One of the biggest problems of the supercapacitors is their self discharge rate. I charged one of them and measured it's changes for a couple of days. The capacitors is D-size SAMWHA Green-Cap 2.7V 500F.
Here are the results:
You can see that @2.7V the capacitor is charged @100%. I managed to charge it only to 80%. The last 20% are the slowest :). In the beginning the voltage drop is very significant! After ~1.8V the graphics are more linear. We lost half of the charge after only 2 days!
So for 2 weeks we drop from 80% - 26%.
I may conclude that, the supercapacitors need something to keep the charge. Otherwise it is lost.
I don't have any Li-Ion supercapacitors for test. They should have low self discharge.
Now! I expected that, but take a closer look to these two capacitors:
Now to tell you the facts:
- 1 & 2 look absolutely the same, even the blue color, markings, numbers, everything is the same except the pins
- 1 & 2 price was the same
Capacitor 1 facts:
- I ordered it few days ago from another online shop
- It looks absolutely new
- It has 4 pins (no documentation, but as far as I know the 2 extra pins are just to keep it stable on the electronics board)
- It charged for almost the same time as number 2, which means the capacity should be the same
- HOWEVER, the discharge time is drasticaly low! It drops to 1 Volt for less than 24h. I have 2 of them and I tested both of them. I recently noticed people complaining about that on some online shops forums.
Capacitor 2 facts:
- The first that I ordered
- It looks worn-out (that was mentioned on the shop's web site)
- It has 2 pins
- and now I remember it was written "low leakage" in the offer, that is the capacitor that I measured with the graphic above. I thought discharging time is bad, but it seems it could be worse :)
My advice is to look for "low leakage" phrase. You cannot be 100% sure. I think the number of pins does not matter and very often you recieve something different from the picture on the online shop (in my case it was with 2 pins, but I received with 4 . upgrade!? :) )
I will definetely order more of the first type, however the web site moves from domain to domain. I hope they don't have legal problems. That is why I do not mention links.