Batteries and supercapacitors are often compared in terms of their energy and power. Batteries have a higher density (meaning they are able to store more energy per unit mass), but supercapacitors have a higher power density (meaning they can release energy more quickly). This makes supercapacitors the best for storing and releasing large amount of power more quickly, but batteries are still the masters for storing large amounts of energy over long periods of time.
Supercapacitors have much higher capacitance values compared to the other capacitors (but lower voltage limits), so they are basically the bridge between the capacitors and the batteries. They can store a lot more energy per unit mass compared to the capacitors. Due to the fact that they work electrostatically, they can be charged and discharged any number of times. Since they have low internal resistance compared to batteries, they work with an efficiency around 98%.
The best suited applications of supercapacitors are back-up devices for the power shut-down of microcomputers and RAMs, smart meters, POE network devices, alarm systems, heater pumps, etc. Depending on the backup current for the power supply, the supercapacitors have different back up times. The picture below shows main application.
The parameters below need to be defined before choosing the capacitor required.
The customer will need a supercapacitor which will be able to withstand 150 hours back up time under the conditions below:
The basic equation for the requested capacitance is given by:
With all other parameters calculated, it looks like the customer will need a supercapacitor with capacitance around 0.1F.
Kemet offers the supercapacitor series below:
As our FC series are the only series with SMD mounting, we will have to choose this series.
As per our catalog, the maximum operating voltage for this series is 5.5VDC, same as maximum operating voltage.
As the FC series is up to 70°C, the additional cooling will have to be applied to the system.
Most logically we would choose FC0H104ZFTBR24 that has discharge capacitor of 0.1F.
There are additional parameters that need to be considered when choosing the right supercapacitor.
The voltage drop of the supercapacitor is determined by the DC resistance and the back-up current. The DC resistance values of each part number are given in our datasheets.
An approximate voltage drop can be calculated from equation below:
Where 𝑹𝑫𝑪 is the DC resistance of Super Capacitor [Ω] 𝑰𝒃𝒂𝒄𝒌𝒖𝒑 is the back-up current [𝑨]
When the back-up current is 1mA and below, there is no potential voltage drop, means that we can neglect the voltage drop in this case as the back-up current is only 540nA.
The operating temperature should be the factor that mostly affects the life of the supercapacitors. As shown in the graph below (graphs available for all parts), the leakage current rises significantly with the operating temperature.
As the leakage current is additional current drain, you will need to take the sum of the back-up current and the leakage current when calculating the back-up time.
As the cooling is possible in the application, we will consider that the operating temperature will be 70C. We can see that the leakage current in that case is 4uA.
You can then calculate how long the energy will last using the calculation below:
When taking into consideration the leakage current, we can now see that instead of the requested 150hours, the back-up time will be significantly reduced to only 18 hours. As that, a capacitor with higher capacitance value (almost 10 times higher) will need to be chosen.
By choosing FC0H105ZFTBR44, whose capacitance discharge value is 1F, we can recalculate the back-up time:
The back-up time in this case is 183 hours which is higher that the requested 150 hours. Even by calculating 15% margin on the capacitance side, we get to almost 155 hours back-up time.
We should also not forget the “Life Estimation” of the supercapacitors. The life of a supercapacitor is defined as the point at which capacitance is reduced to 70% of the initial value, as shown on the graph below:
The requested needed capacitance of the supercapacitor must be calculated with the equation below, taking into consideration the Voltage drop, Leakage current and 15% capacitance tolerance as well.
