Battery cable came off!

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I think this is getting pretty far afield. Does anybody feel the alternator would be stable under varying load with the battery disconnected?
Mike

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Michael Pardee wrote:

Pretty sure it would, yup, because I once had to drive without one for a week (manual tranny, obviously - you get really adept at finding even the slightest grades to park on).
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Michael Pardee wrote:

Thanks for bringing it back, mike. I've been reading all the posts very closely.
Anyway, as an update, no delayed effects yet, thankfully. Though I'm not willing to repeat the "experiment" anytime soon: Seems like I have a shorted rectifier in the alternator. (Does this mean that I'm getting voltage spikes all the time?) I'm wondering if the battery is now "absorbing" the spikes, so it seems to boil down again to whether the battery is acting as a large capacitor..
Thanks again to everyone.
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In my experience, yes, you can expect to be getting ripple of about 1/2 volt to 1 volt even with the battery connected. It should be measurable with a DVM on AC voltage setting, measuring across the battery with the engine running. If diodes in two phases (out of the three phases most alternators use) fail the AC voltage can be over 1 1/2 volts with the battery connected. On an oscilloscope it looks pretty radical. With one phase out the voltage hangs around 14 volts and drops when the bad phase is called on to put out. With two phases out the voltage hangs around 12 volts and spikes upward.
Mike
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Applying AC straight from one phase of the alternator's output would harm the battery,causing excessive heating.On one-half of a cycle,the battery would charge just like the other phase outputs,but on the 2nd half cycle,it would discharge(thru the ALT),maybe even provide a path for damaging currents to be drawn from the battery.It depends on whether the diode failed open or short/leaky.
Open failure would just remove that phase winding's output,lowering the alternator's total output current,and giving more ripple.
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Jim Yanik
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On Sat, 16 Dec 2006 07:08:34 -0700, "Michael Pardee"

test
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The plates of a battery have capacitance. They are charged by the chemical reaction.
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Jim Yanik
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I feel like Kerry... "What I meant to say is, 'Uh, the battery is nothing more than THE EQUIVALENT of a large capacitor.'"
I think that it's time for a belt of Pinch!
JT
Jim Yanik wrote:

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Jim Yanik wrote:

A car battery has many farads of capacitance. In other words, it's a very stout capacitor.. MK
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snipped-for-privacy@wt.net wrote:

Not even close. The smaller the plates of a capacitor, the less the capacitance. The further apart they are, the less the capacitance. Lead-acid battery plates are EXTREMELY small and EXTREMELY far apart compared to a true capacitor's.
If you were to drain the water from a battery and measure the capacitance, I suspect you'd find it in the low microfarads, if not picofarads.
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Matt Ion wrote:

Sure. The capacitance I mention is more of an apparant capacitance. It's not true capacitance per say, but the normal operating car battery does provide a large apparant capacitance to the system. But this would
not be the case with a non functioning battery. I use car and deep cycle marine batteries to run radios here in the house. My chargers are unfiltered, but yet I have little noise to my radios. The use of my battery as a cap is a bit different in operation vs a true capacitor, but the final apparant filtering is still there. If the battery were not acting as a cap of sorts, I would have hash and trash out the kazoo.. I don't know if this makes any sense, as it's hard for me to describe stuff like this off the top of my head.. As a quite dangerous test you could try running a car radio off the running alternator with no battery connected. I bet it will be quite noisy, fairly unregulated as far as volume, etc vs rpm. IE: if the rpm dropped too low, the radio might totally drop out due to the low voltage. Hook the battery up, and all is smoothed out. Both as far as regulation, and also filtering. If thats not acting like a large "apparent" capacitor, I don't know what is. The operation is different, but the end results are about the same. This is not something I've really thought about too much, but I've always considered the usual operating car battery to have many farads of capacitance, at least as far as overall function. Maybe not true in the strict sense, as far as true caps go, but as far as the end results of placing it in the system. I dunno if this makes any sense or not.. :/ MK
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Jim Yanik wrote:

A car battery has many farads of capacitance. In other words, it's a very stout capacitor.. MK
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I'm happy to be wrong on this one, Matt. Thanks, guys. *Whew*.
Mike: Lightning strikes, really? Wow.
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sharx333 wrote:

I've taken two direct strikes to my antenna mast in the last 5 years. No damage at all. But...I ground out my feedlines when storms are in the area. The strikes hit about 15 ft from where I'm sitting at this puter. I was sitting here both times. My mast is very well grounded, with low resistance. When a strike hits that mast, it's very quiet. Sounds about like a light bulb being thrown on the ground, and then a loud sonic boom directly overhead. Pretty wild.. But I have no trouble at all. My puter doesn't even flinch. You can set the station up for full time use, even with direct strikes, "all broadcast stations are set up this way", but it takes a detailed installation using a ground bulkhead, careful single point grounding, suppressors, etc, etc.. I'm too cheap and lazy to mess with all that. :/ I just manually ground the feedlines at the bulkhead. As far as the car, yes, it's not good to unhook the alternator while running. You got lucky. Many cars would have done a toasting of the alternator in record time. You dodged the bullet this time it seems. If it did have a problem from doing that, it will usually be blown diodes I would think. MK
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Yep - mostly mountaintop sites, two or three a year since I moved to the mountains. For some reason the storms prefer holidays and my anniversary!
Mike
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Try Central Florida,the lightning capital of the US. Also,the power density of Florida strikes are on average twice that of northern lightning strikes.
I've had a pine tree about 300 ft from my apartment(tallest in the area) get struck(and killed) while I was watching. The bolt travelled right down the side,blowing a channel of bark off the tree. It took out a surge protector on my phone line and my modem,didn't harm the phones or the TeleZapper.
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Jim Yanik
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The problem is that alternators are very inductive. Without a battery, changes in current produce wild fluctuations in voltage; suddenly reducing the current draw by half should roughly double the voltage for a moment. At low current I would expect the alternator and regulator to go into oscillation without a battery to stabilize it.
Mike
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The battery does as Jim said. But another condition called load dump could have created a destructive transient. Load dump is defined by the ISO to be as large as 270 volts on the 12 volt system. SGS Thompson defines it as 80 to 100 volts. But then better automobiles are designed with electronics that make load dump not destructive.
Of course when you connect a computer to the car using a discount inverter, does it have load dump protection - or did you just save some pennies?
The damage, if it occurred, would be complete in milliseconds. Apparently you did not suffer load dump damage. But you, like many others who replied here, should know of load dump and what automotive electronics (properly constructed) cost more money.
sharx333 wrote:

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