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Capacitors

Gryd3

Jun 25, 2014
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What happens to the current in this case? Does a fully charge capacitor even have enough current for when charging the battery despite using a boost converter?
You should not need to worry if a capacitor can output enough current... they can provide a SCARY amount of current if they are allowed to.
The question here becomes:
-Can the Capacitor put out current 'long enough', to create an acceptable charge on the battery?

In a perfect device, a boost converter will take an input voltage and double it, but will also halve the input current... ie. 2.5V and 1000mA into the device, becomes 5V and 500mA leaving the device.
In reality, you end up loosing a little in the device, so you may only get 5V and 450mA out of the device.
A capacitor can almost be compared to a very tiny battery. The voltage will start out high, but will rapidly fall as you pull energy from it. The larger 'Farad' reading a capacitor has, the more charge it can hold at a time.
Think about filling a cup of water with drops of water. A smaller capacitor will be able to provide a much smaller drop than a large capacitor. It will take a LONG time to fill the cup, but you will eventually get it full. (This also depends on how many drops per second you are putting in) However, there is an issue. Batteries and Capacitors suffer from 'self-discharge'. Now pretend that cup has a very small pin-hole in it that the water leaks out. You notice that if the drips don't come in fast enough, the cup will remain empty.
The same principle applies here. A smaller capacitor will need to cycle much quicker than a large one to fill a battery to overcome losses. This would also mean that the left over energy in the capacitor would simply be added to instead of waiting for it to completely empty.
 

TBadertscher

Apr 14, 2015
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You should not need to worry if a capacitor can output enough current... they can provide a SCARY amount of current if they are allowed to.
The question here becomes:
-Can the Capacitor put out current 'long enough', to create an acceptable charge on the battery?

In a perfect device, a boost converter will take an input voltage and double it, but will also halve the input current... ie. 2.5V and 1000mA into the device, becomes 5V and 500mA leaving the device.
In reality, you end up loosing a little in the device, so you may only get 5V and 450mA out of the device.
A capacitor can almost be compared to a very tiny battery. The voltage will start out high, but will rapidly fall as you pull energy from it. The larger 'Farad' reading a capacitor has, the more charge it can hold at a time.
Think about filling a cup of water with drops of water. A smaller capacitor will be able to provide a much smaller drop than a large capacitor. It will take a LONG time to fill the cup, but you will eventually get it full. (This also depends on how many drops per second you are putting in) However, there is an issue. Batteries and Capacitors suffer from 'self-discharge'. Now pretend that cup has a very small pin-hole in it that the water leaks out. You notice that if the drips don't come in fast enough, the cup will remain empty.
The same principle applies here. A smaller capacitor will need to cycle much quicker than a large one to fill a battery to overcome losses. This would also mean that the left over energy in the capacitor would simply be added to instead of waiting for it to completely empty.
So I have a capacitor that is fully charged and I need to "transfer" that energy to that battery. Could you lead me to some products that best suite what I am doing.

Capacitor:
capacitance: 3000 F
Rated voltage: 2.7 volts
Emax: 10,944 J

Battery:
Nominal voltage: 22.2 v
233 amp hours
5.3 kWh
19 080 000 joules

Is there any products that best fit what I am doing. I would imagine maybe connecting the dc to dc converters in parallel potentially. I can also connect more than one capacitor together in series or parallel as well if need be. The battery does not have to become fully charged, I just wanted to get most/all ( I know the conversion is not done at 100%) the energy from the capacitor to the battery and then repeat until battery is charged. Thank you in advance.
 

Gryd3

Jun 25, 2014
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So I have a capacitor that is fully charged and I need to "transfer" that energy to that battery. Could you lead me to some products that best suite what I am doing.

Capacitor:
capacitance: 3000 F
Rated voltage: 2.7 volts
Emax: 10,944 J

Battery:
Nominal voltage: 22.2 v
233 amp hours
5.3 kWh
19 080 000 joules

Is there any products that best fit what I am doing. I would imagine maybe connecting the dc to dc converters in parallel potentially. I can also connect more than one capacitor together in series or parallel as well if need be. The battery does not have to become fully charged, I just wanted to get most/all ( I know the conversion is not done at 100%) the energy from the capacitor to the battery and then repeat until battery is charged. Thank you in advance.
This has gone from theory to designing a product...
Can you explain what you are trying to accomplish? Big picture please, what are you trying to make?
The issue here is that the capacitor is less than 3V which is pretty close to the lower cutoff voltage for many step up converters. You can use two capacitors in series to help remedy this, but even with completely draining the cap, you will need to take many thousand cycles to charge the battery... It's not practical.
You may be better off directly charging the battery instead of worrying about a capacitor inbetween.
 

BobK

Jan 5, 2010
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What is the point of trying to transfer energy from a capacitor that holds 1/1900 of the energy of the battery to the battery?

Bob
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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@BobK: The point is the capacitor may be receiving its initial charge from a potential much less than the charged (or partially charged) battery potential, say from a weak power source in the environment (energy harvester application). Remote sensors can be configured to operate in this fashion, sipping power from the daylight, or pressure changes in the atmosphere, or temperature gradients... whatever can produce sustainable electrical energy, but at very low levels. The power could be stored in an ultracapacitor, no battery needed, so the sensor can "phone home" periodically, but adding a battery will smooth out the fluctuations in available power from the environment. The trick is how to boost the voltage from the energy harvester to a useful potential to operate electronics. Powerful brains are working on this as we consider the ramifications, including body-powered implantable electronics. Make sure you remain on good terms with Big Brother.
 

BobK

Jan 5, 2010
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My point is that the battery holds nearly 2000 times the energy of the capacitor. Under what scenario would this make sense? The battery would likely self discharge at a rate higher than you could cycle in charge from the capacitor.

Bob
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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My point is that the battery holds nearly 2000 times the energy of the capacitor. Under what scenario would this make sense? The battery would likely self discharge at a rate higher than you could cycle in charge from the capacitor.

Bob
Gee, Bob... where are you getting your numbers? The battery and the capacitor are two separate entities, and neither has been fully specified in this thread. In my energy harvesting example, practical versions do exist! As long as energy is being placed in the capacitor at a greater rate than it is being depleted from the battery, whether by self-discharge or by an electrical load, there will be a net gain in battery energy. The battery will be charged. That the battery holds more energy than the capacitor, by any factor you choose to name, is irrelevant to the process. At least one manufacturer makes a battery suitable for this application.
 

BobK

Jan 5, 2010
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The capacitor and battery are fully specified in post number 22.

Do you really think it makes sense to harvest energy into a capacitor with a battery backup that has 2000 times the energy of the capacitor? If it takes a day to harvest the energy into the capacitor, you will charge the battery in about 8 years.

Bob
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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The capacitor and battery are fully specified in post number 22.
Oops. I missed that.

Do you really think it makes sense to harvest energy into a capacitor with a battery backup that has 2000 times the energy of the capacitor? If it takes a day to harvest the energy into the capacitor, you will charge the battery in about 8 years. ...
Good point. I would either reduce the size of the battery backup or increase the energy into the capacitor, aiming for perhaps three or four days to recharge instead of eight years, depending on the average energy available from the environment.

My only point is this: as long as the energy used to charge the capacitor exceeds the energy used to for re-charge, self-discharge, and load-discharge of the battery, the energy harvester will be self-sustaining. I never said that would be "easy" to accomplish or economically attractive. Solar-powered walkway lights are a good example: cheap Chinese solar cell, POS NiCd battery, and cheap plastic housing for a white-light LED. No supercapacitor necessary. They work until the NiCd fails, which happens sooner than you might expect. Expensive junk IMHO, but they sure are convenient to install and replace when they are stolen or fail, whichever comes first depending on your neighborhood.
 

BobK

Jan 5, 2010
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The newer solar lights have Li ion batteries and work much better than the old NiCad ones.

Back to the original problem. Energy harvesting will be most efficient if the right sized battery is used. The only reason I can think of for using a large capacitor at all is if you can either get away with no battery, or you want to minimize the charge cycles so that the system can run unattended for a longer time.

Bob
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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The newer solar lights have Li ion batteries and work much better than the old NiCad ones.
Yeah, I looked at some with Li-ion cells. Expensive. And the stores I visited (Wal-Mart, Home Depot, and Lowe's) don't tout the life advantage of Li-ion over NiCd, perhaps because they now have tons of stock with NiCd cells installed they want to get rid of.
Back to the original problem. Energy harvesting will be most efficient if the right sized battery is used. The only reason I can think of for using a large capacitor at all is if you can either get away with no battery, or you want to minimize the charge cycles so that the system can run unattended for a longer time.

Bob
I agree completely with your statements.

I must confess that I have never built an energy harvesting application, but your statements make sense. Why even use a large capacitor, if all you want to do is store energy? Is the self-discharge rate of super-capacitors a lot less than the self-discharge rate of current battery technology? I do know that supercapacitors can be charged and discharged over more cycles than a battery without significant change in their ability to accept or deliver a charge, but does the application make that necessary? If we get back to the original poster's question:
I am new to all of this and was wondering if there is a way to dump the energy from a fully charged capacitor to a battery. And what could be used to time this process automatically. Thank you in advance
The answer is no. There is no way to recover all the energy stored in a capacitor and transfer that energy to a battery. But some, maybe even most, of the energy can be recovered, transformed with additional circuitry to a voltage suitable for charging a battery, and this process automatically controlled. Now, why you would want to do this depends a lot on what your project goals are.

At the company I used to work for, one department was developing remote biological sensors for air deployment in the thousands. A later overflight of the same area where sensors were deployed would use a powerful radar beam to illuminate the sensors, commanding them to transmit their stored data. The sensors would "sip" power from the radar beam and send their stored data on another radio frequency. I don't know whether or not they actually got this to work. This bio-engineering department concentrated on sensor development and hired a third-party engineering firm to perform the task of sensor signal conditioning, data storage, and transmission of the data. The sensor systems could have used a disposable battery for all I know, and dispensed with the idea of harvesting RF power from a radar beam illuminating the sensor. But it would have been an intriguing application for energy harvesting, not necessarily using RF power to provide the energy. It sort of reminded me of RFID badges, but at a longer interrogation range.

Hop
 

Gryd3

Jun 25, 2014
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But it would have been an intriguing application for energy harvesting, not necessarily using RF power to provide the energy. It sort of reminded me of RFID badges, but at a longer interrogation range.

Hop
Perhaps with a directional antenna Hevans? I've seen some very interesting tech that they have made public using directed energy : http://en.wikipedia.org/wiki/Directed-energy_weapon
If you can remotely heat up a person, I'm sure you can power a sensor . I'm unsure what kind of range or what weather effects there may be though. (You dont want to pop your balloon either...)
 

hevans1944

Hop - AC8NS
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@Gryd3 when the sensor project was still in the concept stage a few years ago, one of the bio-engineering people consulted with me for ideas. Using active radar to remotely request data transmission seemed to be a viable approach. This would only require a small "patch antenna" to pick up the microwave radar emissions. As for energy harvesting the radar beam, that was pretty much "blue sky" speculation as to whether it would be practical or even possible, given the relatively short time the aircraft would be over the sensor deployment area. I don't think they wanted to hover there with a helicopter while the sensor "re-charged" whatever was going to be used for electrical energy storage. In any case, they went their way and I went on with my work, pretty much oblivious to whatever the bio-engineering department was doing. This sort of thing requires a systems-level design approach because it is multidisciplinary in nature. Costs can get out of hand quickly without specific goals and design parameters, along with periodic reviews to make sure everything is proceeding according to plan. Even then, it is important to know when to "give up" and "abandon" if insufficient progress is made. Sometimes the technology isn't available yet, or it's not affordable.

The directed-energy weapon you cited is real, but AFAIK has not been widely deployed. It was once touted for use in prisons for riot control, but there are political consequences to using it on the general public. Not to say that will never happen, but there are low-tech methods of crowd control that will be used first.
 
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