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Battery Warning light comes on a 2500 plus RPM

I have a 2013 Impreza 2.0L. Recently while driving at highway speed, 75 mph the battery warning light came on and then went off. It would come on and go off for the last couple weeks. Then this past week while going to work I lost just about all of the power in the car. I was able to get to work and get a new battery put it. The light didn’t come back on until the way home, while on the highway. I’m thinking the alternator is going and I’m looking at replacing it on Monday. Is my thinking correct in replacing it and what do I need to know before tackling this little project?

Replace the serpentine belt first to see if it solves the problem.

A serpentine belt can look good when it actually needs replacement.


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First, make sure the problem is the alternator. From your post it seems pretty likely, but best to do a quick double check before taking on the job and expense of replacing it.

First thing in the morning, after the car has sat overnight, with the engine off , measure the voltage at the battery. It should be in the 12-13 volt range, usually around 12.5 volts. Now start the engine and let it idle. Measure the voltage at the battery again. This time it should be in the 13.5 to 15 volt range. If so, the alternator is probably working and doesn’t need to be replaced. Good idea to briefly rev the engine a little, 2000 rpm say, and make sure the voltage doesn’t suddenly drop at higher rpms.

Sounds like the alternator to me too, but I liked Tester’s idea of changing the serp belt. If it is slipping, it could be reducing the output of the alternator. The alternator and the AC pump probably each create the greatest mechanical load per-component on the alternator. A belt could be slipping on the alternator and not on the other components.

Tested it this morning. Volt range was good at start and at idle. It did drop when reved to 2500 rpm. I did take the car to the auto store yesterday and they tested the alternator, said it was good, but I’m still not sure.

dropped by how much? and did it stay that way when the RPMs returned to idle? Note that the voltage will drop with time as the battery becomes closer to fully charged. After you start the car, it will be higher, perhaps 15 volts or up, then as it charges, that voltage goes down, perhaps to as low as 14 volts when fully charged.

Did you check the belt?

I thought the alternator would not be bad given your symptoms. Why would it fail only at high RPMs? But the belt could slip at high RPMs.

Changed the belt, light doesn’t come on now at low speed or idle. battery drops to 12.5 (or half charge according to my cheap gauge) when reved up to 2k plus. Just so you know on the highway the engine is at about 3k rpm

That seems right to me. Are you getting any warning lights at all now?

Just the battery one at 2k plus rpm, not at idle

Someone else here knows, but I don’t. Does this car have a separate voltage regulator or is it inside the alternator or what?

I just looked on rockauto, and it appears to be internal

I say “appears” because they only have the regulator, not the entire alternator assembly. And the regulator appeared to be the kind that is only accessible if you partially disassemble the alternator.

I’m curious as to this “auto store” . . . is it an auto parts store or an auto repair shop? If it’s an auto parts store, I’d be a little wary of their test results. Some of the lower end testing equipment produces ambiguous test results, in my opinion. Not to mention, you have to know how to use the tool(s) correctly and perform the test correctly

Most likely it’s the “brushes” on the voltage regulator that is very close to be worn out. They can give the symptoms You describe. In theory, You can change them, but it might be a short lived solution. I would recommend a new (refurbished) alternator if this scenario is the case.

This is when they are worn out:

This is when new:

Yeah, I know it’s from two different vehicles.

Ahh, found this:

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I don’t think it is within spec for the alternator’s battery charging voltage to drop to 12.5 volts above 2500 rpm. 12.5 volts is just the battery itself, no charging going on at all. Which is why the battery light comes on. Anybody here have a voltage gauge in their car and can double check that’s not normal? To me anyway, that’s an indication of an alternator related problem. Could be a slipping belt, loose connection, but more likely there’s a fault inside the alternator of some sort, likely to do with the brushes. Make sure the belt is the correct one, and tension; that the wiring between the alternator and the battery is good on the positive side, and the negative side; that the alternator and battery are grounded to the chassis with a good, tight, rust free connection. If that all looks ok, I’d just replace the alternator.

The question remains why it tested “ok”. My guess, the fixture doesn’t test it at high rpms, like 2500+ rpm. Physics-wise, it’s harder for an alternator to do its job at lower rpms, so it would be natural for the test fixture to focus on low rpm testing. Plus low rpm testing is safer.

Perhaps they didn’t test it properly

Perhaps they just measured 14V across the battery terminals at idle, and declared the alternator to be good

I think you are right, they only tested it at idle rpm. Have you ever experienced an alternator that fails only at higher rpm? I can’t think of a failure mode where that would happen, except possibly centrifugal force is doing something that prevents the brushes from making contact with the slip ring. Or maybe the alternator bushings are worn.


I don’t have any kind of scientific background, but here goes

The alternators I’m thinking of did just fine at idle with no loads

yet at the test rpm and with the proper loads applied, the alternators failed. Couldn’t do their jobs. I never disassembled them to discover exactly what was the problem. In any case, remanufactured alternators solved the problem. I retested to verify the repair. Night and day difference