Ultracapacitors Boost Battery in Power Tools

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

In portable applications where battery life is important, designers may evaluate various battery chemistries to determine the best energy source. For some designs, traditional alkaline batteries will provide acceptable performance and battery life. In other cases, higher performance and thus higher-cost batteries must be considered. However, in designs where alkaline batteries are insufficient, there is a lower cost alternative to high-performance batteries. That alternative is to parallel ultracapacitors with the batteries. This option is particularly useful in portable power tools where peak current demands are high.

Consider the example of a tube-cutting tool designed by Superior Tool Co. (Cleveland, Ohio). This product puts demands on the battery for high peak loads experienced during the onset of cutting. This loading is typical of many portable devices where the peak load is much higher than the average power demand.

For any portable device, the first step will be to specify the required power source. In 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 necessitated a ½-in. copper pipe to be cut in less than 10 sec, with the ability to make at least 100 cuts using either primary alkaline cells or rechargeable NiMH cells as the energy source. Lab testing determined the work required to cut a piece of ½-in. copper tubing in 10 sec to be on the order of 50 Wsec.

Once energy figures are determined, the next step is to work out the kind of batteries that could be suitable. For the tool cutter, initial calculations determined four series-connected alkaline AA cells could provide the necessary energy. A typical alkaline AA battery is rated at 2500 mAH. Assuming 33% efficiency, four cells can produce roughly 54,000 Wsec yielding 360 cuts. So, the energy storage for the specified 100 cuts is not a problem.

However, the internal impedance of the alkaline cells and the high initial current load demanded by the cutting tool were not considered in these calculations. With the typical internal impedance of a AA alkaline battery at 200 mΩ and a peak load current draw between 4 A and 5 A, it was quickly determined the initial voltage drop rendered the tool inoperable. In the tool-cutting application, it was determined — as a rule of thumb — that peak voltage sags higher than 0.5 V were too demanding and caused undesirable effects on the battery sizes required for this application.

This example shows how it is vital to consider both the overall energy-storage requirements on the batteries and the peak power characteristics of the application. For many applications, the battery has 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.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.