Supercaps keep getting smaller in size and larger in capacitance…!
What is the self-discharge rate of these? If I charge one up and leave it alone for a day, how much will it have leaked away?
I would like to know how to figure out if I used two in series and charged them to 5v, then placed a 10ma load on them how long they would last until the voltage got down to 1.2v
Well with the 2 in series at 5V thats 5F. From 5V to 1.2V is 3.8V drop which is a 19C discharge. With a 10ma (or 0.01 C/s) draw, that’s 1900s, or 31.667 min. Thats not too bad.
This linear calculation works only because it’s a constant current draw. With a resistor it would be an exponential drop as the drop in voltage would lessen the current draw, which maybe more realistic. I had formula that would calculate the Amp-sec of cap from full charge to 1/3 charge under a resistive load, but that was a while back.
thanks, but how does one figure that out?
With this calculation, q= CV , and I = dq/dt. so in english the charge on a capacitor is capacitance times voltage, and the current is the rate of flow of charge. You provided both so it wasn’t that hard.
For the non-constant discharge (resistive), you need the RC constant and after a bunch of math and a little calculus, you’ll find that the it turns out to be something like: Capacity = C*V(1-1/e). I might be missing a term in there, but I’ll try to recalcate after work.
Thanks, no need to go the extra length. i just wanted a figure to see if i was in the ball park of somthing i wanted to do.
I don’t have an answer for Wiml either. The self-discharge rate depends on the temperature and “historyâ€
Guys, it was just an example of what I was wondering. its not something I was going to do.
The truth be known I was wondering if I could replace a 8.4v battery pack I use for my RC car with a bunch of super caps. this way instead of spending 30-40 minutes recharging the pack I would only need to spend 2-3 seconds to charge it.
There’s a datasheet link on the sparkfun product page, but the datasheet is really skimpy. It doesn’t list leakage or anything as far as I can tell. Maybe there’s a more complete datasheet available somewhere else.
seulater:
Guys, it was just an example of what I was wondering. its not something I was going to do.The truth be known I was wondering if I could replace a 8.4v battery pack I use for my RC car with a bunch of super caps. this way instead of spending 30-40 minutes recharging the pack I would only need to spend 2-3 seconds to charge it.
A battery can store far more charge than a capacitor. It may only take you 2-3 seconds to charge it but it will dischage just as fast if not faster in such a high drain application
-Bill
yes & no, i went to supercaps web site and if i got a few of their 4500F caps and series them together i could get 2-3 minutes of run time with a 30A discharge rate. it could easly be done. the only problem is that the cost would be so great as to not even thing of it.
the technology is out there, its just a matter of the cost coming down.
not to mention, charging a 4500F bank would be a very interesting experience.
4500 F!?!? A single cap? What’s the voltage rating and cost of that?
That’s riding under 2Ah if it’s at 2.5V (assuming only 1 time constant worth of resistive drain… which under about 30A is about a 3.95min as a theoretical max, of course)
I once made a temperature monitoring device, measuring 16 temperatures and sending them via bluetooth to the PC. Supplied by 6 supercaps (Maxwell PC10 / 10F 2.5V) and transformed to 3V3 using a DC-DC converter. Total product height 9mm. Charged within 2 minutes and operating 45 minutes, which was perfect for my application. Works great !