True, but nobody is suggesting that you start your car with a capacitor. At issue was the batteries ability to absorb spikes from the alternator. Given that, batteries do have capacitance. I don't know how much it should be to be effective though, but its there.
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:
A car battery has many farads of capacitance. In other words, it's a very stout capacitor.. MK
A car battery has many farads of capacitance. In other words, it's a very stout capacitor.. MK
Sure they do. A pair of wires running side-by-side do too. Not enough to consider them functional "capacitors" though.
Yes, they store energy... but not ELECTRICAL energy. That's generated out of a chemical reaction.
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.
I didn't say he was wrong.
Matt Ion wrote in news:lQygh.485570$5R2.202156@pd7urf3no:
I've seen such a "pair of wires" literally used in a electronic circuit for a capacitor,by DESIGN.
Engineers modeling components like a battery for computer aided design,the battery model definitely has capacitance. Even the electronic symbol for a battery is two plates,just like a capacitor.
yes,it is electrical energy. What other sort of energy would it be?
differing from electric charges generated by friction;static electricity. They both are ELECTRIC charges,though.Both are electric current,too.
Sure, usually in RF circuits where very tiny amounts of capacitance are needed. Sometimes a "capacitor" even exists as just a pair of interlaced traces on the circuit board. And in high-frequency designs, particularly network cables, the inherent capacitance of the wires must be taken into account.
That doesn't mean a pair of wires *are* "a capacitor", or that in most cases the inherent capacitance is of any concern or use, any more than the capacitance in a lead-acid battery is of any concern or any real use.
Well, usually a series of stacked plates of alternating lengths. But that's true of the the symbol for ANY battery, including your good old carbon-based flashlight batteries.
They don't STORE electrical energy. When you charge a battery, the electrical (kinetic) energy you feed into it creates a chemical reaction; the electrical energy is converted to chemical (potential) energy. When not charging, the inverse chemical reaction converts chemical energy back to electrical energy.
In a very simpistic sense, sure, a battery is "storing" electricity (as opposed to "electrical energy"). At the physics level, it's merely converting one form of energy to another.
Matt Ion wrote in news:AxBgh.485714$5R2.255447@pd7urf3no:
When you DRAW current from a battery,how does the chemistry knows to start converting chemicals to electric current? Where does that initial current come from?
Simple,the *charge on the plates* decreases and the chemical reaction adds more electrons to fill the depletion of the plates charge.
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
Very good, you get a gold star.
I think this is getting pretty far afield. Does anybody feel the alternator would be stable under varying load with the battery disconnected?
Mike
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).
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.
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
"Michael Pardee" wrote in news:U7-dnQ-8-Lps_xnYnZ2dnUVZ snipped-for-privacy@sedona.net:
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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