Supercapacitors

From CPFWiki

A supercapacitor is a low-voltage, high-capacity charge storage devices that works by allowing a buildup of electrostatic charge between two electrodes with very high surface area.

They are also called ultracapacitors, supercaps, or electric double-layer capacitors.

They often look similar to the old electrolytic capacitors, but they differ in very important ways. The main way is, of course, that while a larger electrolytic might be 1000 microfarads, a larger supercap may be 100 farads -- higher by a factor of 100,000!

Supercaps have lower energy densities than chemical rechargable cells like lithium-ion, but they are still in the same order of magnitude. They have many benefits that make them better than chemical cells for some purposes.

A lithium-ion cell may have 150 to 200 Watt-hours per kg, while a supercap may have up to 30 Watt-hours/kg.

However, compared to the lithium-ion cell, the supercap:

• can be charged and discharged millions of times, while the lithium-ion degrades in proportion to the number of charge cycles.
• can be charged and discharged at extremely high rates, hundreds of times faster than li-ion.
• has a longer shelf life and calendar operating life, at decades compared to 3 to 7 years.
• is much less affected by cold temperatures, while li-ion nearly ceases to work and becomes very inefficient in cold temps.
• is much easier to read the state of charge, as it's a direct proportion to the voltage. Reading a li-ion state of charge is much more involved.

So, if you want raw power density, there is currently no better off-the-shelf alternative to li-ion. But otherwise, supercaps offer many benefits.

A current price (Feb 2009) on a 100F supercapacitor is in the range of $20 if you buy 10 or more. This more than the standard lithium-ion cell, which currently runs about $8.

One limitation of supercaps is their voltage limit. Typical supercaps have an upper voltage limit of 2.5V. You can string supercaps in series, to increase the voltage limit. If you string more than two or three in series, however, then you should be aware of the need to balance the charges in the cells. They could become imbalanced, and even if the total voltage of the series string is lower than the allowable limit, one of the supercaps may exceed its voltage rating.

Supercaps have a higher rate of self-discharge, or leakage, than li-ion and most battery technologies. Thus, they are not ideal for long-term power storage needs. They are more useful when there is an available small charging supply to trickle-charge them to full charge. This is often done to power a real-time clock in a computer, or other small UPS applications, in which they can replace a coin-cell with a permanent solution.