Charging supercapacitors with Buck converter. Do I need to cut power once charge is reached?
Charging capacitors isn't very complicated. The capacitors will charge up to, and hold at, the voltage of your source. Really no need to turn off the source once the capacitors are fully charged. Once the capacitors match the voltage of the source, no more current will flow and the system will be stable*.

If you do desire to have some kind of cutoff, there is any number of ways to do it. A simple mechanical switch is probably the best option. That way the capacitors are physically isolated from the power source. The switch could be something you manipulate with your finger or on a relay that can be electronically switched based on logic.

Here's a few pointers that come to mind:


I wouldn't recommend charging 2.7V capacitors up to 2.7V. Like any other system, if you operate it at its rated maximum capacity, it may shorten its lifespan. Plus, if the voltage source fluctuates a little high for some reason, it could cause the caps to explode in spectacular fashion. Give yourself a little margin to play it safe.
A large bank of uncharged capacitors will look like a dead short to ground the instant you attach them to the charger. Unless you're working with heavy equipment, the buck converter you're using probably won't like the initial current surge. A simple resistor between the voltage source and the capacitor bank will limit the current, but still allow the capacitors to get fully charged. It will just take a little longer. Just chose the maximum current you want and apply Ohm's Law to calculate the resistor. Don't forget to account for power dissipation!
I hopefully don't need to mention that large capacitor banks can become very dangerous when handled. Lots of stored energy and very low ESR is a recipe for an exciting day if you're not careful.


*In reality there will be a small amount of leakage current, so the system will never truly reach zero current.