In today’s electronics world there is a constant cry for more power. This is a direct result of several factors. Society demands more information and functions out of electronic devices, and engineers and designers are forced to respond to these requests in order to stay ahead. This can be seen by more processor speed on PCs, i-Pods with the capability of storing thousand of files, household appliances with full electronic controls, and many other day-to-day examples that are taken for granted. Compare this with 20 years ago when PCs were not a common household item, cell phones were not in use, and appliances at best only had a digital clock. In most cases, energy storage needs were limited to primary alkaline batteries. There was little need to recharge and no need for high-power energy devices.

Today, the world has changed, and almost every device has an energy storage need. Because of the complexities mentioned above, these energy needs are not easily fulfilled by common alkaline batteries. That has led to a variety of energy storage media. These include, but not limited to, nickel-cadmium batteries (Ni-Cd), nickel-metal-hydride batteries (Ni-MH), lithium-ion batteries (Li-ion) and ultracapacitors. The ultracapacitor, a fairly new device in the energy field, is increasingly being specified into many new applications.

Defining ultracapacitors

Enlarge this picture Fig. 1. Illustration shows how an electronic double-layer capacitor (EDLC) is formed at each electrode. Ions in the electrolyte remain in charge balance, but when an external electric field is impressed, they will diffuse to the oppositely charged electrode. The electrodes are highly porous (3,000 m2/g) and act as very efficient electron and ion accumulators.

In a broad sense, electrochemical capacitors comprise a wide class of electrical energy storage components in which the symmetrical carbon-carbon cell is the most notable and is now commonly referred to as an ultracapacitor. The prefix “ultra” comes from the fact that, unlike conventional electrostatic field storage components, the ultracapacitor takes the two main contributors to capacity, surface area A, and charge separation distance d, to the extreme. Fig. 1 illustrates how an electronic double-layer capacitor (EDLC) is formed at each electrode. Ions in the electrolyte remain in charge balance, but when an external electric field is impressed, they will diffuse to the oppositely charged electrode. The electrodes are highly porous (3,000 m2/g) and act as efficient electron and ion accumulators.

Ultracapacitors can be used by themselves as a primary energy source, or in combination with batteries as a secondary energy source. The main attributes of ultracapacitors are high power capability and long life. Desirable characteristics for use in high power applications include extremely long cycle life, wide operating temperature ranges, low weight, flexible packaging, zero maintenance, and environmental friendliness.

Ultracapacitors often become the component of choice for engineers and designers with applications requiring short-term or peak (burst) power. Ultracapacitors are ideally suited as stand-alone solutions for short-term power requirements ranging from a few seconds to a few minutes. Applications requiring many minutes to hours of backup reserve energy require an additional supplemental energy source.

Ultracapacitors have been around for decades and first appeared as a low-power, low-energy, long-life backup in consumer electronics such as VCRs and alarm clocks. During the last 10 years there have been substantial advancements in this technology, including materials and construction, as well as manufacturing processes, that have made ultracapacitors an acceptable solution in many mission critical applications.

Ultracapacitors range in capacitance from several farads to several thousands of farads. With this spectrum of available capacitance, designers have the ability to customize energy storage to their exact needs, thus reducing size and overall cost of the system. A key advantage of ultracapacitors compared to batteries lies in their safety and ease of use. Batteries, especially Li-ion and Ni-MH, require extensive monitoring and charging circuitry, whereas ultracapacitors are easily maintained, and require little or no care when used. State of health is easily monitored during any cycle, and the life expectancy is not a surprise to the end user.

Benefits

Enlarge this picture Fig. 2. Run time test of a device with alkaline batteries with and without the use of ultracapacitors.

In addition to features and speed, the demand for power out of an energy storage device is also a priority. This need for power also comes with the requirement for reliability and longevity for the device. There have been many advances made in the battery industry in last few years. Powerful Li-ion batteries have found their way into some portable applications and Ni-Cd and NiMH batteries are improving their power and life capabilities. But the truth still holds that if one cycles batteries with high charge/discharge rate, their life is shortened.

This leaves most designers with two choices: to over-size their battery design or try to augment it with a power source that removes the high peaks from the battery. The first choice is fairly straightforward. But in most cases it is undesirable because of added cost and size to the device.

The need for an augmenting technology is easily fulfilled with the choice of ultracapacitors. In most cases, if one looks at the overall energy requirement of an application, the battery can be sized accurately for it. But, what limits the life of the battery is not the energy, but rather the internal impedance that prevents it from delivering the energy as needed.

In Fig. 2, the run time of a device with alkaline batteries is shown with and without the use of ultracapacitors. There is significant increase in the run time as a result of keeping the voltage drop low. Ultracapacitors do not create energy, but allow for a more efficient way of extracting the energy that is already available in a battery solution, allowing the maximum benefit.

Example

Superior Tool Co. UltraCut™ cordless tubing cutter with view of internal components.

Consider an application example of a handheld battery-powered power tool, and in particular an example of a tube-cutting tool designed by Superior Tool Co. of Cleveland. This product puts demands on the battery for high peak loads experienced during the onset of cutting, which is typical of many portable devices where the peak load is much higher than the average power demand. For the tube-cutter, it was determined that battery life was important for successful product acceptance, and that power design was a significant part of the product-design process.

Having determined that battery life is important, a designer needs to evaluate various battery chemistries to determine the best solution. For some designs, traditional alkaline batteries will be acceptable, but if they do not provide the right life and performance, then higher-cost batteries must be considered. For the tube-cutting tool, trials with the ultracapacitor design enabled a significant improvement in battery life with all chemistries tested.

Having decided to incorporate ultracapacitors, the next step is to evaluate the requirements for the battery and the ultracapacitor cells. For the tube-cutting example, Superior Tool planned to produce tools capable of using either primary alkaline cells or rechargeable NiMH cells. The initial target specification for the cutting tool required a 1/2-in. copper pipe to be cut in less than 10 seconds with the ability to make at least 100 cuts, utilizing either primary alkaline cells or rechargeable NiMH cells as the energy source. Lab tests determined that the work required to cut a piece of 1/2-in. copper tubing in 10 seconds to be on the order of 50 Watt-sec.

For the tube-cutter, initial calculations determined that four series-connected alkaline AA cells could provide the necessary energy. A typical alkaline AA is rated at 2,500 mAH. Assuming 33 percent efficiency, four cells can produce roughly 54,000 watt-sec, yielding 360 cuts.

So, the energy storage for the specified 100 cuts was not a problem. However, the internal impedance of the alkaline cells and the high initial current load demanded by the cutting tool was not considered in the initial calculations. With the typical internal impedance of an AA alkaline cell being at 200 mOhms, and a peak load current draw between 4 A and 5 A, it was quickly determined the initial voltage drop rendered the tool inoperable. It was determined as a rule of thumb that peak currents higher than 0.5 A were too demanding and caused undesirable affects on the battery sizes required for this application.

This shows how vital it is to consider both the overall energy storage requirements on batteries, and the peak power characteristics of the application. For many applications, the battery has, in fact, traditionally been sized for the peak-power demands, meaning that a larger and heavier battery has been used than is required for the overall energy storage demands. This is where ultracapacitors, connected in parallel, can help manage the peak power demands.

For the tube-cutting application, the peak power demand was the limiting factor. While different battery chemistries provide equivalent life and capacity for primary cells, their behavior at high currents was significantly different. At 4 A, the AA alkaline cells produced a small fraction of their rating. In rechargeable NiMH cells, both capacities per charge and life cycles decline as current increases. Lithium cells tend to be internally protected against high power demands to avoid excess cell heating. Conventional lithium-ion rechargeable cells (for example the cells used in laptop computers are rated below 2 A) are also limited in power.

The tube-cutter is a good example of the kind of power design challenge engineers face, where alkaline cells are unable to supply the peak power, and lithium cells are too expensive. In such cases, one must next consider another option. NiMH batteries can provide a good compromise solution, offering improved performance over alkaline cells, but at a lower cost than lithium.

Superior Tool found that a 6-cell NiMH battery configuration was required, but this still meant that they had to reduce their original cut-time specification from 10 seconds to 5 seconds. In practice, this was considered acceptable. However, the battery life was too marginal for the 100 usable cuts per charge requirement. Therefore, NiMH cells were considered unsuitable.

Ultracapacitor solution

Enlarge this picture Fig. 3. Battery/ultracapacitor schematic for cutting tool application.

In the past, design engineers would have been scratching their heads at this point in the design cycle, and would have been required to compromise on functionality or cost in the final product. This is where ultracapacitors can help provide an acceptable solution.

By connecting one or more ultracapacitors in parallel with the batteries, the ultracapacitors can provide the peak power demands of the application, and can be recharged from the battery when the power demands are lower. The low impedance of the ultracapacitor means that it can provide high power from a relatively small device, and it can be recharged quickly or slowly, as required.

Superior Tool tested an ultracapacitor/battery parallel arrangement with alkaline cells. Initially, alkaline cells tested alone produced under 10 cuts, and applying ultracapacitors in parallel meant the design goal of 100 cuts with 3 sec. to 5 sec. cut times could be met. This was a phenomenal change in performance brought about by allowing the ultracapacitors to shield the battery from high currents.

Similarly the NiMH solution was tested, which produced less dramatic, although impressive improvements. The number of cuts between recharges was increased by over 30 percent. At a minimum, this would also translate directly to a 30 percent increase in life, as the life of a rechargeable chemistry is related to the number of recharging cycles. It is anticipated that the life may be at least doubled due to the reduced peak current demanded by the battery. With these results, it is clear that the addition of ultracapacitors to the design enable the application specifications to be met.

The final tube-cutter design (patent pending) includes three 10 F, 2.5 V ultracapacitors in series. This allows six alkaline cells rated at 7.2 V and the three ultracapacitors rated up to 7.5 V to be placed in parallel.

Conclusions

Designing ultracapacitors into portable devices does not add additional complexity to the overall product design. The best methodology is determined by the application needs, which includes run-time requirements, frequency of charging, operating voltage range of the electronic components, and peak-load requirements. Identifying these requirements will guide the proper selection of the described arrangements for batteries and ultracapacitors, charging scenarios, and output conditioning.

For more information email: info@Maxwell.com