PLEASE HELP ADVISE Battery Dead but weird as I jumped it and it started but then...
 

Hey everybody- 1979 911 sc. Has a relatively new battery.Has been running great! Friday I parked it in the garage and believe i left the glove box light on. Came home Sunday night from outta town and there was NO juice at all. Pulled in my sedan next to it and hooked up cables. Lights went on, fuel pump loud and clear but too slow of a starter. Let it sit on the cables about 15 minutes, got in and started it up. Disconnected the cables and let it idle about 10 minutes. Got in it to drive around teh block, the SECOND I Pulled on the headlights - car died. Tried to restart-no juice at all?

So I guess the question is did I just run off the battery charge for those 10 minutes and then wqhen I tried to add load like the lights the battery was drained again? Does it sound like my alternator went out?

Thanks!

I think that would be the alternator. I'm sure someone will chime in with more info on how to test it. Don't just buy a new battery. I did that the first time I had a similar problem and threw out a perfectly good battery.

Have you got a multimeter that you can test the voltage at the battery? Put the battery on a charger over night, then start the car in the morning. Measure the battery voltage first. Then with a helper revving the engine to about 3000 RPM, put the multimeter on the battery. The voltage should peak at 13.5 volts or so. If the voltage does not go up from the battery's initial voltage while the car was not running, then the alternator is probably not charging.

with no accessories on, you should get almost 14 volts at the battery.

btw, 15 minutes on some jumper cables DOES NOT CHARGE UP A DEAD BATTERY!

(Sorry for yelling, it just seems this is already just so obvious).....

charge up your battery with a battery charger. even a cheapo El Walmarto one is better than trying to charge it up with your alternator. You'll just end up shorting out your battery, or even worse, burning out the alternator.

Check the obvious as well. Make sure the battery terminals are connected well.

You guys thanks for the info. Rusnak, I thought maybe a 15 minute charge on a cable wiould top charge the battery? So if that did not supply a charge at all. and the battery was so dead, the fact that the car ran fine for 10 minutes then died cto a completely dead status seems weird. I thought it would run off the alternator, but I guess if the battery is bad then it cannot run from the alt with any load? Well I do have the ability to charge it up so I'll givethat a go. Thanks again!!!

If your battery is not taking a charge, but the alternator is working, you will have problems starting it, but once started the car WILL run off the alternator. You do not want to run the car off the alternator w/o a battery in place though (the EE's can explain, but it's not healthy for you alternator)

Get back to basics. Put the battery on a charger. For your posts, I get the feeling that your battery is fine, but your alternator is not.

So go back to my first post.

Is this a Diehard battery? I had one fail open. You could charge it but the minute a load was applied it would open.

I have never seen this before or even heard of this type of battery failure.

Oh... and the 90 amps coming out of your sedans alternator was charging your dead battery. I don't know what that rant was about.

You can't recharge a dead battery with the alternator. It must be placed on a charger.

I think most alternators need at least 12v to excite the field windings before you can get any output amperage.

As Zoanas said, charge the battery overnight, remove the charger for about 2 hours and then test for battery voltage. You're looking for a reading of 12.8v. 12.0v is just about a dead battery.

Nonsense. I have probably a hundred times charged dead batteries with the alternator (or, before that, the generator) without the slightest problem, over the last 55 years of driving. Never hurt an alternator or a battery. Did it a couple of weeks ago with our Boxster, when somebody left the headlights on. Battery dead as the proverbial doornail (whatever that means...), jumped it with cables from the Audi, drove away, did my errands and recharged the battery at the same time. Thousands of people every day get jump starts of dead batteries and drive away while recharging.

With all due respect, Stephan, if you recharged it, it wasn't dead, and you were doing damage to the diodes in your alternator, whether you believe it or not. Alternators are not designed to take the load of a dead battery, they're designed to keep it topped off. Also, finding the battery "dead" from leaving the lights on overnight doesn't kill the battery, especially a good battery. It will simply drain to the point that your lights won't stay on, and no more current will flow. The battery isn't dead, it's simply dropped below the threshold of your electrical load. When you jump it and drive down the road, the alternator is struggling to recharge it the 1 volt or so that it dropped. Check it with a voltmeter before you jump it. It will show more than 11 volts most likely.

Ditto!

Well, then it's essentially irrelevant, if it's impossible to create a truly dead battery in normal leave-the-lights-on situations. All I know is that since 1952 I've owned dozens and dozens of cars, from a 1932 Ford to the '07 Volvo I'm in the process of buying. I can't remember ever replacing an alternator and I've had maybe one or two batteries fail prematurely in my life. And I have never done anything but jump-start what is commonly called "a dead battery" and then gotten it back up to speed fairly quickly by driving the car. So we seem to be talking about a theoretical situation--a battery that is utterly and totally devoid of electricity. I've never experienced that and will assume that if I ever do, I won't be able to recharge it with an alternator...

But then isn't the guy who started this thread talking about having run a battery down by leaving a glovebox light on? How _would_ you make a battery truly "dead"?

Happy trails :)


You do have AAA don't you?

No.

take the batt to advance after charging, they test them and alt's for free!

My ditto was to F.S.W.
I've had flat batteries and I frequently recharged after jump starting by a good drive.
Anyone who has a car with a modernish alarm is familiar with the flat batteries that occur after leaving them standing for a period. This is especially fun with some 996s and Boxters. Jump starting is not ideal but it's frequently the most practical option. A fortnight ago I jump started a 912 from my 3.2. It had been sitting for over 18 months, we expected the battery to be totally dead. It started like a champ and was taken for a spin and moved to its new home where it was put into hibernation until the spring. We wanted to avoid putting in a new battery and were lucky, it gets a new one in the spring.

Question: How many theoreticians does it take to charge a dead battery?

Answer: Two. One to hook up the charger, the other to tell you that what you've been doing all your life is impossible.

ok thanks again for all the advice, information, specifications, etc...

I am going to charge tonight, but I think perhaps i blew out the alternator somehow. I'll post what i find out.

Yeesh,

I'm sorry for introducing a mini-debate on this.

Your alternator is NOT designed to charge a dead batter, as Zoanas said.

You WILL damage an alternator this way.

In fact, the alternator will not keep your battery charged if you constantly run heater blowers or AC, headlights, stereo, etc all at the same time. And in extreme heat or cold, the battery may not hold on to a full charge.

It is a good idea to top up the charge on your battery with a maintenance charger, or trickle charger at least once a month. This will make your car's electronics happier, and your battery last longer.

Those who will disagree, do whatever. It's your car and your wallet.

I drive with headlights on (legal requirement on many roads in Italy) and either heating or aircon on and almost always stereo on with my d.d. 3.2. Assuming the alternator is not keeping the battery fully charged how long would you expect the battery to last. I also see temperatures of up to 38°C in summer and -20°C in winter. battery was fitted 2 I/2 60K km ago. Has not been charged.

I agree a trickle charger is better for a car not in daily use and I agree that jump starting is not good for our electronics but needs must when the devil drives. I have seen both Porsche and Ferrari dealers as well as the AA and RAC in the UK,called out to cars with flat batteries and notwithstanding the risks they jump started.

Gee, I'm glad I don't know all the damage I'm doing to my alternator and battery, because apparently they don't know either. So we're all happy.

Post scriptum.
my other 3.2 is a bit of a garage queen. When not used regularly ( at least once a month) her battery often needs a charge if I haven't disconnected it. Easy when I'm in my own garage.

an alternator is not designed to recharge a DEAD batt. it is there to put back what was used during the start up and run the electrics in the car. unless u have 2 sets of 100 watt bulbs, a 1000 watt stereo system. the stock alt will run what is stock to the car plus a few extras with no problem. i had an interstate batt in my 9114-6 that had a hole in it and the cell was dry. i drove it almost a year like that.....til i left the lights on.

if your batt is going dead while the car sits for a month, you may have a current draw. disconnect the neg terminal and clip a test light to the cable, then touch it to the batt post, if it lights, you have something drawing current.

i did electrical work on cars for a little while. we would even get ones from the dealers that they couldnt fix.

Ssssh--don't tell my alternator. It thinks it's supposed to freshen up the battery rat quick no matter how I kill it.

Oh, and did you know that turning the steering wheel while the car isn't moving will grind holes in the bottoms of your front tires and cause the steering rack to crumble?

Absolutely right! It's caused by the bastard alarm and anti-theft system my insurance company forced on me. A friend with a 996 has an even more sophisticated version that only manages a week! My gf's Boxster has a different make and sat for almost three months without problem.

alarms were always a big problem. especially if they butchered the wiring putting it in, not to mention coins in the rear ciggerette lighters (kids). glove boxe and hood/ trunk lights

So every car that AAA comes out and jump starts has a damaged alternator? Are they liable?

Next time my regulator goes out should I leave the lights on and have AAA jump it and have them buy me a new alternator?

I wonder why they continue to do this?

if the batt is dead, recharge it with a charger. if you jump it and let the alt do it, it will recharge it, IF, it is a good batt, BUT you will put a big strain on the alt. and most likely you will get away with it. im not saying do or dont. im sure everyone out there has at one time or another jumped their car just to get home.
jumping is not so much dangerous to the alt as it is to the computer, although i have never actually heard of either one going bad from jumping.
if you leave the lights on and drain the batt, if it is a good batt it will "recharge" itself some what after a while, so, if you do ever leave the lights, wait a while before starting, you may only get one shot at it. (im sure im going to hear it for saying that)

"bad" as in "bad battery" is a relative term. Easily cured for the less bad batteries by leaving it on a charger. Not cured at all, no matter how much charging is done on a really "bad" battery.

not too terribly expensive but a pain in the butt to deal with a bad battery.

"bad" as in "bad alternator" is an absolute term. no matter what, you're going to have an expensive repair. It's not convenient either. Why would you deal with a "bad" battery in a way that leads to a bad alternator? Never mind, I don't want to upset any delicate egos.....I'll just ponder that one myself.

So why have I never had a bad alternator? Or a bad battery, for that matter? You people spend way too much time worrying about things that never happen. And in my case, as I said earlier, "never" is since 1952. If I can make it through another 15 years, maybe--just maybe--I've beaten the crybaby odds.

Wow, are you guys seriously arguing that using the alternator to recharge the battery will not harm anything? The charging system in a car is, and has been pretty simple for a long time. There is no debate about the purpose of the alternator is there? Is there debate about how the battery operates? About how current and voltage work? What about diodes? Have they changed recently?

Just because "you've done it for the last 20 years", it doesn't change the fact that the charging system isn't designed for that. You can get away with it forever, theoretically, but to argue that it isn't doing any damage is simply wrong. You can argue that the damage caused is acceptable to you, and that you haven't had any problems as a result of your misuse of the system, but there is no questioning the design.

Glad I spotted this thread. News to me. Being a complete ignorant when it comes to car electrics I am afraid I had no idea the alternator were susceptible to damage jump starting the car. Like S.W. I have done it for decades and 'recharching' the battery by driving.

I wonder how many alternators I have fried without noticing it.

I really need to read up on car electrics..

OK. so I got home tonight and was prepared to remove battery to charge it. noticed that the negative cable was really quite loose. SO I thought perhaps the loose cableis my problem. I proceeded to jump start it again and got it running. but by then my friend had come over with his multimeter. As the car was idling I disconnected the jumper cables and thought I'd see how long it ran. While doing this My friend put on the multimeter and we watched the numbers dive from 13 to 4 with the jumper cables off and at 4 the car shut off.

The battery is now on the charger for an overnight charge.

My questions are:

Where is the voltage regulator located on this (1979 911 SC)
How do I check that with a meter?
How best to check the alternator with a meter?
How do you remove the ALternator in case thats what I need to do??

Could the loose negative cable on the battery have created these problems if the alt or regulator is bad?

I have never seen anything that says charging a completely discharged battery (Vbatt <12.2 volts) will in any way harm a typical automobile alternator.

Perhaps you could steer us to some of this alternator design info (data sheet) that you have knowledge of that states the design limits. Or is this just anecdotal evidence also.

Your battery went from 13 volts to 4 in a matter of seconds? The battery does not sound healthy

Google it yourself. Facts are facts.

I had batteries with internal shorts which sounds like your problem. It appears to have 12 volts one minute, then it drains down to nothing. Shock it with a battery charger, and it goes up to 12 volts again. Problem as I understand it is debri/corrosion shorting the plates. Toss the battery.

I have... came up with nothing.

You are the one that professes to have hard facts. Please share.