SMALL, LIGHT-WEIGHT, BUILT-IN BATTERIES OFFER MORE ENERGY AND IMPROVED COSMESIS
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One might think that with over a hundred Li-Polymer cells on the market, one could find just the right cell for every application, but this is far from the case. Most cells are made in Asia by companies that only sell in large quantities. Dimensions are driven by the size and shape that is appropriate for 3.6V hand held devices. For these devices thin and flat are paramount, while the linear dimensions are driven by the size of the device and are usually too wide for a prosthesis. Figure 1 shows four cells that have proven usable in prosthetics. Left to right, the first cell is a mere 200 mAh, but is really small. The second, 250 mAh, is the smallest rectangle that will mount crosswise at the end of a child prosthesis. Unfortunately, it has been discontinued by the manufacturer. The third cell, 450 mAh, is slim in two dimensions so it fits well with one cell each on side of a wrist. The 750 mAh cell on the right is good for a full day’s operation of an adult transradial prosthesis. Cells must be selected for their electrical characteristics as well as for size. For instance, many of lithium ion cells have a high internal resistance limiting the current that can be drawn. Recent changes in the chemistry of some of the lithium polymer cells have opened up new high current applications. To move an elbow or shoulder with weight in the gripping device requires substantial current even when extra cells are added to increase voltage. With battery operation every component must be as efficient as possible and this includes the batteries.
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