Batteries vs. Supercapacitors? The Answer is Both. Batteries and supercaps

Replace Batteries in Power Ride-Through Applications with Robust Supercaps and 3mm × 3mm Capacitor Charger

Supercapacitors (or ultracapacitors) are finding their way into an increasing number of applications for short-term energy storage and applications that require intermittent high energy pulses. One such application is a power ride-through circuit, in which a backup energy source cuts in and powers the load if the main power supply fails for a short time. This type of application has been dominated by batteries in the past, but electric double layer capacitors (EDLCs) are fast making inroads as their price-per-farad, size and effective series resistance per capacitance (ESR/C) continue to fall.

In a power ride-through application, series-stacked capacitors must be charged and cell-voltage balanced. Supercaps are switched into the power path when needed and the power to the load is controlled by a DC/DC converter. The LTC3225 supercapacitor charger has a number of features that make it a good choice for power ride-through applications. It comes in a small, 10-lead 3mm × 3mm DFN package and features programmable charging current, automatic cell voltage balancing, low drain current on the supercapacitors and a patent pending, low noise, constant current charger.

Supercapacitors come in a variety of sizes, for example a 10F/2.7V supercap is available in a 10mm × 30mm 2-terminal radial can with an ESR of 25mΩ while a 350F/2.5V supercapacitor with an ESR of 1.6mΩ is available in a D-cell battery form factor. One advantage supercapacitors offer over batteries is their long life. A capacitor’s cycle life is quoted as greater than 500,000 cycles; batteries are specified for only a few hundred cycles. This makes the supercapacitor an ideal “set and forget” device, requiring little or no maintenance.

Two parameters of the supercapacitor that are critical to an application are cell voltage and initial leakage current. Initial leakage current is a misnomer in that the initial leakage current is really dielectric absorption current which disappears after some time. The manufacturers of supercapacitors rate their leakage current after 100 hours of applied voltage while the initial leakage current in those first 100 hours may be as much as 50 times the specified leakage current.

The voltage across the capacitor has a significant effect on its operating life. When used in series, the supercapacitors must have balanced cell voltages to prevent overcharging of one of the series capacitors. Passive cell balancing, where a resistor is placed across the capacitor, is a popular and simple technique. The disadvantage of this technique is that the capacitor discharges through the balancing resistor when the charging circuit is disabled. The rule of thumb for this scheme is to set the balancing resistor to 50 times the worst case leakage current, estimated at 2μA/Farad. Given these parameters, a 10F, 2.5V supercapacitor would require a 2.5k balancing resistor. This resistor would drain 1mA of current from the supercapacitor when the charging circuit is disabled.

An alternative is to use a non-dissipative active cell balancing circuit, such as the LTC3225, to maintain cell voltage. The LTC3225 presents less than 4μA of load to the supercapacitor when in shutdown mode and less than 1μA when input power is removed. The LTC3225 features a programmable charging current of up to 150mA, charging two series supercapacitors to either 4.8V or 5.3V while balancing the voltage on the capacitors.

Power Ride-Through Applications

To provide a constant voltage to the load, a DC/DC converter is required between the load and the supercapacitor. As the voltage across the supercapacitor decreases, the current drawn by the DC/DC converter increases to maintain constant power to the load. The DC/DC converter drops out of regulation when its input voltage reaches the minimum operating voltage (VUV).

batteries, supercapacitors, answer, supercaps

To estimate the requirements for the supercapacitor, the effective circuit resistance (RT) needs to be determined. RT is the sum of the capacitors’ ESRs and the circuit distribution resistances.

Assuming 10% of the input power is lost in the effective circuit resistance when the DC/DC converter is at the minimum operating voltage, the worst case RT is

The voltage required across the supercapacitor at the undervoltage lockout threshold of the DC/DC converter is;

The required effective capacitance can then be calculated based on the required ride-through time (TRT), and the initial voltage on the capacitor (VC(0)) and VC(UV).

The effective capacitance of a series connected bank of capacitors is the effective capacitance of a single capacitor divided by the number of capacitors while the total ESR is the sum of all the series ESRs.

The ESR of a supercapacitor decreases with higher frequency. Manufacturers usually specify the ESR at 1kHz, while some manufactures publish both the value at DC and at 1kHz. The capacitance of supercapacitors also decreases as frequency increases and is usually specified at DC. The capacitance at 1kHz is about 10% of the value at DC. When using a supercapacitor in a ride-through application where the power is being sourced for seconds to minutes, use the effective capacitance and ESR measurements at a low frequency, such as 0.3Hz.

Figure 1 shows two series connected 10F, 2.7V supercapacitors charged to 4.8V that can hold up 20W. The LTC3225 is used to charge the supercapacitors at 150mA and maintain cell balancing, while the LTC4412 provides an automatic switchover function. The LTM4616 dual output switch mode μModule DC/DC converter generates the 1.8V and 1.2V outputs.

Figure 2 shows a 12V power system that uses six 10F, 2.7V supercapacitors in series charged by three LTC3225’s set to 4.8V and a charging current of 150mA. The three LTC3225’s are powered by three floating 5V outputs generated by the LT1737 flyback controller. The output of the stack of six supercapacitors is set up in a diode OR arrangement via the LTC4355 dual ideal diode controller. The LTM4601A μModule DC/DC regulator produces 1.8V at 11A from the OR’d outputs. The LTC4355’s MON1 in this application is set for 10.8V.

Supercapacitors are meeting the needs of power ride-through applications where the time requirements are in the seconds to minutes range. Capacitors offer long life, low maintenance, light weight and environmentally friendly solutions when compared to batteries. To this end, the LTC3225 provides a compact, low noise solution to charging and cell balancing series connected supercapacitors.

Author

Jim Drew joined Analog Devices Inc. as a Senior Applications Engineer at the company’s Boston, MA Design Center in 2007. He was responsible for application support of Application Specific Power ICs. His area of interest is power conditioning applications for solar power, energy harvesting, supercapacitor chargers and active battery balancing. Jim was a consulting engineer at EMC, Hewlett Packard, Compaq and Digital Equipment Corporation responsible for power system development. He has also been an Adjunct Professor of Electrical Engineering at the University of Massachusetts Lowell where he now teaches since his retirement in 2017. Jim received his BSEE and MSEE from Lowell Technical Institute, now the University of Massachusetts Lowell.

Batteries vs. Supercapacitors? The Answer is Both.

Batteries have a weakness stemming from the way they are designed. Batteries struggle to deliver energy quickly. Delivering energy quickly, which is needed to level a load or provide the burst of current needed to turn on a motor, can waste the battery’s capacity and put stress on the battery that shortens its operating life. For this reason, batteries are said to have a low power density or a low specific power. Put another way, using a battery in applications where there is a short burst or pulse of power may be problematic.

Alternatively, supercapacitors are designed specifically to deliver energy very quickly, making them perfect complements to batteries. While batteries can provide ~10x more energy over much longer periods of time than a supercapacitor can (meaning they have a higher specific energy), supercapacitors can deliver energy ~10x quicker than a battery can (meaning they have a higher specific power). Batteries and supercapacitors, working together as a team, are the ideal energy storage system for many applications in renewables, electric vehicles, and more.

Batteries vs Supercapacitors

But before we look at some other applications, let’s compare batteries and supercapacitors:

The most important points to keep in mind are that batteries have a slower charge and discharge relative to supercapacitors and supercapacitors cannot discharge for nearly as long as batteries. One of the challenges that designers face is finding the physical space to use both batteries and supercapacitors in their product or system. This challenge ends up forcing tough engineering and design tradeoffs.

Flexible Supercapacitors in Automotive, Transportation, and Mobility Applications

Supercapacitors are getting used more and more in transportation for electric vehicle power supplies, infotainment centers, connected vehicles (sensors), regenerative braking systems, and switching power supplies. For example, there are busses that use regenerative braking storing the recycled energy in supercapacitors. Also there are trollies and trams that use supercapacitors to store enough energy to get from station to station.

Flexible supercapacitors could also be used for regenerative braking for traditional automotive systems as well as in bicycles and e-bikes. Bicycles and e-bikes also benefit from active suspensions based on flexible capacitors. In both cases, the flexible capacitors can take on unusual shapes within the structural elements of the vehicle, bicycle, or electric motorcycle to help save space and overcome the tradeoffs.

Designers and engineers don’t have to decide if they should place batteries or supercapacitors, or some kind of combination of the two, inside space dedicated for energy storage systems. In the electric vehicle example, filling this space with batteries may be the best option to achieve maximum range, but would not be the best option to achieve maximum service life or the best-in class acceleration. Filling the space with supercapacitors achieves a great service life and acceleration, but terrible range. Using fewer batteries to make room for supercapacitors, like Lamborghini did, forces end users to make tradeoffs and can limit the market for the vehicle.

Capacitech’s flexible supercapacitor modules allows designers to fill the space dedicated to energy storage systems with energy-rich batteries and connect power-rich supercapacitors within the wiring harness distributed throughout the vehicle.

The Answer is Both

There are a range of other applications where the flexible supercapacitors could add value such as robots and exoskeletons to have flexible power available for more fluid movements of limbs. There are also many wearable items such as sports injury monitoring helmets, wearable medical devices, prosthetics, fitness bands and Smart glasses that can benefit.

batteries, supercapacitors, answer, supercaps

The answer to batteries or supercapacitors, is often times both, but where is there enough room for both? There is enough room when the supercapacitors are flexible and integrated into the product or system. The answer is to fill the space dedicated to energy storage systems with batteries and complement those batteries with flexible supercapacitors within the existing power cords, distributed throughout the rest of the infrastructure.

Supercapacitors – A Viable Alternative to Lithium-Ion Battery Technology?

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Introduction

Supercapacitors, also called Ultracapacitors, double-layer capacitors, or electrochemical capacitors, are a type of energy storage system attracting many experts in recent years. In simple terms, they can be imagined as a cross between an ordinary capacitor and a battery; still, they are different from both.

Before we get into the nuances of whether Supercapacitors can make a difference on their own in terms of how energy can be stored in the future, it’s worth knowing more about how they work and how they are different from a lithium-ion battery.

Supercapacitors and batteries, they are both storage methods. If we look at lithium-ion batteries, they rely entirely on chemical reactions. They consist of a positive and negative side, technically called an anode and a cathode. These two sides are submerged in a liquid electrolyte and separated by a micro-perforated separator, allowing only ions to pass through. During batteries’ charging and discharging, the ions tend to flow back-and-forth between the anode and cathode. While this ion transfer process occurs, the battery gets heated up, expands, and then contracts. These reactions gradually degrade a battery, resulting in a reduced lifespan of batteries. However, a significant advantage of battery technology is that it has a very high specific energy or energy density to store energy for its use later. But Supercapacitors are different; they don’t rely on a chemical play to function. Instead, they store potential energy electrostatically within them. Supercapacitors use dielectric or insulator between their plates to separate the collection of positive (ve) and negative (-ve) charges building on each side’s plates. It is this separation that allows the device to store energy and quickly release it. It basically captures static electricity for future use. The most significant advantage of this is that a 3V capacitor now will still be a 3V capacitor in 15-20 years. In contrast, on the other hand, a battery may lose voltage capacity over time and repeated usage.

Also, unlike a battery, they have a higher power throughput, which implies it can charge and discharge in a fraction of the time. Still, they have a very low specific energy as compared to batteries. Supercapacitors are best suited for very small bursts of power. Ultracapacitor and battery company Maxwell for over 200 million. It wasn’t clear if it was for the company’s main business, Supercapacitors, or its latest battery technology IP, like a new dry electrode technology for battery cells.

A fantastic example can be seen in Switzerland on how effective Supercapacitors are. A fleet of buses is exposed to charging stations at various stops along their daily commutation route. Just 15 seconds can top the energy-charge off, and only a few minutes would suffice for a full charge. With frequent top-offs, it makes up for the lack of energy density and storage. And because Supercapacitors draw a lower current for a few minutes at a time, this puts less stress on the grid.

Why Supercapacitors are gaining a lot of interest, and how does it compare with Lithium-Ion Batteries, for example?

The answer to this question can depend a lot on the applications they can be used for. There are indeed a few clear advantages and disadvantages of each technology. As mentioned earlier, batteries have a much higher energy density than Supercapacitors.The Difference between Battery vs Supercapacitor

  • How Supercapacitors work?
  • Supercapacitor vs Battery – Comparison and Case Study
  • Can Supercapacitors replace batteries?
  • Supercapacitors versus batteries- will batteries last the test of time?
  • Will capacitors replace batteries?
  • New materials make Supercapacitors better than batteries
  • Discover how the Supercapacitor can enhance the battery
  • Could Ultracapacitors Replace Batteries in Future Electric Vehicles?
  • Set up a discussion with our consultants on topics related to Battery technology and how FutureBridge can assist you.

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    Ultracapacitor Overview

    A rapidly emerging and increasingly applied technology, ultracapacitors are capable of storing and discharging energy very quickly and effectively. Due to their many benefits, ultracapacitors are currently being utilized in thousands of different applications, and considered in an equally diverse range of future applications. Ultracapacitors complement a primary energy source which cannot repeatedly provide quick bursts of power, such as an internal combustion engine, fuel cell or battery. The future horizon looks brilliant for ultracapacitors, which already rank as a powerful alternative energy resource.

    Where Ultracapacitors Work

    Harvest power from regenerative braking systems and release power to help hybrid buses accelerate.

    Reliably crank semi-trucks in cold weather or when batteries are drained from repetitive starting or in-cab electric loads.

    Provide cranking power and voltage stabilization in start/stop systems, backup and peak power for key automotive applications – and serve as energy storage in regenerative braking systems.

    Capture energy from regenerative braking systems and release power to assist in train acceleration, and used for vehicle power where overhead wiring systems are not available.

    Open aircraft doors in the event of power failures.

    Used in blade pitch systems and to help increase reliability and stability to the energy grid.

    batteries, supercapacitors, answer, supercaps

    Capture energy and provide burst power to assist in lifting operations.

    Provide energy to data centers between power failures and initiation of backup power systems, such as diesel generators or fuel cells.

    Provide energy storage for firming the output of renewable installations and increasing grid stability.

    How Ultracapacitors Work

    PRIMARY ENERGY SOURCES like internal combustion engines, fuel cells and batteries work well as a continuous source of low power. However, they cannot efficiently handle peak power demands or recapture energy in today’s applications because they discharge and recharge slowly.

    ULTRACAPACITORS deliver quick bursts of energy during peak power demands, then quickly store energy and capture excess power that is otherwise lost. They efficiently complement a primary energy source in today’s applications because they discharge and recharge quickly.

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