I will be talking a bit about double-layer capacitors and why they are useful. Double-layer capacitors are sometimes called ultracapacitors or super capacitors. I will be calling them super capacitors. What makes a super capacitor super ? Capacitors have two conductors separated by an insulator. Super capacitors also have two conductors which are coated with active carbon instead of having an insulator separating them they are immersed in an electrolytic solution.
Super capacitors can be charged and discharged thousands of times with out any affect on the capacitor itself. Batteries have a limited number of cycles before they start degrading. Super capacitors store charges electrostatically and batteries use a chemical reaction . Super caps have some advantages and some disadvantages, advantages include high cycle life, low resistance provides high load currents, they are not greedy they only draws what energy they need to charge with no worries of overcharge, and charges in seconds . The disadvantages include only holds a fraction of energy as a battery, higher self discharge than most batteries, and low cell voltage you have to connect multiple caps in series , which also decreases capacitance, and each cell has to be balanced.
So what can I do with these things!? you might be asking. They are a green alternative to batteries in certain applications. I worked on a project for a smart power manager. The solar panel input had a super cap bank tied to the output, when the solar panel output would drop due to clouds or other variables the voltage on that output remained constant for a period of time until the solar panel started outputting it's maximum again. At home I play around with super capacitors and leds, solar charging and arduino boards, and fans.
I believe that super capacitors are the future of green energy storage. I hope you enjoyed my first Instructable and learned something along the way.
As I mentioned before when you connect two or more super caps is series you will need to add balancing to each cap. This is done because you will be charging the capacitors at a voltage that is higher than each cell is rated therefore the one or more of the caps may try to take on more voltage than they can handle, this usually happens when the capacitors are weak.
There are two way to balance the simplest is passive. Passive balancing is pretty simple, you place a resistor in parallel with each capacitor , I have always been taught to size the resistor value 10x the leakage current of the capacitor for slower balancing or 100x the leakage current for faster balancing.
The seconed method is active balancing. Active balancing uses semiconductors to balance each cell. Active is the way you want to go when efficiency is important. I hope that shines a little light on the subject of balancing super capacitors.