Hi Everyone
I am considering using a Lithium Ion Capacitor as a power source for my next project. This will be embedded in clothing, so I don't want it to spontaneously heat up or something like that.
Can anyone suggest some reading on capacitors as power source (in regards to power regulation etc.) for me?
Thanks
p.
"Lithium Ion Capacitor", I have never heard of that.
Lithium Ion battery, yes.
Super Capacitor, yes.
LiIon batteries and LiPo batteries are safe as long as you're not trying to overcharge or draw excess current.
Sparkfun has a bunch of charge boards for them.
It will probably only work if you need very-little power.
A capacitor has a logarithmic discharge rate, which is the opposite of what you want in a battery. A capacitor discharges quickly at first, and then more slowly as the voltage approaches zero. An ideal battery would be like a fuel tank in a car... You would like to have the full voltage/power available until the end when it suddenly gives-out. Real batteries are not quite like that, but under normal (low drainage) operating conditions the discharge curve can be fairly flat.
So yeah... you'd need LOTS of regulation... Maybe charge-up the capacitor to 5 or 10 times the voltage you need, and regulate-it down. (Of course, a switching regulator is more efficient than a linear regulator. So, the power would last longer with a switching regulator.)
I don't know anything about lithium ion capacitors, but most capacitors are nearly 100% efficient. You get-out almost as much total energy (Watt-hours) as you put-in... I don't know how efficient batteries are, but I'd guess less than 50%.
Lithium Ion Capacitor:
thanks for the input, DVDdoug (and yes, I am looking to running the entire system on maybe 2V - i guess that could be considered low power)
@crossorads
super capacitors, then. Any useful tutorials on how to power them and how to use them to power say a vibration motor? (I plan on charging it wire-less through a coil, so I a) probably cant use off the shelve products and b) really need to know what I am doing before I start some fruitless trial and error process...)
Capacitors, even 'super' types don't make very good power sources for other then very low current consumption devices like ram chips and such. Assuming your going to power something like leds on your wearable project I would think you should look at high power density miniature Li-po battery cells.
Your best off just proceeding with building the project and writing/debugging the code first using wall wart supply and once you are completed you can take a current reading of the running project and you will be in a better position to size the power source for the size Vs duration goal you seek.
Lefty
@lefty - I know what you are saying makes sense, however I am working on this together with someone who really wants to use these capacitors. I would prefer LiPo cells myself. However, I want to read up on them in order to make sure that it really cant be done with a capacitor.
Also, the trickiest part of our design is the power supply. We have some pretty extreme constraints on it, so we need to design around it. (yeah, I know, its sort of lame letting you guess, but the application is so far out, that I rather show a working prototype in the end, or fail silently and not talk about it.)
So, I guess, the question still stands. Does anyone have any recommended readings for me, so I can become a self taught expert of using super-ion-mega-duper-capacitors as power supplies?
Oh, and while we are at it. Any recommended links in regards to wireless charging?
So, I guess, the question still stands. Does anyone have any recommended readings for me, so I can become a self taught expert of using super-ion-mega-duper-capacitors as power supplies?
Your pretty likely on your own on this regard. However to get any chance of someone being able to help you need to give a few specification requirements:
What must the voltage range be from this 'magic power source', example 5vdc only, 12 to 3 vdc as charge decreases, or some other voltage specification.
What will be the current draw of the project, 20ma constant, 20ma to 1 amp depending on 'events'. or other current draw characteristics.
How long a duration must this power source power the project, 1 min, 1 hour, 1 week?
No matter if you design it yourself or someone here takes a stab at it for you, without filling in the above unknowns it's just not possible to proceed with anything other then 'brainstorm' type ideas, not practical solutions. As in most engineering efforts it's mostly about the math.
PS: As you already seem to have the caps, a link to their datasheet would also be required to proceed.
Lefty
Okay, given what wikipedia shows, I would put whatever charging circuit you need in front of the cap (does it just need 5V thru a feedback preventing diode like a supercapacitor?) and then the output thru a boost regulator such as Linear Technology LTC3525, which only needs 2 caps and 10uF inductort (see last page of datasheet) to maintain the output voltage while the cap discharges.
LTC3525fb voltage_boost.pdf (541 KB)
This is quite an amazing forum. Instead of a "nope, nothing we know of" I get a "we can help you if you just tell us more"
I was not aware of how esoteric my question was. I thought someone would just be able to point me to some tutorials.
Also, I too am waiting for a spec sheet. However, apparently we have the options to hand in specs and get a custom capacitor.
Lets turn the game around. I am interested in your suggestions on how you would go about this.
We have following constraints:
There are two usage scenarios
a) 25 minutes, constant 100mA, - Charging speed is an important factor
b) 48 hours, max 110mA, depending on 'events'. Average current is approximately 40mA
For usage scenario a) I was thinking that capacitors might work, especially since they can be charged very fast. For scenario b) I would go with a LiPo, but I am really interested in any alternate suggestions, or whether it might be possible to cover both usage scenarios with one system...
@crossroads. Thanks for the suggestions. I was thinking something more or less along those lines for the general setup, just didn't know if I was on the right track or not. I will check out the specific regulator you suggested.
Cheers
p.
48 hrs, 40mA avg, thats 1920mAH.
25 minutes 100mA, that's 42mAH. Li-Ion-Cap can do that?
Maybe have a cap for fast charge/short term use in parallel with a LiPo for long term use.
Well this sounds very intriguing but I have one question: where can you buy these things? Just spent a few minutes googling and came up with nothing. Even downloading a datasheet yielded blank pages. Seems like the hardest part of this project will be figuring out how to build it with an unobtainable part.
Being someone who sells caps for a living, I think I can (kinda) help.
First, "Lithium-ion capacitor" is a form of super cap. High capacitance, very low operation voltage. I'm not aware of any manufacturer producing these in anything beyond samples-volume.
fkeel:
However, apparently we have the options to hand in specs and get a custom capacitor.
Realize the cheap prices you see in capacitors is because the largest capacitor company in the world produces more capacitors in a day than Hershey's makes Kisses in a day. And then, most of the volume is ceramics, which account for about 10% of that companies total sales. (Which is to say, if it isn't in a catalog, it isn't going to be affordable.)
So a custom capacitor can be tens of thousands of dollars, assuming it is a technology (e.g. film) that lends itself to "custom" work.
Maybe just use a big-ass capacitor until they are purchasable.
Digikey has some large capacitance caps in the $30 range.
http://www.elna.co.jp/en/capacitor/double_layer/catalog/pdf/dz_e.pdf
Unfortunetly for unique things like this simple tutorials don't exist(feel free to make one when u figure it out for the next guy)
being a capacitor it can charge very quickly which is useful, look around for tutorials on that wireless power
most of it includes a fairly large coil and capacitor in a tuned resonant circuit and 125khz driving frequency
you'll need a way to get as much power from that at a low enough voltage to not blow your cap,
but idk if its easily possible on a small size with capacitors, I just did a project with a 1F supercap at 5v and my atmega 328 wich drew exactly 33ma @ 5v lasted about 3 minutes before the brownout stopped it at I think 1.71(or 2.71 I forget) volts
At that rate its gonna be alot of caps to get the time you need