Overvoltage bizarreness



I have what I think is a bizarre alternator and/or battery problem. It makes no sense.

A couple of days ago my wife attempted to start our 2000 Subaru Outback 4 cyl automatic. The battery was dead.

I used a car charger to charge the battery. My $20 battery tester said that the voltage from the alternator was 14.5V. This is as it should be.

The battery ? a 6-year-old Interstate MT-35 ? did not seem to charge the way other batteries seem to charge. Every other battery will trickle at about an amp after about 8 hours of charging. This battery ?trickles? at about 3 amps after 16 hours.

I took the car into NAPA and the guys were a little uncomfortable with their $400 electrical system tester. The tester?s display said that there was overvoltage. I don?t see overvoltage on my cheapo tester.

Another, possibly related, problem is that my ABS light flickers on and off. Can this be caused by a bad voltage regulator?

A new alternator will set me back about $400. A new battery, about $80. Call it $500.

Can one just put on a new voltage regulator? Why do manufacturer?s bury a regulator in the alternator?

So the question is: Why did the battery die? Is it true that a bad battery can cause an alternator to go bad and vice-versa ? so that you have to replace both in order to be sure that you won?t have to replace both again in the near future?


I would assume the 6 yr old battery is bad. The alt may show an over voltage condition trying to charge a shorted battery and the fluctuating voltage could play havic on the eletronics. But I agree at 14.5 volts it is not an overcharge. I have found that car batteries being made today are more likely to short than to just get lower voltage as they used to. Anyway, I would replace battery and then if problems occur have the alt rebuilt at a electric motor shop, half as much as new one and they usually more reliable.


i had a battery go bad in my 96 geo one time and the air bag light was on and then it would go off when i turned the car off and restarted it then 1 week later car wouldn’t start jumped the car and took it to a battery place and they tested it and said it was a bad battery replaced the battery and the air bag light when’t out and never came back on again .go to a place where they only deal with batteries and have them put the car on a machine and they will tell you if it’s the battery or alternator.


Oh, wow, I didn’t even think about a short!


I forgot to thank you for your answer.

Question: If the alternator is bad, won’t that destroy a new battery?

And can a battery “have a short” and then “not have a short”? Could it be heat related?

Also, anyone know why newer batteries develop shorts and older ones didn’t?


You might just try a new battery because it is 6 years old and shows anomalies during charging. The battery does a lot to moderate the system voltage levels. Remember that the alternator produces a pulsing DC voltage which the battery smooths out. Then you should be able to see if the alternator needs to be replaced. In addition to DC voltage readings taken at various engine speeds and durations, you also could put an oscilliscope on the line to see any AC ripple that may be upsetting the computers.


So the battery is acting like a capacitor, too? The things they don’t teach a EE. (I graduated as a EE 40 years ago but am barely qualified to screw in a light bulb.)

What should the silly scope trace look like? How much of a ripple is enough and/or too much?


In my experience, cheap meters are very easy to confuse. Put a little high frequency noise with the signal and watch the readings skew. On top of that, they aren’t calibrated very well to start with.
Second of all, cheap meters can be dangerous. Most aren’t fused on the ammeter scales, so guess what happens if you try to measure voltage with the meter accidentally set for amps? The wires can melt almost instantly and cause serious injury.

As for what the problem is, borrow a good meter, set it to measure AC, and connect it. That will measure the ripple. Chances are, it’s too high.

And yes, the battery is also a capacitor. Consider what a battery is - many metal plates separated by electrolyte solution. A capacitor is made up of many metal plates separated by a dielectric medium.


Well, I took the car into another shop. They got the same readings that NAPA did. This time, though, I brought my cheapo meter with me.

The nice guy at Fuji Motors in Boulder walked me through the diagnostic and said “There! We’re seeing 17V! It should never been about 15. It’s a bad alternator.”

So I slapped on my el cheapo (http://search.harborfreight.com/cpisearch/web/search.do?keyword=battery+tester&Submit=Go) and for the first time saw the meter read off scale.

The problem, obviously, was intermittent and it was likely frying the battery.

Since the battery was old (and likely failing, anyway), I bought a replacement battery. We went through it again and there was the 17V with the new battery. It had to be the alternator.

The nice man, John, at Fuji Motors told me that replacing the alternator was a simple job on the Subaru. He walked me through which screws and which order and then sold me the alternator.

By doing the work myself, I’ll save a couple of hundred bucks.

Well worth it to me.


Just curious. Are you measuring the 17 volts at the alternator plug or the battery terminal?


Just curious. Are you measuring the 17 volts at the alternator plug or the battery terminal?

Battery terminals.


Just curious. Are you measuring the 17 volts at the alternator plug or the battery terminal?

Battery terminals.


Well, before I swapped out the alternator, I attempted to recharge the (new!) battery. After 16 hours, the charger was still sending out 3 amps.

I swapped out the alternator on my Subaru, (about 40 minutes of work having never swapped out an alternator on a Subaru), I then charged the new battery again.

After 30 minutes the charger was trickle charging the battery. It would seem to me that the alternator was sucking up the charger’s charge. Indeed, the alternator was warm to the touch.

Thus, I guess, one symptom of a bad alternator is that a charger never goes to trickle charge. I would guess that a/the diode went bad.

How hard would it have been for me to replace just the diode instead of spending $200 on a rebuilt alternator?


You can repair the alternator but from experience I have had doing one for my Subaru it is better to get a remanufactured unit from Bosch. All the normal wear parts are replaced in it and you get a warranty if something goes wrong later on. You should be able to get a replacement for around $160 dollars.