Battery Life (and Death)
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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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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. This page describes the factors influencing
battery life.
Battery life can usually
only be extended by preventing or reducing the cause of the unwanted parasitic
chemical effects which occur in the cells. Ways of improving battery life and
hence reliability are also considered below.
Battery performance
deteriorates over time whether the battery is used or not. This is known as
"calendar fade". Performance also deteriorates with usage and this is
known as "cycle fade"
·
Battery
Calendar Life is the elapsed
time before a battery becomes unusable whether it is in active use or inactive.
There are two key factors influencing calendar life, namely temperature and
time, and empirical evidence shows that these effects can be represented by two
relatively simple mathematical dependencies. A rule of thumb derived from
the Arrhenius Law describes how the rate at which a chemical
reaction proceeds, doubles for every 10 degrees rise in temperature, in this
case it applies to the rate at which the slow deterioration of the active
chemicals increases. Similarly the t1/2 (or √t )
relationship represents how the battery internal resistance also increases with
time t. The graph below illustrates these effects.
·
Battery
Shelf Life like calendar life
is the time an inactive battery can be stored before it becomes unusable,
usually considered as having only 80% of its initial capacity. See also Battery Storage
·
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. Key factors
affecting cycle life are time t and the number N of
charge-discharge cycles completed. An obvious example is the Depth of Discharge (see below) which is a simple reciprocal mathematical
relationship, but there are many more complex factors which can also influence
performance.
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 number of cycles the battery can perform before
its internal resistance increases by an agreed amount. Usually 1.3 times or
double 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.
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