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

FinallyGotOne · FinallyGotOne's Garage

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!

rdrr · 12-10-2006, 08:54 PM

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.

MotoSook · 09:12 PM

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.

rusnak · 12-10-2006, 10:32 PM

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.

Milu · 12-10-2006, 11:01 PM

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

FinallyGotOne · FinallyGotOne's Garage

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!!!

MotoSook · 12-11-2006, 07:26 AM

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.

rick-l · 12-11-2006, 07:49 AM

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.

ZOA NOM · ZOA NOM's Garage

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

CliffBrown · 12-11-2006, 08:58 AM

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.

Formerly Steve Wilkinson · 12-11-2006, 10:12 AM

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.

ZOA NOM · ZOA NOM's Garage

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.

Milu · 12-11-2006, 10:25 AM

Ditto!

Formerly Steve Wilkinson · 10:43 AM

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"?

ZOA NOM · ZOA NOM's Garage

Happy trails

You do have AAA don't you?

Formerly Steve Wilkinson · 12-11-2006, 10:44 AM

No.

T77911S · 12-11-2006, 10:55 AM

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

Milu · 12-11-2006, 12:26 PM

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.

Formerly Steve Wilkinson · 12-11-2006, 12:35 PM

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.

FinallyGotOne · FinallyGotOne's Garage

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.