RE: How Do Supercapacitors or Batteries Fare in IoT Devices?

michaelkellett — Wed, 01 Oct 2025 10:10:12 GMT

This article contains so many errors and misleading statements that it must be challenged.

I'll pick just three things out of many:

"Due to their high energy density, long cycle life, and rapid charge"

The energy storage density of supercaps is very low compared with Lithium Ion batteries: for example the Wurth 10F 2.7V cap is 10mm diameter and 25mm long = 2cc. It stores 36.4J = 18.2J/cc

An AAA, 500mAh size Lithium Ion battery is 3.52cc and stores 6600J = 1892J/cc

The Lithium Ion battery has 100x the energy storage by volume as the supercap.

"it is important to understand that supercapacitors serve as high-energy storage units with low equivalent series resistance (ESR)"

This is deeply misleading - big supercaps may well have very low ESR and be able to supply huge currents - but they are capacitors and this means that the output voltage drops as they supply energy and when discharged to 25% of the total stored energy the output voltage will be halved.

Small supercaps (such as you find in small IOT devices) may have quite a high series resistance, for example the Eaton KR-5R5V105-R ( the first 1F supercap that Farnell's search engine found) has 30R series resistance.

"Since supercapacitors are like a short circuit at low voltages, charging circuits must handle high inrush currents effectively while maintaining efficiency."

Well, no - supercapacitors are like capacitors - no more a short ciruit at low voltages than at full voltage. The ESR only varies by small amounts as the voltage across the supercap changes.

But you can't just connect them to a voltage source to charge them - in low power designs a series resistor might do to limit the current but the time to charge to full voltage will be much longer than it needs to be, and energy is wasted in the resistor.

In high power designs supercaps need current mode charging using a switching design, and in most cases will need a switching regulator to drive the load as well.

I could go on but three points is enough.

Supercaps are available to cover a wide range of applications and if you plan to use one then read the data sheet.

And read some application notes for management circuits.

Beware of generalisations about cycle time and max temperature - big supercaps have quite limited cycle lives and they all seem to have quite short full voltage lifetimes. This is important because voltage ratings of supercaps are low (2.7V or 3V are common) so you are far more likely to run a supercap at very close to its rated voltage than you would run a standard cap.

When you put several supercaps in series a whole new can of worms opens up...........