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Lead-Acid Storage Batteries DOE-HDBK-1084-95 BATTERY COMPONENTS AND OPERATION Rev. 0 Page 5 Batteries BATTERY COMPONENTS AND OPERATION Cells vs. Batteries A battery is a device that converts the chemical energy contained in its active materials into electrical energy by means of an electrochemical reaction.  While the term "battery" is often used, the basic electrochemical element being referred to is the cell.   A battery consists of two or more cells electrically connected in series to form a unit.  In common usage, the terms "battery" and "cell" are used interchangeably. Primary and Secondary Cells and Batteries Batteries are either primary or secondary.  Primary batteries can be used only once because the chemical reactions that supply the electrical current are irreversible.  Secondary (or storage) batteries can be used, charged, and reused.  In these batteries, the chemical reactions that supply electrical current are readily reversed so that the battery is charged. Primary batteries are common since they are cheap and easy to use.  Familiar primary battery uses are in flashlights, watches, toys, and radios.  The most common use for secondary (storage) batteries is for starting, lighting, and ignition (SLI) in automobiles and engine-generator sets.  Other applications include uninterruptible power supplies (UPSs) for emergency and backup power, electric vehicles (traction), telecommunications, and portable tools.  The remainder of this Primer will be concerned only with storage batteries except where general operating characteristics of batteries are discussed. Battery Components A cell has five major components as shown in Figure 1. The negative electrode  supplies electrons to the external circuit (or load) during discharge. In a fully charged lead-acid storage battery the negative electrode is composed of sponge lead (Pb).  The positive electrode accepts electrons from the load during discharge.  In a fully charged lead-acid battery the positive electrode is composed of lead dioxide (PbO  ).  It 2 should be noted that the electrodes in a battery must be of dissimilar materials or the cell will not be able to develop an electrical potential and thus conduct electrical current.  The electrolyte completes the internal circuit in the battery by supplying ions to the positive and negative electrodes.  Dilute sulfuric acid (H  SO  ) is the electrolyte in lead-acid batteries.  In 2 4 a fully charged lead-acid battery, the electrolyte is approximately 25% sulfuric acid and 75% water. The separator is used to electrically isolate the positive and negative electrodes.  If the electrodes are allowed to come in contact, the cell will short-circuit and become useless because both electrodes would be at the same potential.  The type of separator used varies


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