Lets say one has this big super capacitor with 2.5V and 200-400F of charge. And you want to run 12V / 1A DC motor off that super cap. How is it done?
DROBNJAK:
Lets say one has this big super capacitor with 2.5V and 200-400F of charge. And you want to run 12V / 1A DC motor off that super cap. How is it done?
Unless the motor is really tiny, or you have a really special super capacitor, you probably can't. Regular super capacitors can only provide microamps of current and you need 10A. This is why people still use regular batteries in RC cars, etc.
Thanks for the quick answer. But I do not need 10A, only 1A.
I thought caps can supply immense quantities of current. Don't they use them in magnetic rail guns, where caps discharge torrents of current in a millisecond?
DROBNJAK:
Thanks for the quick answer. But I do not need 10A, only 1A.
You need 1A at 12V but you only have 2.5V so you have to pass it through a transformer.
Maybe not 10A, but 6-7A at least, depending on the transformer. Still thousands of times more than cheap super capacitors.
DROBNJAK:
I thought caps can supply immense quantities of current. Don't they use them in magnetic rail guns, where caps discharge torrents of current in a millisecond?
They use a different type of capacitor and/or put hundreds of them in parallel.
eg. You might be able to run your motor if you put a thousand of them together.
And in any case, how long would it take to discharge the cap?- a few seconds?
Pehh, this is a real disappointment. I hoped to harvest energy with say, solar panel, store it in super-cap and than run few DC motors. So, the batteries are the only option?
Is it true to say that even 400F capacitor is to small to run a typical 12V/1A DC motor for any significant period of time?
JimboZA:
And in any case, how long would it take to discharge the cap?- a few seconds?
Days. Or weeks.
Those eBay supercaps have ESRs in the thousands of Ohms, it's like charging/discharging through a 2K resistor.
DROBNJAK:
Pehh, this is a real disappointment. I hoped to harvest energy with say, solar panel, store it in super-cap and than run few DC motors. So, the batteries are the only option?
Yep.
DROBNJAK:
Is it true to say that even 400F capacitor is to small to run a typical 12V/1A DC motor for any significant period of time?
It's nothing to do with time. It won't be able to turn it at all. Not a single rotation.
Imagine trying to power your motor with a button battery.
OK, those are the military ones. Very bulky/expensive.
(ie. Far more expensive than the equivalent capacity in batteries)
I paid $16 for one of those, second-hand. Like he did.
How sensitive are they to overvoltage, etc?
A bank of those might work if you could find some (but even at that price it would still be much more expensive/difficult than regular batteries).
And you have to charge the buggers up in the first place..... and then put the lid on very quickly to keep those pesky Coulombs in until you need them
DROBNJAK:
Lets say one has this big super capacitor with 2.5V and 200-400F of charge. And you want to run 12V / 1A DC motor off that super cap. How is it done?
2.5V at 400F is 1250 Joules. The motor is approx 12W. Time in seconds = Joules/Watts so I calculate that assuming 100% efficiency you'd be able to run the motor for 1250/12 = approx 104 seconds. Assuming 60% efficiency, sounds like your motor would run for just about one minute if you spun it up to speed before applying power.
This is all assuming the ESR of the caps is low enough to flow the current required.
This is an off the cuff calculation and could be way wrong since I don't often deal in Joules or Coloumbs.
This is approximately .0003472 kWh, worth about .003 cents. You need allot more caps.
afremont:
2.5V at 400F is 1250 Joules. The motor is approx 12W. Time in seconds = Joules/Watts so I calculate that assuming 100% efficiency you'd be able to run the motor for 1250/12 = approx 104 seconds. Assuming 60% efficiency, sounds like your motor would run for just about one minute if you spun it up to speed before applying power.This is all assuming the ESR of the caps is low enough to flow the current required.
This is an off the cuff calculation and could be way wrong since I don't often deal in Joules or Coloumbs.
This is approximately .0003472 kWh, worth about .003 cents. You need allot more caps.
Well, the engineer who knows theory always wins. This analysis really paints a full picture. When one now calculates a typical super-cap cost and weight, it becomes too uneconomical. Back to batteries.
DROBNJAK:
afremont:
This is all assuming the ESR of the caps is low enough to flow the current required.
...Well, the engineer who knows theory always wins. This analysis really paints a full picture. When one now calculates a typical super-cap cost and weight, it becomes too uneconomical. Back to batteries.
I didn't bother doing that part because I know the ESR of cheap supercaps isn't low enough.
fungus:
DROBNJAK:
afremont:
This is all assuming the ESR of the caps is low enough to flow the current required.
...Well, the engineer who knows theory always wins. This analysis really paints a full picture. When one now calculates a typical super-cap cost and weight, it becomes too uneconomical. Back to batteries.
I didn't bother doing that part because I know the ESR of cheap supercaps isn't low enough.
What about those gigantic 1F caps that car stereo freaks use next to their KW amplifiers. They can surely flow some current or they are worthless; that's the whole point they even exist. I'm not saying they work, I'm just saying that high ESR surely wouldn't do with those.
afremont:
What about those gigantic 1F caps that car stereo freaks use next to their KW amplifiers. They can surely flow some current or they are worthless; that's the whole point they even exist. I'm not saying they work, I'm just saying that high ESR surely wouldn't do with those.
Those people don't mind spending money on low ESR caps.
Not all caps have high ESR, only the ones that are cheap enough to replace batteries. 
So what about simply trickle charging a battery with the solar instead? Unless the demand for the motor is very high, that may be an option.
And is a 12V/1A motor really necessary for your application? Is it speed or torque that you need from the motor?
Retroplayer:
So what about simply trickle charging a battery with the solar instead? Unless the demand for the motor is very high, that may be an option.And is a 12V/1A motor really necessary for your application? Is it speed or torque that you need from the motor?
Oh, most likely I do not need that much. I was asking more for the sake of self education. I watched recently some YouTube videos about energy harvesting, so I though that was an interesting angle for many new projects.
On the end of the day, the old Mars Rover, was using solar panels to store power in a battery, not capacitors.