We want to share some knowledge with you so you don't make a mistake during your supercap selection process. Supercaps are an excellent source of power that are an excellent replacement for a rechargeable battery. Over charging via trickle charge is not an issue, so long as you don't exceed the voltage limit for your supercap!
While there are different capacitor ratings that you have to determine if it will suit your needs, there is one rating of a super capacitor that will impact your project if not factored in during the selection process. It is the ESR rating, aka the Equivalent Series Resistance. In a nutshell, ESR is the internal resistance of the supercap itself.
The more expensive supercaps will most likely have a low ESR rating, down into the milliohms! What this means for you is that little power and time is wasted during the charge cycle. In addition, the response time of the supercap is lower, since there is very little resistance between the electrons stored in the supercap and the waiting circuit that demands them.
We tested various super capacitors during the design phase of the SuperBurst board to see if we could use the higher rater ESR supercaps. What we found was the supercap didn't always perform as needed, during high load times it was not 100% available, since it could not charge fast enough or respond fast enough. We experienced occasional brownouts of the circuit we were attempting to bolster, not good. When we replaced our high ESR supercap with a low ESR one (40ohm ESR to 120mOhm ESR), our circuit no longer experienced large dips in voltage due to high current demands.
If you need an instantaneous, high current source, then a low ESR supercap is an excellent solution. If you need a "ride through" power source with low current demands, then a high ESR supercap may be all you need.