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<B><P>Lithium Polymer Cells</P>
</B><P>Engineering Notes</P>
<P>Cells are selected on the basis of <B>charge capacity</B> and <B>current capacity</B>. Charge capacity tells you how much electricity the cell can energize (in other words, the size of the cell, or how long it will last) and is specified in milliamp-hours (mAh).</P>
<P>The endurance of a cell equals capacity divided by discharge current (<I>t</I>=<I>C</I>/<I>I</I>). When the cell voltage reaches 3V, discharge must be interrupted in order to prevent damage. A fully charged cell provides about 3.7V to the load when current is low, and averages 3.5V over a discharge cycle in high current applications.</P>
<P>Current capacity (a.k.a. current rating, or maximum continuous discharge rate) tells you how much current the cell can continuously supply and still deliver the rated charge. If the load draws more current than the rated current capacity, endurance and voltage will be low. </P>
<P>Current capacity is expressed in milliamps (mA), amps (A), or in multiples of <B>C</B>, where "<B>C</B>" is the amount of current the cell can supply for one hour.</P>
<P>Example 1: The capacity of a cell is 130mAh, so C=130mA. (Note: The term "capacity" used by itself is understood to mean <I>charge</I> capacity.)</P>
<P>Example 2: The current capacity of a 130mAh cell is 15C. Therefore the maximum continuous current is (15 x 130mAh=) 1.95A. </P>
<P>The current capacity for small high rate cells ranges from 10C to 20C or more. The higher the current rating, the more efficient and powerful a given system will be.</P>
<P>A cell is never referred to as a "battery".</P>
<P>You don�t have a battery until you connect two or more cells together (usually in series).</P>
<P>When two cells are connected in series (sometimes denoted by a "<B>2S</B>" suffix),</P>
<P>the positive terminal of one cell is connected to the negative terminal of the other cell, and the two remaining leads are brought out to the battery connector. The positive battery wire is color-coded red, the negative wire black, and the center tap is blue. The battery center tap lets you independently charge both cells simultaneously if you have a balancing charger.</P>
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<P>When cells are connected in series, the voltage multiplies, and the current and charge capacity <I>stay the same</I>. A 3-cell battery, for example, would have a nominal voltage of 11.1V and two taps. </P>
<P>Charge does not multiply in a series battery because cells do not store charge. Each cell supplies energy that makes the charge inside the circuit flow in a closed loop.</P>
<P>Cells are normally charged at a 1C rate, so it takes a little over an hour to recharge fully discharged cells. Never charge cells unattended or near flammable materials because a malfunction can cause explosion and fire.</P>
<P>Most micro lithium cells are manufactured by Kokam, Full River and AHA.</P></BODY>
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