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Bad battery or bad alternator test


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27 replies to this topic

#1 Rooster2

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Posted 20 September 2012 - 01:08 PM

My battery won't stay charged. I have heard that pulling off one of the battery cables, while engine is running, is a good test to tell if the alternator or battery is bad. I was told using this test, that if the car stays running with a battery cable pulled off, then this indicates the battery is bad. If engine stops, then this indicates that the alternator is bad.

Is this a reliable test? Could this damage the alternator doing this test?? Anyone know?

#2 92_rugby_subie

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Posted 20 September 2012 - 01:59 PM

You can cause a short.... I wouldnt do that.

Pick up a cheap multimeter from your parts store and check it.

I believe I use DCV 20 on my multimeter.

Youll want to do this while the engine is running. At idle is fine. Hook up your multimeter: One end on the positive terminal and one on the negative. Over 12.5 volts should charge the battery but youll want at least 13.5 volts for reliability. My reading is 14.34V on my EA82 and 14.3V on the Legacy.

Chances are if your car is running once jumped, the battery isnt holding charge and needs to be replaced because that means its running off the alternators power which is NOT what the alternator is designed for and can fry the alternator... then youll have to replace the battery still AND replace your alternator.

Here is the output. And it is "DCV 20V" that you want to use.
Posted Image

Edited by 92_rugby_subie, 20 September 2012 - 02:18 PM.


#3 jp98

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Posted 20 September 2012 - 02:50 PM

Why not just go down to a automotive parts store such as Autozone and have them test the system. They can do a load test on the battery and a charging test on the alternator while they are on the car. Then if one or both of them are bad you can decide on what you want to do. You don't have to buy a thing from them to get this done.

#4 92_rugby_subie

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Posted 20 September 2012 - 03:13 PM

Multi-meters are multi-use lol. Not bad to have one... and its not like its an expensive tool...

#5 jp98

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Posted 20 September 2012 - 03:25 PM

Multi-meters are multi-use lol. Not bad to have one... and its not like its an expensive tool...


You are right there. It is also good to learn how to use one also. It is also nice to have a analog meter also just for the times when the digital is jumping all over the place with ghost readings.

#6 Ricearu

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Posted 20 September 2012 - 05:22 PM

pull the negative cable. I have done it a million times. The books say don't but I have never had a problem.

test with a mulitmeter goes as follows
check open circuit voltage on the battery (key off engine off) full charge is 12.6v at rest

start engine should be 13.6v-14.6v charging voltage.

If you pick up a used FLUKE multimeter from a pawnshop (about $30) you can do an amp test in line of the negative cable with key off engine off to see if you have a parasitic drain. This can kill your battery overnight or over a few days of not running.

set your meter to dcA (dc amps) and move the red lead on the meter to the amp port. Unhook the negative cable on the battery and hold one lead to this cable and the other to the negative post of the battery. any more than about 30ma isn't acceptable. depends if you have an alarm system. some of them can pull up to 40ma by theirself. anything more than 300ma can drain your battery over just a few days.

#7 MilesFox

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Posted 20 September 2012 - 05:29 PM

you can do this as a proven method. there could be a voltage spike by electrical theory, and theoretically cause a voltage spike. The battery acts as a ballast for the alternator's output and loads on the system.

Do the negative battery thing. if the car stalls then how can there be a voltage spike? If the car does not stall just shut it down or reconnect the battery right away.

to reduce this risk turn off all lights and heater, etc. you are probably more likely to get a volt spike while re-connecting the battery. maybe just turn the car off.

hwever, if the car remains running, the alternator could still have an intermittent charging pattern, so further troubleshooting is recommended

#8 TOONGA

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Posted 20 September 2012 - 06:39 PM

While pulling the negative terminal will tell you which item is faulty it can also destroy a perfectly good alternator. the diodes and/or regulator can be damaged. this will either cause the alternator to stop charging or overcharge. either way a 5 dollar multi meter will give you the answer immediately.

With the car stopped and ignition off check the batterys charge it should be between 12.5 and 13.5 volts, hold the probes on the terminals for at least 30 seconds to a minute to check for current fluctuations, as this will give you a better indication of the batterys health.

start the engine and put the probes on the battery teminals again. you should have 14volts + 15 and above is a problem as the regulator is faulty and is overchargin the battery. below 14 is a problem as the alternator is not charging the battery.

This video on youtube has safety over kill but it does show you a way to test the alternator.



TOONGA

Edited by TOONGA, 20 September 2012 - 06:40 PM.
added stuff


#9 jp98

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Posted 20 September 2012 - 07:35 PM

You really need to have the battery load tested with a load tester to see if it is bad or not. I have seen dozens of batteries that tested just fine for voltage but as soon as you put them to the load test they failed.

#10 john in KY

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Posted 20 September 2012 - 08:23 PM

Only have limited electrical experience but my understanding is since the alternator requires a small amount of current to function, once the battery is out of the circuit the alternator should stop producing electricity.

#11 Fairtax4me

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Posted 20 September 2012 - 08:46 PM

The alternator needs current to excite the field windings to begin charging. Once it starts it is self-exciting and will continue to charge.

Pulling the negative cable can work but it is NOT smart. Electronics do not respond well to rapid voltage fluctuations, you can cook the ECU as soon as the cable is unhooked. Newer vehicles are especially prone to this since nearly every system of the car has some type of control module.

Just take it to autozone or whatever your local parts store is and have them test the battery and charging system.

#12 Cougar

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Posted 20 September 2012 - 10:14 PM

I agree. Disconnecting the battery while the alternator is working is a very bad idea. People get away with it but you won't do that on my car. Using a voltmeter to check the charging voltage will tell you pretty much all you need to know about the charging system. It can also tell you something that the other method won't tell you. You can check for bad diodes in the output by looking at the AC ripple voltage across the battery. If there is more than .1 volt of AC volts then the alternator needs to be replaced.

#13 heartless

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Posted 22 September 2012 - 08:05 AM

I would also suggest going to your local auto parts house to get a load test done on the entire electrical system. The testing equipment they use will check the battery at rest, the battery & starter when starting, the alternator output, and the entire charging system under load - ie: lights and blower motor on. This can usually offer a much more detailed look at your car's electrical system than you can do on your own - even with a good multimeter.

the battery cable trick was used, and is fine, on older cars that do not have sensitive electronics, and while many do still do this successfully on newer cars, there is a big risk of frying those electronics when it is done.

If you are poor, like me, I do not suggest doing it, because Murphy's law will kick in, and you will more than likely cause more damage than you can afford to fix. :-p

Now, all of that said - there are times when the parts house check will not reveal the problem, such as intermittent alternator failure under heat - which i dealt with earlier this year. Even if I was experiencing the problem when I arrived at the parts house to get it checked, opening the hood would allow the excess heat to escape, and the alt would test fine. It was exceedingly frustrating.

#14 uniberp

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Posted 22 September 2012 - 03:01 PM

Which is exactly what I am dealing with now. Marginal low voltage condition, showing up in funny ways: volume knob on stereo not responding, cruise control and remote start working intermittantly, expecially when I was using wipers, lights, fan and defrost (a/c compressor).

Stopped by autozone on the way home, warm afternoon, he put his tester on the battery, it showed 14.3 v charging, battery "good". The battery was 138 degrees he said, which seemed kinda hot indicating charging.

So I suspected intermittant charging due to heat/vibration/something. Replaced alternator. Seemed better at first but then radio/cruise started acting up again. I checked connections for a third time. It appeared the battery block gorund cable was potentially weak, very fine wires may be susceptible to corrosion. I twisted them a bit for better conection and it may have improved things. I then bolted in an additional ground strap from the battery to the block (New cable bolted to same mounts.)

Not happy about buying that alternator, but I doubt the battery because it was replaced last winter. If it was seriously discharged and recharged it may be damaged, but there was never an alternator light.

1999 Forester

...Now, all of that said - there are times when the parts house check will not reveal the problem, such as intermittent alternator failure under heat - which i dealt with earlier this year. Even if I was experiencing the problem when I arrived at the parts house to get it checked, opening the hood would allow the excess heat to escape, and the alt would test fine. It was exceedingly frustrating.


Edited by uniberp, 22 September 2012 - 03:04 PM.


#15 Cougar

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Posted 22 September 2012 - 09:47 PM

If you are still having problems with the accessories, like the wipers, I suggest you have the alternator checked out for AC ripple voltage while the engine is running around 1,500 RPM. From what you say, it sounds like the new alternator may have a diode problem.

#16 uniberp

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Posted 23 September 2012 - 11:39 AM

If you are still having problems with the accessories, like the wipers, I suggest you have the alternator checked out for AC ripple voltage while the engine is running around 1,500 RPM. From what you say, it sounds like the new alternator may have a diode problem.


New alternator seemed no diff than the old, but I think I have to wait for the battery to be properly charged. If the battery was working a lot (Discharging and charging) It may be worn out, but like I said ther alternator light never came on during running.

I do suspect the alternator and or the diodes. Machinery and electronics re so long-lived now we may see new failure modes, like the ripple you refer to.

I gave the car back to the driver, and will wait for a report next week.

#17 Cougar

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Posted 23 September 2012 - 04:12 PM

After reading post 14 again things look different to me and it now seems to me that the real problem you are having is due to excessive noise in the power bus system. You stated that the trouble is more pronounced when you have accessories like the wiper or blower motors are on and they can create noise on the power system. Clean the engine and chassis ground connections along with the battery connections to see if that helps. There may also be a noise filter that needs to be checked. If you have a good digital meter you should be able to measure the AC voltage and check the amount of noise on the bus. You can also see if things clear up while the engine isn't running. If the trouble is significantly less then excessive electrical noise is a good bet.

Edited by Cougar, 24 September 2012 - 10:21 AM.


#18 edrach

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Posted 23 September 2012 - 11:34 PM

You can check for bad diodes in the output by looking at the AC ripple voltage across the battery. If there is more than .1 volt of AC volts then the alternator needs to be replaced.

I never disagree with what you post and I don't disagree with this either, but I should add that a fully charged battery acts as a capacitor on the alternator. If the battery is sufficiently discharged,the ripple voltage would be significantly higher than 100 mv AC.

#19 Cougar

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Posted 24 September 2012 - 10:17 AM

I never disagree with what you post and I don't disagree with this either, but I should add that a fully charged battery acts as a capacitor on the alternator. If the battery is sufficiently discharged,the ripple voltage would be significantly higher than 100 mv AC.


Hi Ed, well I would have agree with you there if the battery is severely discharged, so good call. I'm not sure what the peak to peak voltage would be but the average AC voltage level still should be pretty low.

Edited by Cougar, 24 September 2012 - 10:24 AM.


#20 wateraeroplane

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Posted 24 September 2012 - 11:13 AM

What is the year, model and the last time your replace your battery?

#21 uniberp

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Posted 26 September 2012 - 08:52 AM

What is the year, model and the last time your replace your battery?


1999 Forester, replaced battery within the last 12 months. Replaced alternator, added a second ground cable from battery to block (doubled up, basically). Cleaned and retightened battery to body connector.

I will know by this weekend if things are is still working.

#22 wateraeroplane

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Posted 27 September 2012 - 06:57 PM

Measuring the voltage at the battery terminals is a good starting point. Battery should carries around 12.1-12.5 volts when the car ignition is off and about 13.5-14.2 volts when the car is running. My 2005 Forester is 13.7 volt when running, my 2006 Impreza is 14.1 volt.
Although the alternator was changed, it still a possibility the alternator goes bad (was a brand one or re-condition). Also there is a regulator controlling the charging current from the alternator to the battery when the car is running.
A regulator is a rectifier & filter proving a stable voltage to charge the battery.
Another possibility: I seen CD changer/player mechanism jammed and the motor kept on spinning tried to un-jam the mechanism to eject/exchange CD, thus keep drawing current from the battery even your car is off. Try disconnect your stereo to verify.
Another factor is how far you drive daily? If you only drive 10-15 minutes that may not have enough time to re-charge your battery then your battery is having deficit every day.

Cheers

#23 uniberp

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Posted 27 September 2012 - 08:00 PM

Another factor is how far you drive daily? If you only drive 10-15 minutes that may not have enough time to re-charge your battery then your battery is having deficit every day.
Cheers


If this is true, then for some reason Subarus have heavier electrical loads than all the other cars that are used only for short drives with no problems.

It's 75 amp alternator. That should be massively enough to start it, keep the lights fan compressor and wipers running and charge the battery in 15 minutes.

#24 edrach

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Posted 28 September 2012 - 09:40 AM

If this is true, then for some reason Subarus have heavier electrical loads than all the other cars that are used only for short drives with no problems.

It's 75 amp alternator. That should be massively enough to start it, keep the lights fan compressor and wipers running and charge the battery in 15 minutes.

Not quite the case. Starting current can be very high; I've actually never measured it, but I would suspect 300 amps for 15 seconds is not unusual; that would draw 75 amp/minutes (300Ax1/4min) of charge out of your battery. Now you have a 75 amp alternator which supplies all of your energy to drive the car down the road; remember the battery is only a reservoir which will supplies energy to start the car. Now drive the car for 15 minutes which uses 50A from the alternator to drive the car down the road. That leaves 25A available to recharge the battery and replace what was lost. That's 375A/minutes (25Ax15min). That's just barely enough to replace what was used in starting the car. Now add in other factors: an alternator that's not quite up to snuff, a battery that's not taking a full charge anymore, running your headlights during your 15 minute drive, or your stereo, etc. and you can see that a short daily drive will eventually deplete your battery and give you problems.

If you only drive short distances for each start of the car, it would help if you buy a trickle charger and plug the battery into that overnight. A trickle charger that supplies two amps per hour overnight would bring your battery up to snuff pretty well. (2A x 480min = 960A minutes of charge). At $15 to $20 cost for the charger, that's a pretty cheap solution until you find what is actually causing your problem.

#25 edrach

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Posted 28 September 2012 - 09:42 AM

Not quite sure how I double posted this, but no need to see this twice. Deleted.

Edited by edrach, 28 September 2012 - 10:36 PM.





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