A FET is a field effect transistor. Smart batteries must have a series FET (field effect transistor) switchs to open and protect the battery’s cells.
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A FET is a transistor that uses an electric field to control the conductivity of a particular 'channel' in a semiconductor material. FETs at times are used as voltage-controlled resistors. As such field effect transistors are chosen based upon their designed ability to dissipate on demand power.