Dash Battery light on - Subaru Outback - Subaru Outback Forums
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post #1 of 20 (permalink) Old 04-11-2019, 12:44 PM Thread Starter
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Dash Battery light on

09 LGT. What condition might cause the battery light on the dash to be on just after starting? Car has been inside ~66* all night so it is not cold like outside. Started it to swap two cars and the battery light is on now. Battery was replaced in January. I hit a huge pothole yesterday during the snowstorm and was worried about suspension but that all looked fine. I don't see how an alternator issue could be related. Thoughts?

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post #2 of 20 (permalink) Old 04-11-2019, 12:48 PM
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A bad alternator that is not charging the car can create a BAT light.


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post #3 of 20 (permalink) Old 04-11-2019, 12:51 PM
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The light means the alternator isn't charging. Hard to say whether it is related to the pothole.



I had a pothole ruin a battery by breaking an internal connection, but since your car started, I doubt that is your problem. Old batteries are more susceptible to mechanical failure.
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post #4 of 20 (permalink) Old 04-11-2019, 01:03 PM Thread Starter
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I started it and disconnected the battery and it died so yep I think I need a new alternator...
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post #5 of 20 (permalink) Old 04-11-2019, 01:13 PM
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I started it and disconnected the battery and it died so yep I think I need a new alternator...

If you have a multi meter you can check it if your alternator is working, your reading should be higher than 12. Volt maximum 15.6 volt when you rev your engine. If the the reading stay 12 volt or lower, yes your alternator I'd bad.
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post #6 of 20 (permalink) Old 04-11-2019, 02:01 PM Thread Starter
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My son and I just picked up a reman alternator and I will pull my car into the shop again for the swap when my daughter leaves for work in a few hours. The car she drives (08 2.5i) was totally packed and piled with snow from this spring blizzard so it is thawing out in shop now. I will check the running voltage when I get it back in here. I am not a fan of reman parts but hopefully this does the trick. It certainly looks like reasonable quality.
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post #7 of 20 (permalink) Old 04-11-2019, 02:14 PM
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I started it and disconnected the battery and it died so yep I think I need a new alternator...

Ooof! careful when doing that, that can spike the system. Not recommended with modern cars.
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post #8 of 20 (permalink) Old 04-11-2019, 03:04 PM
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Ooof! careful when doing that, that can spike the system. Not recommended with modern cars.
Yea, the battery acts sort of like a capacitor for your vehicle's electrical system; It smooths out many voltage spikes that can originate from the alternator. Disconnecting the battery did confirm indeed that you have a bad alternator that is not putting out enough current to even keep the car alive but generally you are gambling whenever you disconnect the battery like that.

Remanufactured alternators are usually a safe item to buy; When alternators go bad it is usually a mechanical problem like bad bearings. Electrically it will be bad diodes or a bad winding. Diodes can be replaced... bad windings... those are more challenging and most people who attempt to fix windings are mere "hacks" who have no idea what they are doing.

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post #9 of 20 (permalink) Old 04-11-2019, 03:21 PM Thread Starter
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Head Scratcher...

Put the car in the shop a few mins ago and my son and I checked it out. This is before starting any work on it. No alt/batt light now and it runs fine with battery neg disconnected. Unsure what the issue (if any) might be. Argh...
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post #10 of 20 (permalink) Old 04-11-2019, 04:33 PM
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Not unusual for alternator regulators to develop an intermittent problem. If it works now, it might not work later. But I guess you could watch it for a while to see if the symptom reappears.

Back to the disconnected battery. The alternator could be applying higher voltage to the rest of the car's electrical system -- something in the range of 17-24 V, or more. That could damage critical functions, as well as stress simple things like light bulb filaments.

The alternator regulator works by controlling the current through the alternator rotor winding. As the rotor turns, the magnetic field generated by that current passes over the field winding, which generates an AC voltage and corresponding current that is connected through rectifying diodes to the alternator output terminal. The voltage from the field winding itself is not controlled; instead, the regulator turns the rotor winding current on and off fairly rapidly, with the on times longer when the battery is not fully charged, and shorter when it is. This results in a pulsed output from the alternator that goes between zero and somewhere above ~17 V. The pulses can be accompanied by spikes of voltage far higher. When the battery is connected, as was mentioned by others, the battery smooths out the pulses by absorbing the excess energy being output by the alternator. When the battery is disconnected, the pulses, and spikes, are not controlled.

Disconnecting the battery is not a way to test the alternator. A simple multimeter would be adequate to check if the alternator is working.

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