Can a Supercapacitor Replace Your Battery for Backup Power?

Can a Supercapacitor Replace Your Battery for Backup Power?

From smart meters to machines to vehicles, supercapacitors are popping up in a variety of applications. Will they replace batteries? Perhaps not in every application at the moment, but supercapacitors do provide an advantage over batteries in certain designs.

In his recent webinar, "Simplifying Supercapacitor Backup Applications Using MAX38888," V.A. Krishnan, product definition director in Maxim's Core Products Group, provided an overview of supercapacitors and highlighted when they would be advantageous to use.

So, what is a supercapacitor? A high-capacity capacitor, a supercapacitor bridges the gap between electrolytic capacitors and rechargeable batteries. There's no dielectric. As Krishnan explained, its structure is such that a solid electrode and a liquid electrolyte form an electrical double layer with a separator to prevent a short circuit. You get twice the junction as you would in a typical capacitor. The capacitor action is formed by the physical movement of ions. Unlike in a battery, said Krishnan, there's no chemical reaction, so supercapacitors have an excellent charge and discharge lifecycle. Supercapacitors also have an extremely high capacitance due to fine pores in the electrode that increase the active surface area of the electrode.

Krishnan then outlined the advantages and disadvantages of supercapacitors for backup applications. Advantages include:

  • Longer backup time than conventional batteries due to high capacitance
  • Potentially longer life than batteries due to excellent charge/discharge lifecycle
  • Easier shipping and handling process than batteries (particularly compared to lithium-ion types)
  • Support for distributed backup in complex wired applications

Some disadvantages to consider include:

  • Expensive backup beyond 12V, as the maximum rated voltage is limited to 3V per supercapacitor
  • Higher cost than batteries
  • Equivalent series resistance (ESR) at cold temperature is 3-4x higher than ESR at room temperature
  • Current technology is not ubiquitous for elevated temperature operation (lifetime is reduced at elevated temperature)

Based on all of this, when is a supercapacitor a better choice than a battery for backup applications? According to Krishnan, the answer lies in your answers to these questions:

  • Is it OK if the power source disappears for >100ms? Do you have a limited power source, like USB?
  • Are the main system voltages <12V?
  • Are backup times <120s?
  • Is your ambient temperature <105°C?

If your answer to any of these questions is yes, then a supercapacitor is worth considering over a conventional battery. Supercapacitor costs will continue to decrease over time. With a supercapacitor, "you can take a complex wired system and back up nodes you need to back up, instead of having a giant battery back up a rail and running an entire system on it," Krishnan said.

Applications that Benefit from Supercapacitors

Power requirements during backup have evolved; as systems have grown smarter, they're requiring more functions. This is driving up the amount of capacitance needed. A bus or taxi surveillance device (like a black box), for instance, might require data storage on an SD card during backup. A more complex application, like a fire safety actuator, might need data storage as well as communications and other core functions when the main power source goes out.

The load current profile during backup isn't constant. Periods of peak power, when all systems are running, determines the power transfer size. The total load profile and energy delivered during backup determines the size of the supercapacitor. And the size of the supercapacitor and the frequency of backup determine the size of the charging circuit, explained Krishnan.

During his webinar, Krishnan highlighted a few applications where supercapacitors would be valuable. A portable barcode scanner provides a good example. Typically, these scanners are powered by a lithium-ion battery and are in their charging cradles most of the time. Krishnan noted that a battery isn't really needed here, as a supercapacitor can provide the necessary power. "The biggest benefit of doing this is that your shipping and handling costs go down significantly. Batteries…can be an added system cost. A supercapacitor is a lot more forgiving—you don't need to trickle charge and so on," he said.

Figure 1. Portable barcode scanners are an example of an application where a supercapacitor can replace a battery for backup power.

To simplify the development of supercapacitor backup applications, a reversible buck/boost regulator like the MAX38888 can help. The MAX38888 backs up from 0.8V to 4.5V capacitor voltage range. No power down is needed while swapping out the battery, and backup power operates in buck and boost modes. Figure 2 shows a bus/taxi surveillance application without and then with the MAX38888. As you can see, using the MAX38888 eliminates one buck converter as well as a linear charger.

Existing Bus/Taxi Surveillance Application

Bus/Taxi Surveillance Application with MAX38888

Figure 2. Since the MAX38888 has 2% regulation accuracy and is a bidirectional system, a buck converter and linear charging aren't required in this bus/taxi surveillance system when rearchitected with the MAX38888 reversible buck/boost regulator.

"The Maxim solution offers the simplest and most optimized solution for backup applications," said Krishnan.

Watch Krishnan's on-demand webinar, "Simplifying Supercapacitor Backup Applications Using MAX38888," to learn more about the advantages of supercapacitors for certain backup applications.