Capacitor question: How much would ~10kJ/3Wh worth of useful storage cost?

[DeathRayLoveMachine]

So I have a circuit that draws about 200-300mA at right around 1 volt, and I want to be able to power it off a rechargeable circuit for around 12 hours. What would be the cheapest option to supply this with capacitors? For full credit, please also describe your preferred method of voltage regulation and include the cost and inefficiency of that system in your estimate.

The reward for answering my question is my undying admiration (and also I might post pics if I ever get everything working).

Batteries aren't a good option because of limited charge/discharge cycles and poor performance in very hot or cold environments (the system needs to remain outdoors, possibly in direct sunlight).

 

[Raghu]

Any limits on size? Supercaps are the only ones that come within a comparable range of batteries for similar energy/size.

1) Pick supercapacitor (0.5CV^2 is the energy stored)
2) Buck regulator to convert capacitor voltage to ~1V
3) Regulator only works till capacitor voltage is above ~1.1V. (0.5C(1.1^2) energy left in capacitor cannot be utilised)
4) Well designed regulator is 80% efficiency
5) Work out what supercaps you need and allow margins for self-discharge

Improvements/Options
1) Design a buck-boost regulator. Allows cap to discharge lower than 1V. This is difficult for other reasons. MOSFETs switches dont turn on easily at this point. You might need a secondary supply running on BJTs to switch MOSFET gates. Doesnt get you much improvement in capacitor values anyway, since the energy left in capacitor might be too little.

I would still prefer a battery design - just use more/bigger batteries than needed. If regular battery has 1000 cycles and you need 10k cycles, then design with 10 batteries. Compensate for temperature too, so that you have 12 hours of operation at worst conditions (hot or cold).

Supercaps have a cycle life too. Lower than normal caps, more than batteries.

Normal caps tend to become way larger for similar energy capacity.

Can I have my cookie now?

 

[PottedMeat]

here's 16kJ @ 5V for $34.00. Add $5-10 of parts and labor for a buck converter for 5V->1V @ 0.2A.

http://www.goldmine-elec-products.com/prodinfo.asp?number=G18108

They look like parts from surplus transportation modules.

I don't know the internal discharge rate but i'll just guess that starting fully charged and being loaded with 200mA ( a buck converter would load it less ) for 12 hours it'll still be >3V.


1V@300mA isn't much and a nice battery would probably work fine. Outdoor/Very Cold/Very Hot environment requirements have been around a long time and there's probably a battery for this.

 

[CycloWizard]

What's the application? A cheap solar cell can deliver that current at that voltage even under incandescent lights. Of course, if you're using it in a diamond mine, that's not going to help you.

 

[William Gaatjes]

Any limits on size? Supercaps are the only ones that come within a comparable range of batteries for similar energy/size.

1) Pick supercapacitor (0.5CV^2 is the energy stored)
2) Buck regulator to convert capacitor voltage to ~1V
3) Regulator only works till capacitor voltage is above ~1.1V. (0.5C(1.1^2) energy left in capacitor cannot be utilised)
4) Well designed regulator is 80% efficiency
5) Work out what supercaps you need and allow margins for self-discharge

Improvements/Options
1) Design a buck-boost regulator. Allows cap to discharge lower than 1V. This is difficult for other reasons. MOSFETs switches dont turn on easily at this point. You might need a secondary supply running on BJTs to switch MOSFET gates. Doesnt get you much improvement in capacitor values anyway, since the energy left in capacitor might be too little.

I would still prefer a battery design - just use more/bigger batteries than needed. If regular battery has 1000 cycles and you need 10k cycles, then design with 10 batteries. Compensate for temperature too, so that you have 12 hours of operation at worst conditions (hot or cold).

Supercaps have a cycle life too. Lower than normal caps, more than batteries.

Normal caps tend to become way larger for similar energy capacity.

Can I have my cookie now?

Here is your cookie :thumbsup: :

 

[William Gaatjes]

So I have a circuit that draws about 200-300mA at right around 1 volt, and I want to be able to power it off a rechargeable circuit for around 12 hours. What would be the cheapest option to supply this with capacitors? For full credit, please also describe your preferred method of voltage regulation and include the cost and inefficiency of that system in your estimate.

The reward for answering my question is my undying admiration (and also I might post pics if I ever get everything working).

Batteries aren't a good option because of limited charge/discharge cycles and poor performance in very hot or cold environments (the system needs to remain outdoors, possibly in direct sunlight).

Every component has a temperature range. With that in mind you have to take into account the maximum temperature. Your circuit may drift in design specification as well when exposed to an extreme temperature range.
You could also opt for a hybrid design. As Cyclowizard mentioned, solar panels can be found at the rated current and voltage, however, if you combine an over dimensioned solar cell with a small rechargeable nimh accu you will be fine. The excess current from the solar cell can be used to charge the battery. When the solar cell is no use, you have the rechargeable accu.

Linear Technology has some "ideal" diodes in usable sot23-5 cases : The LTC4411.
Instead of a wall adapter you could use the solar panel.

These can still be soldered by cutting a standard 1e (2.54mm) pad into 2 halves.I do this myself all the time for DIY soldering on single pad prototype boards.

If you use your battery at say (3* 1.2 Volts) 3.6 Volts and your solar cells at a bit higher range, you can use a battery charger from LT (linear technology for it). LT have a free simulation program LTspice where you simulated your design and compare efficiency. May i ask why you need 1 Volts at 200 to 300 mA ? It sounds as if you are using a circuit that has a step up converter in it.
Electronics working at that low voltage are rare. Forgot to mention that efficiency higher then 90 percent is easy, and the current drain on your source will go lower when you use a buck smps to output a lower voltage. Thus reducing the current you draw.

If more information is required, we will help...

 

[DeathRayLoveMachine]

Meat and Gaatjes get nothing, NOTHING.

The application is a line-of-sight IR Ethernet transceiver, installed in a place where it would prohibitively difficult to install wiring (duh, or else I wouldn't be using wireless networking). Power consumption is about 150mA at 1.5v. The plan is to hook it up to a thermoelectric generator for power, with a battery or capacitor bank for backup operation.

Considering my usage scenario, one charge/discharge cycle = exactly one day of service. When dealing with extreme temperatures and poor power management, you'd be lucky to get even 200 c/d cycles out of a battery, which would translate into about six months of operation. For these reasons I want capacitors, just as God intended.

 

[DeathRayLoveMachine]

Oh yeah, since the device will be permanently installed and never moved, I don't care about size or weight.

 

[Jimmah]

RONJA, yes? I like all the ideas here, a mix of them might be your best bet, just know you will have to replace parts eventually.

 

[herm0016]

thermoelectric generator? like rtg? I have always wanted an old Russian RTG.

what is your heat source?

i agree with others here, you could use caps, or you could use a rechargeable battery. or a combo of both. what sort of temperature extremes? the solar garden lights in front of my parents house have AA nimh batteries and they have been out there 2 winters and still work, at a bit reduced light time from new.

 

[DeathRayLoveMachine]

I assume just a Peltier, based on the shape. It is huge, optimized for harvesting energy from relatively small temperature differences (i.e. 5-50 F), and works in either direction. Give it a 20 degree difference and it will happily supply you with almost one watt of power from now until the end of time.

I should mention that it is Russian and possibly Soviet.

Right now my plan is to attach one end of it to a giant heatsink wrapped in insulating foam and the other end to a dark-colored heat exchanger. During the day the heat exchanger will get hot, heat will flow into the heatsink, and that will generate electricity; and at night the heat exchanger will be cooler than the heatsink, the flow of temperature will reverse, and that will also generate electricity.

I haven't worked out the math properly yet but I need at least 6Wh/day and that's only about 20BTU/day (enough to heat one pound of water 20 degrees Fahrenheit), which seems eminently feasible.

 

[videogames101]

Meat and Gaatjes get nothing, NOTHING.

The application is a line-of-sight IR Ethernet transceiver, installed in a place where it would prohibitively difficult to install wiring (duh, or else I wouldn't be using wireless networking).

Out of curiosity, why are you using IR for ethernet? 😕

 

[DeathRayLoveMachine]

RONJA, with a few modifications.

 

[alkemyst]

Give it a 20 degree difference and it will happily supply you with almost one watt of power from now until the end of time.

You know what's scary these kinds of devices were sort of laughed about as cool, but what could they really power.

With LED lighting and other 'green' innovations, 1 watt goes long way.

 

[Jimmah]

Knew it. We've been trying to finish a laser diode RONJA for almost a year now, found our most efficient power source was an ebay solar panel with battery backup. Your situation is likely different though, since you never intend to touch the thing once it's running.

*We used a 6v utility battery with 2x 9v .3a solar panels in series and a cheap 7807 regulator another student cobbled up, works fine in lab powering approx dummy load dunno how it would do in the field for months at a time.

Good luck

 

[William Gaatjes]

Meat and Gaatjes get nothing, NOTHING.

The application is a line-of-sight IR Ethernet transceiver, installed in a place where it would prohibitively difficult to install wiring (duh, or else I wouldn't be using wireless networking). Power consumption is about 150mA at 1.5v. The plan is to hook it up to a thermoelectric generator for power, with a battery or capacitor bank for backup operation.

Considering my usage scenario, one charge/discharge cycle = exactly one day of service. When dealing with extreme temperatures and poor power management, you'd be lucky to get even 200 c/d cycles out of a battery, which would translate into about six months of operation. For these reasons I want capacitors, just as God intended.

To our defense, you did not supply the information what your circuit is doing.
You where also not specific about the energy consumption which can be a very big difference in the world of electronics. If you want to use thermal electric generators, you must have a situation of a large temperature difference all the time. In my defense, you can still use the ideal diodes from LT. 😛

Only capacitors similar as the original Leiden Jar have a very high temperature range. Practically you can use high temperature electrolytic capacitors. These can be used up to 105 degrees Celcius (and minus -40 degrees celcius) Since size is not your limiting factor, just put a few hundreds in parallel.

But i still do not get the 1.5Volts. Leds work at a higher voltage and most electronic circuits made from non special designed or non custom components do not run on 1.5 Volts but use a boost converter for a common voltage as for example 3.3 Volt or 5 Volts. Or are you using very low power mcu's together with boost circuits ?


And RONJA still uses at least 1 ethernet cable when not using wireless transmission to hook up to the network. I do not know if wifi chips are available at that low voltage. Unless your intention is to build repeaters, then you would only need to comply to the light system.

I am curious, how is the solar light shielding ? Most photodetectors i have seen even with fancy demodulation circuits go blind when solar light enters directly the detector.

EDIT :

This explains a lot :

http://ronja.twibright.com/

 

[William Gaatjes]

This explains a lot, but i am sure you are not using these circuits or using these circuits without boost converters.

http://ronja.twibright.com/schematics/

http://ronja.twibright.com/schematics/metropolis_transmitter/metropolis_transmitter_00.pdf

http://ronja.twibright.com/schematics/10M_receiver_00.pdf

 

[William Gaatjes]

You know what's scary these kinds of devices were sort of laughed about as cool, but what could they really power.

With LED lighting and other 'green' innovations, 1 watt goes long way.

These guys state impressive numbers :

http://www.tegpower.com/

 

[William Gaatjes]

I forgot one thing. Capacitors have a leakage current. This is similar as the self discharge current from batteries(Not the same mechanism). I have not calculated this, but i think that per joule, the leakage current of capacitors is higher then batteries. This is something you want to investigate as well. Because it is highly likely that this leakage or discharge current is temperature dependent.

 

[DeathRayLoveMachine]

To our defense, you did not supply the information what your circuit is doing.
You where also not specific about the energy consumption which can be a very big difference in the world of electronics.

So I have a circuit that draws about 200-300mA at right around 1 volt

Only capacitors similar as the original Leiden Jar have a very high temperature range. Practically you can use high temperature electrolytic capacitors. These can be used up to 105 degrees Celcius (and minus -40 degrees celcius) Since size is not your limiting factor, just put a few hundreds in parallel.

All the capacitors I've looked at specify operating temperature ranges of at least -20 to +80 centigrade, which is at least fifteen degrees colder and twenty degrees hotter than I will ever need.

But i still do not get the 1.5Volts. Leds work at a higher voltage and most electronic circuits made from non special designed or non custom components do not run on 1.5 Volts but use a boost converter for a common voltage as for example 3.3 Volt or 5 Volts. Or are you using very low power mcu's together with boost circuits ?

I'm using ULV variants on the RONJA (mostly) designed by someone else. I'm not exactly sure what's up with the LEDs but 1100nm LEDs will happily work down to around 1.12 volts if you find the right ones. (Addendum having called to ask the LEDs we will probably use have integrated voltage boosters.)

And RONJA still uses at least 1 ethernet cable when not using wireless transmission to hook up to the network. I do not know if wifi chips are available at that low voltage. Unless your intention is to build repeaters, then you would only need to comply to the light system.

There are two RONJA units plugged into each other to make a repeater pair. (Just to be clear that is two Tx units and two Rx units.)

I am curious, how is the solar light shielding ? Most photodetectors i have seen even with fancy demodulation circuits go blind when solar light enters directly the detector.

Don't have it yet, no idea.

 

[William Gaatjes]

As posted in your above posts, you mentioned different levels for voltage and current. Please, it helps a lot if you are specific sometimes. I know it is not always easy or possible but please try. ~1 Volt or 1,5 Volt and 200mA or 300mA or 150mA make a lot of difference because it can be reason for a redesign and it limits or expands your choice of components.

All the capacitors I've looked at specify operating temperature ranges of at least -20 to +80 centigrade, which is at least fifteen degrees colder and twenty degrees hotter than I will ever need.

Ok, do you also know that often the lifetime is only for 40 degrees celcius and that IIRC extreme temperature seriously degrade the electrolyte of the elco ? This is not always clear and only a few manufacturers actually state these numbers because there is no need too (indoor consumer electronic applications have no need for -40 to +105 temperature specs). It is wise to do some research here as well.
And take into account the shelf life. Capacitors laying in a dusty basement do not perform that well after a few thousand hours shelf life.

I'm using ULV variants on the RONJA (mostly) designed by someone else. I'm not exactly sure what's up with the LEDs but 1100nm LEDs will happily work down to around 1.12 volts if you find the right ones. (Addendum having called to ask the LEDs we will probably use have integrated voltage boosters.)

I read about that.
IIRC the light can not be varied fast enough because of the way the led is build, what they do is i think the same principle used in overdrive LCD's. The squarewave signal has an overshoot and an undershoot placed just at the right spot(the edges) to increase the speed (or responsiveness of the led) momentarily. It is a sort of predrive where you provide more power then needed to accelerate the charge carriers faster for a short moment but you do not want to long because then you cause the unwanted side effect of actually reducing the bandwidth or responsiveness(More voltage all the time is more charge carriers, is more things to move around in the same time frame and with the same means of transportation a limiting factor).
With the RONJA nebulus, they use capacitors parallel to the current limiting resistor for the led.
A discharged capacitor acts as a short circuit when a voltage is applied to it. However, after the voltage between the capacitor "plates" begins to rise, the current will decrease until the capacitor is full. Then there is only a leakage current remaining.


I have never heard of integrated booster inside leds. I think what you will buy then is a led and electronics on a small pcb in a special case. Are you sure the booster circuit can provide the bandwidth you need ?

There are two RONJA units plugged into each other to make a repeater pair. (Just to be clear that is two Tx units and two Rx units.)

I expected that ^_^ .

Don't have it yet, no idea.

I had a quick look on the mechanics, it is just a solar cap, a narrow tube which makes the light dependent of the direction. Only in line with the tube the light can enter. It is needed because the sunlight is a very broad noise spectrum.

 

[DeathRayLoveMachine]

I have never heard of integrated booster inside leds. I think what you will buy then is a led and electronics on a small pcb in a special case. Are you sure the booster circuit can provide the bandwidth you need ?

I'm not in charge of procuring them, but I think the plan is to scavenge the LED assemblies off some old ULV fiber optic gear. (This is also where the photosensors will come from, I think.)

They are already integrated into a modular PCB and removing them from it would just be a pointless hassle.

 

[DeathRayLoveMachine]

As posted in your above posts, you mentioned different levels for voltage and current. Please, it helps a lot if you are specific sometimes. I know it is not always easy or possible but please try. ~1 Volt or 1,5 Volt and 200mA or 300mA or 150mA make a lot of difference because it can be reason for a redesign and it limits or expands your choice of components.

The variability in the numbers I gave is to account for overprovisioning of the circuits. I said "about a volt" and later refined it to 1.5 volt, although that might change a little once we get all the components together.

The numbers I gave were entirely specific enough for people to make good suggestions for components or designs. At this point we are explicitly in the design phase so there is no threat of having to redesign anything complicated or already assembled.