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Bad battery or bad alternator test
#1
Posted 20 September 2012 - 01:08 PM
Is this a reliable test? Could this damage the alternator doing this test?? Anyone know?
#2
Posted 20 September 2012 - 01:59 PM
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.

Edited by 92_rugby_subie, 20 September 2012 - 02:18 PM.
#3
Posted 20 September 2012 - 02:50 PM
#4
Posted 20 September 2012 - 03:13 PM
#5
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
Posted 20 September 2012 - 05:22 PM
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
Posted 20 September 2012 - 05:29 PM
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
Posted 20 September 2012 - 06:39 PM
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
Posted 20 September 2012 - 07:35 PM
#10
Posted 20 September 2012 - 08:23 PM
#11
Posted 20 September 2012 - 08:46 PM
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
Posted 20 September 2012 - 10:14 PM
#13
Posted 22 September 2012 - 08:05 AM
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
Posted 22 September 2012 - 03:01 PM
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
Posted 22 September 2012 - 09:47 PM
#16
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
Posted 23 September 2012 - 04:12 PM
Edited by Cougar, 24 September 2012 - 10:21 AM.
#18
Posted 23 September 2012 - 11:34 PM
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.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.
#19
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
Posted 24 September 2012 - 11:13 AM
#21
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
Posted 27 September 2012 - 06:57 PM
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
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
Posted 28 September 2012 - 09:40 AM
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 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.
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
Posted 28 September 2012 - 09:42 AM
Edited by edrach, 28 September 2012 - 10:36 PM.
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