Do I have an alternator problem or not?

Okay, especially for you E.E.s out there. I posted a few days ago about how my 1 year old battery was running down with around town trips. Turns out the battery was undersized. I replaced it with a "correct"' sized battery. I had the charging system checked when I did this. I was told my car was putting out 13.6 volts when running (with a voltmeter - put across the battery terminals). It registered 12.6 volts with the car not running. The voltmeter gage on the dash registered just under 14 volts. It could well (given the somewhat vague nature of the markings on the voltmeter gage) have been 13.6 volts (but definitely under 14 volts). The moron at the battery place was ignorant of the difference between volts, amps and watts. So here's the questions. Let's assume the alternator is putting out 13.6 volts. The battery place guy said the "specs" on the Duracell battery was that it needed between 13.8 and 14.4 volts to "properly recharge". And that if the battery was only getting 13.6 volts that it would not fully recharge and that in short order the battery would need replacing again. Here's what does not make sense about this. My battery tender is clearly marked 12 volts, 750 mili-amperes. This tender would fully recharge any of my vehicle's batteries. So if the tender works at 12 volts, why wouldn't my alternator "work" at 13.6 volts? Perhaps it might take longer to fully recharge the battery if it's only putting out 13.6 volts (vs. say 14 or 14.4) - but it seems to me if the alternator is putting out more than 12 volts (I don't have a clue what amperage is being generated), the battery will recharge. Perhaps that's what happened to the undersized battery. I'd leave the house at "full charge" and after two or three stops and re-starts, the car would crank slowly. But assuming I drove for hours and hours, I don't think I'd be running the battery down - to where it would go flat. I think that the battery would be recharged fully. My wife's BMW is "smart enough" so that it measures what type of driving she does and if it's mostly short stop/start trips, the alternator puts out more amperage to recharge the battery more quickly. The 996 system is not, I think, that smart.

When you start your car in the morning, the voltmeter in the instrument cluster should read about 14v or even a hair higher. As the engine warms up, it may drop lower to 13.8v or so (with accesories off). Do you currently have any symptoms? With low beam and a/c on, can it still maintain 13.6v? 12v battery charger supplies about 13 to 14v+ to charge the battery,

Just returned from an approx. 20 mile highway trip each way. The voltmeter on the dash started out at under 14 volts (probably close to the 13.6 volts the battery place guy says it was putting out at idle). As I was driving down the highway, the voltmeter gradually moved up to exactly 14 volts on the gage. And, again, my battery tender has stamped right on it: 12 volt, 750 mili-amps. Why would it say 12 volt if it was putting out 13 or 14? My original question still stands (for you E.E.s). If I'm getting near 14 volts - as indicated on my voltmeter gage, why wouldn't that re-charge the battery? Perhaps if it was 14.4 volts - it might re-charge it more quickly. But I still think the battery will recharge - as opposed to drawing power down to run the car's stuff (say firing the spark plugs, running headlights, etc).

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My 996 Turbo volt gage has never read over 14V -- the needle hovers around between an estimated 13V+ and just under or sometimes at (barely) 14V -- and it has kept the battery topped up just fine. My info is the voltage reading is the battery voltage. If the alternator wasn't supplying sufficient voltage you'd see the reading down under 13V and under 13V is a cause for concern. But of course the actual value needs to be checked with an accurate voltmeter. Short trips -- especially in colder weather -- with of course the obligatory engine starts is hard on a battery. The battery can tolerate this better in warm weather but in cold weather...not so much. If the battery wasn't being supplied enough volts, if the battery was having to supply electricity to run the car, the engine, the accessories, you'd not get very far at all before warnings would appear, the engine likely started misfiring, then the engine would die if you didn't shut it off before this happened.

I had an interesting problem with my alternator, which I found to be quite common on my 996. I noticed that upon start up and until the engine got warm the voltage would read correctly. Once the engine was warm the voltage would dip to just above 12, maybe 13. I noticed a change though. I took it to the Autozone, they threw it on the meter and said everything was fine. I knew it wasn't. I changed out the alternator and brought it back for the core charge. I told them to throw it on the tester, they checked it out and everything was fine. It was ultimately the regulator on the back side of the alternator, about $60 or $110 for a new alternator. When it got warm the voltage dropped and wouldn't charge the battery.

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So that people who don't know jack about car batteries wouldn't ask where the 12v charger is.

The battery charger says 12v meaning it's for charging 12v battery. You cannot use 12v to charge a 12v battery. It simply won't charge. If the voltmeter indicates 13.6v or higher when both the low beam and a/c are on, as long as it's accurate, your battery will be charged amd your alternator is likely ok. If you can answer my other questions, I could try to help more.

Yes, with everything running at once, lights, defroster (which activates the a/c compressor) in the morning, the voltmeter still registers 13.6-13.8 volts. Ahsai: are you an E.E.? I took my circuit theory courses a long time ago. If you have a device with an U.L. stamp on it (which my battery tender has), the markings are, if I recall correctly, accurate. My tender has right on it: "Output: 12 volt DC, 750 mili-amps". Assuming this is correct, then how does a 12 volt tender charge my 12.6 volt battery? If the tender is putting out less voltage than the battery holds (and I am foggy on my circuit theory here) does this mean that, in theory, the battery should be pushing a charge back out through the tender and into the grid?

Let's just say my degrees encompass more than an E.E. ...but I also trust and use multimeters. If you have a multimeter, you can measure the voltage the charger outputs when it's connected to the battery and you will see that is > 12v guaranteed. As you correctly described, you cannot charge a 12.6v battery with 12v due to physics (circuit theory). Our alternator normally outputs 13.5v to 14.5v. After the voltage loss on the cables between the alternator and the battery, you will see about 14v at the battery. Our alternator also has temperature compensation meaning the higher the temp, the lower its output voltage to avoid over charging the battery. Battery voltage goes down as temperature goes up. http://www.tekrispower.com/pdfs/xant...20Charging.pdf Still my question is do you have any symptoms now? That will determine what to check next.

I have found that listening to Ahsai is a very smart thing to do. Mike, you've got one of the three best experts on here interested in helping you.

No symptoms. The only anomaly was that my former "undersized" battery - would be fully charged (with the battery tender light on green). Then after two or three around town start/stops, the battery would be almost unable to re-start the car. Then back home, put the car back on the battery tender - (took 4-6 hours to come back to the green light steady on) and the same pattern would repeat. It struck me as strange that after a re-charge on the battery tender, the "undersized" battery was fully capable of starting the car. The only "test" I did not try for practical reasons, would have been to take the car with the undersized battery - on a very long road trip. Then seeing if the car would re-start. So far with the new correctly sized battery and my voltmeter gage reading just under 14 volts, the car has been fine - including several around town start/stops. I've read on this forum that if you lock your car while in the garage, that a bunch of systems shut down - reducing the draw on the battery while the car is off. Generally I do not lock the car in the garage. My next test will be to measure the parasitic draw on the battery while the car is off with an ammeter. Many years ago, I was driving from Boston to New York City in an almost new rented station wagon. I noticed the headlights getting dimmer and dimmer - although nothing was lit up on the dash, I stopped at the Hertz office in Hartford Ct. Sure enough, the battery was dead. They provided me with a replacement wagon. That's how I learned that if your alternator is not working, you will eventually run out of the ability of the car to function. I'll keep an eye on the voltmeter on the dash as I drive around town. As an experiment, I'll hook up the battery tender to see how long it takes to get the green light to stay on. That should tell me if the battery is being properly charged in short around town trips.

I really suspect you have an issue with the alternator regulator, the short trips should still charge a battery sufficiently. Take a look at the voltmeter on the dash after the car is good and warm. See if it dips, throw the lights, seat heater, wipers, fan on high, etc. Turn on all electrical items and see if the alternator keeps up.

So basically you suspect your alternator is not charging the new battery properly. When the battery is at rest, it should read ~12.6v when fully charged (it's temperature dependent but it's good enough for the following). All you need to check is in the morning, what's the voltage of the battery before you start the engine. Or simply glance at your voltmeter in the cluster when you turn the key to the last position before crank. It should read a little above 12v. That's one proof that your battery is being charged properly. Another proof is the voltmeter reads between 13.5v to 14.5v at all times when idle/at speed, cold engine /hot engine, with low beam and a/c ON. With the above 2, you can conclusively say that everything below is good: - battery - alternator - ALL cables between the battery and the alternator - Car has no excessive parasitic drain Trust me, if you have issues in your charging system/cables/battery/parasitic drain, it won't be long before you can't crank the car.

Thanks Ahsai: This may be a situation where I'm just over-analyzing things. I had a battery that just was not the correct one for the car. Now I have a correct battery. So far everything seems to be okay. Being an engineer has it's bad points. Sometimes when I fly, I'm sitting there on takeoff and my mind wanders to the guys servicing the plane. Did they remember to put the cap back on the oil reservoir? Were the landing gear tires replaced according to the recommendations of Boeing? Did the guys assembling the turbo fans at G.E. (my father-in-law worked in the engine manufacturing plant at G.E. in Massachusetts) attach the fan blades properly? If one of the blades tears loose, will the containment structure surrounding the turbo fan work as it's supposed to - or will metal fragments tear through the fuselage - tearing my arm off? I will monitor your suggestions and re-post in a month or so.

I think your logic is valid - it appears that the same alternator could not keep the old battery charged properly while the battery tender could. Two assumptions though. Your old battery was in good shape and the alternator and the tender charge the battery the same way. I can see a big difference between the tender and the alternator. The former could output 14+ volt to charge the battery in 4-5hrs where as the latter applies at most 13.6v (after the voltage loss in the large cables between the alternator and the battery) and charges for a short trip duration. The proper way to diagnose the charging system is to load test the battery, load test the alternator, do voltage drop tests on all the large cables, and check for parasitic draw. I use those capacitative battery testers to check the batteries. To load test the alternator, I use a carbon load tester to apply a load of half of the CCA value (hundreds of amps) for 30s or so to see if the battery voltage stays above 10.6v. To check for parasitic drain, I use a DC amp clamp to check if the rest current is 50mA or less. To do voltage drop test, I use a multimeter. As you see, everything is based on measurements so there's no need to guess or ponder. However, most people don't have these testers. If you are really interested in finding out what's going on, I highly recommend this tool for $17. It will give you a definitive answer in a few days. You just plug it into your cigarette lighter and leave it there. It's super accurate to two decimal places and it draws about 7mA so it won't drain your battery. Amazon.com: INNOVA 3721 Battery and Charging System Monitor: Automotive Amazon.com: INNOVA 3721 Battery and Charging System Monitor: Automotive It has colored LEDs built in to tell you ok or not but you can do more. For examle, in the morning before you unlock the car, you can peek through the windows and see the battery voltage at rest. This indicates how full the battery was charged (12.6v equals fully charged). If there's significant parasitic drain or your battery is not charging, you will see low voltage. This is different than checking the voltage after you open the doors and put the key in, which will turn on some lights and computer modules, all consume some current so the voltage will not be a true at rest voltage. Now keep an eye on the voltage while you crank the car. If the battery is healthy, the voltage should not drop below 9.6v or so. Then just drive the car normally and keep an eye on the voltage reading and LEDs. A green LED means everythng is good. With low beam and a/c ON, it should read no less than 13.6v at all times. That tells you your alternator and cables are good.

What makes you think the battery is getting drained during driving?

Battery tender/maintainer.... 12 volt, 3/4 Amp..... The latter, 3/4 amp, is the controlling factor. The 12 volt source might be as high as just enough not to shock a person. 36 volts...??? Actually most are about 16-18 volts. But the tender/maintainer is designed such that it can NEVER supply more than 3/4 amps. Internal resistance, active constant current source, whatever. So, overnight, the battery charge level, voltage, rises to the point that the battery is "absorbing" the 3/4 amp as HEAT(***1). The battery begins acting, literally, as a voltage regulating, voltage limiting(***2), zener diode. ***1,2: The reason the battery electrolyte boils off when it is being seriously overcharged with a high current supply source, say your 90 amp alternator with a defective VR.

Thanks. Clearly you have E.E. training. In fact, it is my understanding that when the battery tender receives a signal from the battery that it is fully charged, it basically goes into a stand-by mode - and only starts charging again when the battery drops below a full charge.

So why is the car's alternator/VR system not adequate when the tender/maintainer is...??? As has already been mentioned, automotive VR's automatically adjust the alternator's output VOLTAGE as a function of ambient temperature. Higher voltage level output, ~1 volt range, as temperature declines. Just as is required to keep lead/acid batteries fully charged throughout the service range. For most cars this works just fine since the battery is located in close proximity to the engine... In your/our case, as the engine warms the nearby VR is affected in a manner adverse to fully charging a remotely located (cool/cold?)lead/acid battery. Porsche makes up for this inherent design flaw via recommending an over-capacity battery.

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The simplistic ones, Harbor Freight, say, simply supply a continuous current flow, generally no more that 2 amps.

Recently, I had noticed my voltmeter needle behaving inversely to engine temp: From a cold start, the needle would move from almost 14V to an estimated 13V. While cold starts were never an issue, hot starts were becoming tentative. Given previous write-ups from Ahsai on subject, I suspected the Y-cable (starter to Alt and to +Jump start terminal) to be increasing impedance and that my battery was getting weak as well (6 yrs old). Battery failed test, so easy course of action here, but cable looked perfect, sans corrosive vert-de-gris showing anywhere on the copper strands near connectors. Since I had the cable already purchased, I decided to replace it. I'm glad I did. Now the voltmeter stays around 13.6v at any temps and all cold and hot starts are equally brilliant. I need to dissect that cable and find out if anything happened between the connectors to create this added resistance. With a voltmeter and a clamp amp meter, I tested the new battery setup. Statically, I have a tension of 12.56V between battery posts and a residual amp current of around 50 mA (.05A). Trickle charging the new battery with a CTEK charger, the voltage increases between 13.56V to 14V at first while current starts at close to 1 Amp but reduces in time to .4A while voltage settles at 13.5V after the CTEK achieves trickle charging steady state. Lesson learned: Glad I listened to Ahsai as these hot starts were really starting to be doubtful. Happy New Year to all!

Glad to hear that you replaced that cable. 50mA of drain at rest is in spec and with a new battery and cable, cold startup should give you very close to 14v. 50mA is 1.2Ah per day. Our battery is about 70Ah and it doesn't take the battery to drain completely before it can't start the car. So my guess is you can probably still be able to start the car in a month w/o trickle charger but but don't bet on it though. Of course it's best to keep the battery fully charged at all times to increase its life expectancy. I also have a CTEK charger and found similar numbers. As I mentioned before even if the charger says 12v output, it will output about 13-14v when it's charging the battery. It's just chemistry and physics. Agree with West's comment on the battery being in a remote location when the compensation measures the alternator temp (not ideal in this setup). I think Porsche got away with that though.

Ahsai; The condition of the Y-cable in my case, was very insidious; in that, its excellent condition appearance made me doubt that it was really at fault. Had I not read any of your previous wrt this problem, I might have left the old one in with unfortunate consequences, as you and I now know. As to the battery, the previous owner did not tell me very much but one thing is said when I bought the car from him was that, given his airline insurance executive job, he would travel for weeks at a time and come back to a dead battery frequently. Triclke charging technology was not part of his background lexicon. So a 6-yr duration for a battery treated in this fashion was, in my view, pretty reasonable. Thank you again for all of the various lessons learned you post at this site. Many of us make better learneth repair decisions on account of your perseverance and unselfish participation at this forum. Cheers! Martin

Ahsai should be charging us for his advice. My mantra is: It Is Always the Y-Cable! The Y cable looks nice and shiny and new on the outside, but the inside is a completely different story. Come to think of it, I wonder if alternators (/voltage regulators) are going bad prematurely because of the increased load caused by the bad Y-Cables.

Should be someone's new sig. "It's always the y-cable." Because it's so true.

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Haha, just trying to help out my 996 brethren.

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Great idea, if SJG allows me as it was his idea!

Go for it JMLavoie! Lucky guy - living on Vancouver Island, beautiful place.