Ultracapacitor-Powered Cordless Drill

Whereas charging of batteries usually take hours, ultracapacitors can be charged in seconds.

The figure depicts a portable, handheld power drill with its attached powersupply unit, in which ultracapacitors, rather than batteries, are used to store energy. This ultracapacitor-powered drill is a product of continuing efforts to develop the technological discipline known as hybrid power management (HPM), which is oriented toward integration of diverse electric energy-generating, energy- storing, and energy-consuming devices in optimal configurations.

Instances of HPM were reported in several prior NASA Tech Briefs articles, though not explicitly labeled as HPM except in the most recent such article, which was “Hybrid Power Management” (lew-17520-1), Vol. 29, No. 12 (December 2005), page 35. To recapitulate from that article: The use of ultracapacitors as energy- storage devices lies at the heart of HPM. An ultracapacitor is an electrochemical energy-storage device, but unlike in a conventional rechargeable electrochemical cell or battery, chemical reactions do not take place during operation. Instead, energy is stored electrostatically at an electrode/ electrolyte interface. The capacitance per unit volume of an ultracapacitor is much greater than that of a conventional capacitor because its electrodes have much greater surface area per unit volume and the separation between the electrodes is much smaller.

HPM offers many advantages over the conventional power-management approach in which batteries are used to store energy. To place the present development in context, it is necessary to reiterate these advantages from the cited previous article:

• Power-control circuits for ultracapacitors can be simpler than those for batteries for two reasons: (1) Because of the absence of chemical reactions, charge and discharge currents can be greater than those in batteries, limited only by the electrical resistances of conductors; and (2) Whereas the charge level of a battery depends on voltage, temperature, age, and load condition, the charge level of an ultracapacitor, like that of a conventional capacitor, depends only on voltage.
• Whereas a typical battery can be charged and discharged about 300 times, an ultracapacitor can be charged and discharged more than a million times. The longer lifetimes of ultracapacitors contribute to reliability.
• The longer lifetimes of ultracapacitors greatly reduce life-of-system costs, including the indirect costs of maintenance and downtime.
• The longer lifetimes of ultracapacitors reduce adverse environmental effects, inasmuch as it will probably never be necessary to replace and dispose of ultracapacitors in most applications, whereas batteries must be replaced frequently.
• Disposal problems and the associated contributions to life-of-system costs can be reduced because the chemical constituents of ultracapacitors are less toxic and less environmentally harmful than are those of batteries. Indeed, ultracapacitors are somewhat recyclable.
• Excellent low-temperature performance makes ultracapacitors suitable for storing energy in applications at temperatures too low for batteries.
• The consistent performance of ultracapacitors over time enables reliable operation not possible with batteries.
• Unlike batteries, ultracapacitors can be safely left completely discharged for indefinitely long times.
• Whereas the charge-discharge efficiency in conventional power management using rechargeable batteries is typically about 50 percent, the charge-discharge efficiency in HPM typically exceeds 90 percent.

For the cordless drill, a dedicated charger is used to fully realize the advantages of the ultracapacitors as energystorage devices. Because of the non-critical nature of charging and discharging of ultracapacitors, this charger is less complex and less costly than would be a battery charger for the same power drill. More spectacularly, taking advantage of the unique charging characteristics of ultracapacitors, this charger can make the ultracapacitor-powered cordless drill ready for operation in seconds, in contradistinction to the several hours needed to recharge batteries.

This work was done by Dennis J. Eichenberg of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category. Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18116-1.