Ultracapacitors Boost Battery in Power Tools

Oct 1, 2006 12:00 PM
By John Dispennette, Application Engineer, Maxwell Technologies, San Diego

For the tube-cutting application, the peak power demand is the limiting factor. While different battery chemistries provide equivalent life and capacity for primary cells, their behavior at high currents is significantly different. At 4 A, the AA alkaline cells produce 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 Li-ion rechargeable cells such as those used in laptop computers, which are rated below 2 A, are also limited in power. Recently, some of the major professional power tool manufacturers have come out with new technology Li-ion cells. These are adequate in environments where power and energy storage are needed, but are not economical in a tool that runs 5 min per day.

The tube cutter is a good example of the kind of power design challenge often faced. Alkaline cells are unable to supply the peak power, and Li-ion cells are too expensive. But can NiMH batteries provide a good compromise solution? Superior Tool found that a six-cell NiMH battery configuration was required to provide the needed peak power. The six-cell configuration was arrived at experimentally. First, a deregulated dc power supply was connected to a prototype of the tube cutter to determine the required operating voltage (approximately 7 V). Then, it was determined that six NiMH cells in series would provide that voltage under load.

However, this arrangement meant that they had to reduce their original cut time specification from 10 sec to 5 sec. In practice, this was considered acceptable. However, the battery life (determined experimentally) was marginal for achieving the 100 usable cuts-per-charge requirement. As a result, NiMH cells by themselves were deemed unsuitable.

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 can be recharged quickly or slowly as required.

In the cutting design, Superior Tool tested ultracapacitor/battery parallel arrangements 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.

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