Naked Science Forum
Non Life Sciences => Technology => Topic started by: Stefan on 18/11/2008 13:39:32
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Quote from article on NET:
Lead-acid batteries are cheap and can store large amounts of energy. But if they are repeatedly and rapidly charged and discharged - the battery's negative plate becomes coated with deposits. That limits its working life to a few years..
By marrying a lead-acid battery with a supercapacitor, the combination stores as much energy as a standard lead-acid battery, but can happily charge and discharge without deterioration.
Can somebody pls. tell me how i can integrate a Lead Acid Battery with a Supercapacitor. I've found a coupe of suppliers of SCs on the NET.
Thanks - Stef
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Tricky as supercapactors have to be charged to a few Kv to get a decent kWh storage capacity and then upon discharge the volts fall exponentially. So you need to have some switched-mode electronics to produce a fairly constant output voltage.
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I read recently of the lead-acid battery/supercapacitor arrangement, but I understood that it was merely a parallel arrangement of the two items, in which case the capacitor voltage could not exceed that of the battery. The claim was that not only did it reduce the ageing effect of repeated partial charging of the battery (this may make sense), it also increased the total capacity of the battery by "about 50%". I don't know if this is true, or how it achieves the improvement, but as Pumblechook says, the voltage on a capacitor does fall continously under load. I would be interested to see the links to SC's.
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There are battery/capacitor packs available.
There seem to be some conflicting figures. Many sites claim poor Wh per kg, worse than lead-acid. One manufacturer is claiming a 7 fold improvement over lead-acid??
I would be concerned about saftey. I have known electrolytic capacitors in even low power circuits to explode with quite a bang.
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function: Time-Constant EMF/..speed-drop/drop-level
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meaning what, nicephotog?
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...integrate a Lead Acid Battery with a Supercapacitor...
Its a capacitor.
The issue is the charging roll-off levels(cycle wave form) over time of the capacitor as the difficult part of matching to a battery.
Choose it(Supercapacitor) by the specifications of the battery and will imply to calculte its time-constant for the cycle and charge.
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It depends on your actual application. They sell Several Farad capacitors for car audio systems (outrageous price for an outrageous noise, if you ask me) when I reckon a small, local, sealed lead acid accumulator would do as well, right at the power supply terminals of the amp, (massive equivalent capacity in many ways) and only cost £15.
To make use of the low reactance of a capacitor as a smoothing component for low frequency, high current loads, the leads must be really short and fat - just like battery leads.
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I think the large capacitors in audio systems are there because, as they have such a low internal resistance, they can respond almost instantly to demands for large currents. Batteries, even ones WITH low internal resistance, cannot always respond that quickly because the chemical reactions require a certain time to build up to supporting a high current.
I still don't understand how connecting a capacitor in parallel with a battery can increase the battery's capacity.
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High C capacitors are fine for 'normal frequencies' but, for sub-woofers at sub sonic frequencies, you're into a different ballpark. The 'dynamic' load changes quite slowly and, as the required Capacity increases, the response time (if it is actually a factor) of an accumulator would start to be less relevant.
You'd need to do the actual sums - disinterestedly.
Putting a capacitor in parallel with a battery deals with the 'instant response' problem. Also, the capacitor is placed right next to the amplifier, eliminating the resistance (and inductance) of supply leads. No increase in capacity but an improvement in dynamic performance.
There's a similar technique used when putting a 0.1 microFarad capacitor right on a circuit board in parallel with a 1000 uF electrolytic, which will have a significant Inductance for some frequencies. Decoupling is an art. (Sorry, it's called engineering).
The in-car audio business suffers from two (ad/disad)vantages. First, it's to do with Hi Fi, which adds two zeros on the price as a matter of principle and in-car, which has cash rich boy racers as customers
A bean-can sized 1F capacitor looks soooo sexy.
I don't know what's wrong with a full size, car battery in the boot if all that people are worried about is headlamps being modulated by the music. A diode splitter and modified voltage regulator will isolate any of these effects. It's a common practice in campervans, boats etc and not very expensive, either.
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do the capacitor will act like an open component [???], because of the formula Xl= 1/(2πFC) so the capacitive reactance would be undefined -because the frecuency is zero- resulting in a 0 flux electrostatic field, not storing energy or adding to the capacitance of the car battery
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The frequency of the current taken by the amplifier is not zero. It goes with the audio signal for a class B amplifier.
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The frequency of the current taken by the amplifier is not zero. It goes with the audio signal for a class B amplifier.
oops sorry i thought we were talking about lead batteries.
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This statement "the chemical reactions require a certain time to build up to supporting a high current." is the oposite of the well documented proprty of batteries called polarisation.
I think the biggest problem of connecting a capacitor to a battery is that the battery voltage is near constant; but a capactor can only lose or gain energy by changing the voltage across it.
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The frequency of the current taken by the amplifier is not zero. It goes with the audio signal for a class B amplifier.
oops sorry i thought we were talking about lead batteries.
We are, in the context of car audio.
BC
I think the biggest problem of connecting a capacitor to a battery is that the battery voltage is near constant
The voltage on the amplifier circuit board is not constant when you take the effect of internal and supply lead impedance and varying load current into account. Decoupling the AC with a capacitor right near to the amplifier can reduce the AC current taken from the battery.
My problem is with expecting a capacitor to have a significant effect at 30Hz.
I think it's probably the King's New Clothes effect.
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ok.. [:D]
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"The voltage on the amplifier circuit board is not constant when you take the effect of internal and supply lead impedance and varying load current into account."
True, but the voltage at the battery is a lot more constant. Connecting a capaitor near the battery won't help much. It needs to be up close and personal with the PA.
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Of course. It's called decoupling.
I, too, cannot believe that the 'effective capacity' of a nice fat battery could be less than that of a 'really fat' capacitor when you're talking about load variations at a 30Hz rate.
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I just had to do the sums.
I found a quoted value for a car battery internal resistance of 0.01Ω.
To find the output voltage variation for a battery and a 1F Capacitor, I assumed a 20A RMS current drawn at 30Hz. That corresponds to more than 100W of audio but I guess it could be higher.
Resistive drop for battery:
V = IR
= 20 x 0.01 = 0.2V RMS
Reactive voltage variation for Capacitor
V = jIX
For a Capacitor:
X = 1/2πfC
V = j x 20/2π x 30 x 1
= approx j0.1V
A bit of a coincidence but it ignores the actual resistance of the Capacitor leads.
For much lower frequencies - say the rate of the beat of heavy music at 3Hz or 180bpm, The reactive voltage variation will ten times that (j1V) but the resistive drop would be the same as before.
The battery wins but it's only marginal.
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I believe we have gotten off subject here.
Stefan's question has nothing to do with audio.
I believe the question is how to integrate a supercapacitor to a lead acid battery in order to extend its life in high charge/discharge environments. As in electric/hybrid vehicles or power generation from wind or other highly sporadic energy sources. Lead acid batteries do not take well to being drained down real low, charged fast, or over charged. These conditions shorten the life of the battery. The capacitor is supposed to act as some sort of a buffer being able to take quick charge/discharge cycles. I am not sure what all is involved, but these people sell the supercaps & related equipment. http://www.maxfarad.com/
Capacitor are used with bass speakers(sub woofer) to filter out the high frequencies so that only the bass frequencies reach the driver(speaker).
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I believe we have gotten off subject here.
Stefan's question has nothing to do with audio.
I believe the question is how to integrate a supercapacitor to a lead acid battery in order to extend its life in high charge/discharge environments. As in electric/hybrid vehicles or power generation from wind or other highly sporadic energy sources. Lead acid batteries do not take well to being drained down real low, charged fast, or over charged. These conditions shorten the life of the battery. The capacitor is supposed to act as some sort of a buffer being able to take quick charge/discharge cycles. I am not sure what all is involved, but these people sell the supercaps & related equipment. http://www.maxfarad.com/
Capacitor are used with bass speakers(sub woofer) to filter out the high frequencies so that only the bass frequencies reach the driver(speaker).
1. Supercapacitors would be useless as a battery backup. No question about that. The exponential discharge characteristic makes it a hopeless way of storing energy for most uses.
2. Supercapacitors are used as a decoupling device to smooth the power supply voltage under dynamic load. Car audio will use dc coupled outputs to drive the speakers and usually would have active crossovers which use small capacitors. If you wanted to use a passive crossover network, the value of capacitor would still only be a fraction of a Farad. (The crossover frequency would be a few tens of Hz and an LC filter would be more effective than a simple C.)
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I have reconsidered my views on using big Cs for energy storage.
It would have to be a special application. It has to be admitted that a battery which goes flat is usually buggered (pretty much any technology afaik) and a flat capacitor is quite happy. This application would be for a situation where power could be missing for a long time and the equipment should pick up when power was restored. There would be no opportunity for maintenance - replacing dead batteries.
So how do you use the stored energy effectively, bearing in mind the crazy charge / voltage characteristic?
You would need a switched mode power supply. The output voltage would be much lower than the fully charged voltage of the capacitor and you would need to take dollops of current out, transferring them to another capacitor, kept at a constant voltage. We're talking about very low power requirement and not a long timescale either - the capacitor could leak worse than a Lithium Ion battery.
So it's not nonsense - just a rare application.
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Hello everyone. first time poster...
I spent some time in vocational school learning electronics. I was wondering if perhaps one might get around the loss of cap voltage due to capacitive discharge with a voltage doubler rigged in parallel with said capacitor(and battery but imagine say a tree with 2 main branches and the left branch which i will arbitrarily say is the capacitor, has 2 branches itself{at this point in the tree there are now 3 total branches but respectively only 2 main branches} if that makes any sense at all)...this seems an impractical solution to this all-to-common problem. but lets do some surmising just the same.
what you would need is an oscilloscope to get an exact trace of your cap's discharge/voltage loss=rate...this would appear as a sort of left aligned peak which tapers down. in this "saw tooth" like valley you will note that most caps' capacities are measured in 5 steps, each step losing 63% of the previous steps peak voltage. IE 1 volt at step 0 is 1v, 1-.63v, 2-63% of .63v and so on...
i think, were one so mathematically inclined...that you could approximate..and ultimately compensate this loss using voltage doubler circuits (which are merely a kind of capacitor that takes the same voltage and discharges it in half the time, thusly with twice the voltage. to oversimplify) thrown at each time interval... probably with the aid of some high duty sampling or(if analog is more your thing) bridge apparatus measuring relative voltage between said cap and battery.
this is, understandably a highly abstract thought and will need some flushing out. feel free to be pedantic about my oversimplification and perhaps generalization of fundamental electronic theory.
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As I said previously. To use the charge in a large capacitor in an efficient way you need to let the charge out as it is needed - using a switch-mode circuit. The losses can be minimal if the switching device has a low 'on' voltage drop.
You should be able to get ENERGY out of a capacitor using a switch mode circuit whether the voltage on the capacitor is greater OR less than required output voltage.
However, you need to get in perspective the total energy which can be stored in a capacitor compared with that stored in a battery. Capacitors have a great future but in a limited number of applications. Not for the main energy storage in motor cars!
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It's also worth noting that Super Caps have a high leakage current. They could drain the battery supplying it if it was not in constant use like in a car driven daily.
You don't get something for nothing. A charged Super Cap can delivery the current into a circuit, but it then needs a finite time to recharge from its source during which constant Boom Boom music will discharge.
Listening to quiet sedate music in your car eliminates this problem completely and probably means you live longer as a result.