Supercapacitors are not only for high power
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People who have at least some interest in electronics have surely heard about supercapacitors or ultracapacitors. The biggest selling-point of them is that they have enormous amount of specific power compared to batteries. Supercapacitors have specific power of about 10 - 20 kW/kg, while, for example Li-Ion batteries have specific power of about 0.3 to 1.5 kW/kg.

One of the popular applications of supercapacitors is in hybrid vehicles, specifically in regenerative breaking systems. The ability of the supercapacitor to charge very fast is perfect for this. If we take, for example, Maxwell K2 3000F 3.0V (3.5 Wh) supercapacitor, it can be fully charged in 0.84 seconds if we have 16 kW DC power source. So, it is clear that supercapacitor are the very well fit for the high-power applications.

 

Supercapacitors compared to batteries

As usual, there is a drawback, which is, ridiculously low specific energy of the supercapacitors. The specific energy of supercapacitors is about 5 - 10 Wh/kg. Compared to Li-Ion batteries which have 100 - 300 Wh/kg, supercapacitors are like some ancient technology from the middle age. Because of that, we don't see phones or laptops being power by supercapacitors. Sure, we could have a supercapacitor bank with our laptop that weights 5 kg and charges in 0.84 seconds, but it doesn't sound very convenient. 

The fast charging time is not the only advantage over the batteries. The cycle life of supercapacitors is usually specified as 500 000 to 1 000 000 cycles or 10 years before their capacity decreases by 20%, compared to 500 - 2000 cycles of the Li-ion batteries. So, you could carry 5 kg of supercapacitors and replace them only each 10 years. Sounds almost fantastic, if not those 5 kg...

Another great thing about supercapacitors is their excellent performance in cold environment, below 0 ºC. It appears that you can still charge them, even when it is cold, without any significant negative influence on the performance. Something, that batteries are not capable of and furthermore, lithium batteries can catch fire if you do so. In fact, if supercapacitors are used only at fraction of their rated power, negative temperatures will increase their performance. The self-discharge current will decrease and the life-time will increase.

The significant practical difference between batteries and supercapacitors is that voltage of supercapacitor decreases from rated voltage (typically 2.5V or 2.7V) to 0V while discharged from full to empty. You can see from the graph, that if, for example, the maximum rated voltage of supercapacitor is 2.7V, then at voltage of 1.5V there will be 30% of energy left. The lower the voltage, the less useful it becomes. However, there are step-up converter solutions that can operate below 1V, for example, TPS61200 from Texas Instruments. 

State of Charge of the supercapacitor compared to voltage.

The wide operating voltage of the supercapacitor may seem as a big disadvantage, because a voltage converter is needed. However, in most low power application that use, for example, Li-ion battery a voltage converter is still required to get stable 5V or 3.3V. So, there is basically no disadvantage compared to batteries, it is just that a different kind of voltage converter is required.

So, supercapacitors are good, but there is just one big BUT, which is, that they take a lot more space than batteries. However, if we can't use them in the portable electronics, that is nothing disastrous. Lead acid batteries are also heavy and chunky, but we still use them in cars and UPS systems, despite the fact that they have very short cycle life. In the applications where size and weight doesn't matter and stored energy is not measured in killoWatt-hours, supercapacitors are a good replacement of rechargeable batteries. 

 

Supercapacitors Batteries
++ Wide temperature range ++ -- Narrow temperature range --
 ++ Long cycle life ++ -- Short cycle life --
-- Low energy density -- ++ High energy density ++
-- High price --  ++ Low price ++

 

Low power applications

When I say low-power applications, I mean applications where the energy stored is measured from couple of hundred of milliWatt-hours to couple of Watt-hours. For example, Arduino typically consumes from 200 to 400 mW depending on the configuration. With 3000F 3.0V supercapacitor it will be able to run for about 12 hours. Add another supercapacitor in parallel and it will be 24 hours. It is not bad, isn't it? If you want to put Arduino or some other device where the power is not available, you can, for example, use solar cell together with supercapacitors. The setup will be maintenance-free and will work even in winter.

Supercapacitors are underestimated in the low power applications, because the market is focused on their ability to provide a lot of power. There are not many supercapacitor products oriented for low-power applications on the market. This is the reason why I developed protected supercapacitor module Beast Supercapacitor 175, because I believe that there is a big potential. The module is perfectly fit for low-power applications, as it provides similar protection to the one found in Li-Ion batteries with some additional features, such as reverse voltage protection and over-voltage indication.

In my next articles I am going to show examples of possible low-power applications, where supercapacitors can be used. I am also going to show how important it is to have a protection circuit for the supercapacitors with some experiments. :)