Is it me or did I learn somthing wrong in Autotech in HS? Most cars run on a 12 volt system. Typically an alternator will put out somewhere between 12.5-13 volts in charge, or am I wrong?

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    You learned technology from 1957.

    The lesson I gave was from '04.

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    JU's Eric Cartman ZmOz's Avatar
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    -=- originally posted by XJay96 -=-
    Typically an alternator will put out somewhere between 12.5-13 volts in charge, or am I wrong?
    No, that's a battery. Alternators should put out 14+ volts.


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    JU's Eric Cartman ZmOz's Avatar
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    -=- originally posted by Ivan Aby -=-
    Yes, a battery has 2.1v per cell, but a healthy battery will hold much more for much longer.
    Wrong again. A fully charged automotive battery in good condition should hold very close to 12.8v. Over 13v and something is wrong. SLA batteries should be above 13v when charged, but most automotive batteries are not SLA.

    Brand new Diehard:

    3 month old Champion:

    ~5 year old Exide:

    All batteries are fully charged.


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    Different meters give different readings, +/- 5% is all I ever expect outta DVM's. In other words, a variation of +/- 0.7Vdc for this situation. 'Could really be 12.9Vdc-14.2Vdc very easily.

    I wouldn't be too worried about absolute numbers but rather, how does the voltage change from off, at idle, with lights-blower on/off, RPM, etc.?

    Use several different "load/operation" conditions while monitoring the voltage. That will provide more useful information than some vauge number on an LCD display.
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    Great check on old batteries that are sulfating. I have a warehouse with 5000+ units that turn in a week than won't leave unless they are over 13.1...

    Oh, BTW, I own the fifth biggest battery distriburtorship on the west coast...

    I guess my information isn't solid, especially against someone who works on ride aboards.

    I'll give details later.......I am bored right now.

    Edit: I can show you hundredes of batteries at 14.0 or above....perfectly healthy......your scewed facts are just that. Don't take facts from tech school and apply them.....the don't always fit.

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    -=- originally posted by joguitar -=-
    Different meters give different readings, +/- 5% is all I ever expect outta DVM's. In other words, a variation of +/- 0.7Vdc for this situation. 'Could really be 12.9Vdc-14.2Vdc very easily.

    I wouldn't be too worried about absolute numbers but rather, how does the voltage change from off, at idle, with lights-blower on/off, RPM, etc.?

    Use several different "load/operation" conditions while monitoring the voltage. That will provide more useful information than some vauge number on an LCD display.
    Well, I am using a very expensive Fluke meter, it should be pretty darn accurate. The voltage isn't really any different at idle or highway speeds, but with heavy loads it goes way down. The gauge in my XJ is allways showing less than 14v also. I know that gauge isn't the most accurate thing, but it is designed to point straight up when everything is working good. When I have the headlights on and the AC on full, the gauge goes down to almost 12v. (but 13.5v measured with my DMM)

    BTW - I agree with ZmOz, I've never seen a car battery over 13v. And I used to collect used batteries, desulphate them, and resell them. I've tested probably 1000 batteries, ranging from brand new to 10 years old.

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    -=- originally posted by Ivan Aby -=-
    Great check on old batteries that are sulfating.
    Old batteries? 1 is a brand new $80 battery. Another is only 3 months old. They are not sulphated. I don't care what you do, a standard flooded lead acid battery is not 13v unless you just took it off the charger. Only specialty batteries are over 13v. I don't know why we're even talking about this, this guy is having an alternator problem, not a battery problem.


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    Not to interrupt this diversion about battery voltage, but the poster asked about ALTERNATOR voltage.

    There IS a difference, and that difference is driven by the state of charge of the battery.

    On a discharged battery (one below approx. 13.6 volts), the alternator will be putting out MORE than the battery, up to about 14.5 volts max. This allows current to flow INTO the battery, since the voltage of the vehicle is HIGHER than the voltage of the battery. Then, as the battery begins to reach it's "full charge" level of 13.8V (actually anything higher than about 13.5V), then the ECU backs off the stator voltage on the alternator, and that, in turn, lowers the voltage put out by alternator.

    So, if one measures a battery before starting, a good one should measure around 12.5V (you experts can argue the exact value), but as soon as the car starts, the voltage at the batt. terminals should rise to AT LEAST 13.8V, maybe as much as 14.5V. This is the alternator doing it's thing, recharging a battery that was discharged during starting. As the battery gets recharged, the alternator voltage will drop back, until it's finally stabilized at about 13.6V, plus or minus a couple tenths of a volt.

    All values I mention are from measurements AT THE BATTERY TERMINALS. Elsewhere in the system, voltages might be higher or lower, due to voltage drop in the wires. Using the guage in the instrument panel is probably the worst way to establish the true health of the electrical system, as the guage is NOT very accurate, and the voltage drop in the wires to the guage makes the readings artificially low.

    When in doubt, use a decent voltmeter directly on the battery terminals to get the best data.
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  17. #17

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    -=- originally posted by OldJeep -=-
    Well, I am using a very expensive Fluke meter, it should be pretty darn accurate. The voltage isn't really any different at idle or highway speeds, but with heavy loads it goes way down. The gauge in my XJ is allways showing less than 14v also. I know that gauge isn't the most accurate thing, but it is designed to point straight up when everything is working good. When I have the headlights on and the AC on full, the gauge goes down to almost 12v. (but 13.5v measured with my DMM)

    BTW - I agree with ZmOz, I've never seen a car battery over 13v. And I used to collect used batteries, desulphate them, and resell them. I've tested probably 1000 batteries, ranging from brand new to 10 years old.
    I've used/compared dozens of meters in my lifetime. Some are Fluke (good meters) some are off-brands. All of 'em deliver different readings measuring the same source. Anybody who thinks their meter is "absolutely correct" is fooling themselves BIGTIME.

    A good electronic technician uses a volt meter to compare readings within a circuit. For most uses, a meter merely provides an "indication" within a circuit and not an absolute value.

    Believe what you want, but I know more than a little "tech school" about electronic testing/repair.
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    alternator output should always be between 13.5-14.5, running, then turn on all acessories, still should be in that range

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    I thought I would add my input to this thread since I have also been wondering about the voltage readings that my cherokee gives. It reads 14v unless I turn the AC which makes it drop(at least the gauge) to what looks to be about 13v. So, today I decided to test my battery, alternator. Here are my findings:

    Battery with engine off(Engine was last ran 12 hours ago) = 12.43V

    Battery with engine on, no acc. = 13.75V

    Turning on lights, stereo or AC caused no more than a .05V drop, even with all 3 on.

    I put the 12 hours since last ran bit in because I noticed after i shut the engine off the battery was still at ~13.5V and steadily dropping. I am unsure how long it takes to become stable.

    So are these readings normal? My voltage gauge used to always read 14V up until a couple months ago which gave me cause for concern. What are your thoughts?

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    Turning on lights, stereo or AC caused no more than a .05V drop, even with all 3 on.
    It's all in the AMPS when it comes to draw. A 12v battery sitting shows 12v, under load it may drop a volt but only due to discharging amps.

    Your alternator is HEALTHY if:

    1) Running voltage is higher than non. Can't charge a battery unless your give it more power than it has.
    2) Running voltage drop is less than non. Alternators should sense the increased draw and therefore supply more power (read: diodes and xciters).

    From what I've read from the Non-scientists it sounds like your alt, however lower than optimum is still fine. Not all alternators are built the same (hence different readings).

    Certain things to look for are AMPs at idle, and at 2500 rpm (average cruise rpm) to make sure the battery is not becoming a backup power source for the system. The battery has 2 purposes in it's life: 1) TO start the motor. 2) to backup the alternator when an increases load is instantly added (like a capictor).

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