Supercapacitor Battery

[superpanda]

I am have small circuit I am trying to design that would power a 3vdc / 70ma device for approx 2 hours. The device can not contain rechargeable batteries of any sort, therefore I want to use a Supercapacitor capacitor to do the job.

I want to have a charge / discharge circuit that would supply the device with a steady supply of 3-3.3vdc. The charge side could be 3.3,5,12 or 24 volts with ample current. I want the charge cycle to take less than 1 minute.

I also want to keep the circuit compact. A garbage can sized cap will not work. By my rough calc I need about 400F.

Any experience out there with this? Can anyone point me in the right direction?

Thanks

 

[IRstuff]

You need to account for a voltage regulator and its associated inefficiency, since you are allowing more than 1V change in voltage.

If you only allow 0.3V change, you could do without the voltage regulator, using 1700F

TTFN

FAQ731-376

 

[zappedagain]

Check the Ultracap web-site. They have good application notes that may point you in the right direction. Illionois Capacitor may have some app notes too.

John D

 

[BobM3]

That's a lot of capacitance. What's the ESR? Can you even pump enough current through them to charge them up in a minute?

I'm currently struggling with a similar application. I decided on a simple buck switcher. I thought the transformer was needed because I was going from 110 VAC way down to 10 VDC with 3 amps charging current. Still more parts than I'd like though. If you find something slicker can you post what you found?

 

[superpanda]

I am looking at a couple options...One is Maxwell and the other is Nichicon. The Maxwell has a better ESR of 2.2. The Nichicon is 7. I purchased a 400F Nichicon unit to test with becuase the packaging is better for me. I think I may actually be able to work with a runtime of less than an hour so my capacitance need is somewhat less than origianll thought. I have not done any caluclations yet to see how long to charge the cap @ 3A (the max current I have available to me)

I am plannig on using a small boost switcher the level out the discharge.

 

[IRstuff]

2hr*70mA/400F = 1.26 V, which means that you need to have charged it to at least 1.26V + 3V = 4.26V

400F*4.26V/3A = 568s initially

400F*1.26V/3A = 168s when cap is already beend precharged and you are simply replacing the 2hrs worth of current draw.

TTFN

FAQ731-376