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Battery Life and How To Improve It
http://www.mpoweruk.com/life.htm (1 of 13)5/14/09 11:36 AM
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For product designers, an understanding of the factors affecting battery life is vitally important for managing both product performance and warranty liabilities particularly with high cost, high power batteries. Offer too low a warranty period and you won't sell any batteries/products. Overestimate the battery lifetime and you could lose a fortune.
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Battery Life (and Death)
That batteries have a finite life is due to occurrence of the unwanted chemical or physical changes to, or the loss of, the active materials of which they are made. Otherwise they would last indefinitely. These changes are usually irreversible and they affect the electrical performance of the cell.
Battery life can usually only be extended by preventing or reducing the cause of the unwanted parasitic chemical effects which occur in the cells. Some ways of improving battery life and hence reliability are considered below.
Battery cycle life is defined as the number of complete charge - discharge cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity. Lifetimes of 500 to 1200 cycles are typical. The actual ageing process results in a gradual reduction in capacity over time. When a cell reaches its specified lifetime it does not stop working suddenly. The ageing process continues at the same rate as before so that a cell whose capacity had fallen to 80% after 1000 cycles will probably continue working to perhaps 2000 cycles when its effective capacity will have fallen to 60% of its original capacity. There is therefore no need to fear a sudden death when a cell reaches the end of its specified life. See also Performance Characteristics.
An alternative measure of cycle life is based on the internal resistance of the cell. In this case the cycle life is defined as the numer of cycles the battery can perform before its internal resistance increases to 1.3 times its initial value when new.
In both cases the cycle life depends on the depth of discharge and assumes that the battery is fully charged and discharged each cycle. If the battery is only partially discharged each cycle then the cycle life will be much greater. See Depth of Discharge below. It is therefore important that the Depth of Discharge should be stated when specifying the cycle life.
When battery systems are specified it is usual to dimension the battery in terms of its end of life capacity rather than its capacity when new. Battery shelf life is the time an inactive battery can be stored before it becomes unusable, usually considered as having only 80% of its initial
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