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Chapter 8: Choosing the Right
Battery
What causes a battery to wear down — is it mechanical or
chemical? The answer is ‘both’. A battery is a perishable
product that starts deteriorating from the time it leaves
the factory. Similar to a spring under tension, a battery
seeks to revert to its lowest denominator. The rate of aging
is subject to depth of discharge, environmental conditions,
charge methods and maintenance procedures (or lack thereof).
Each battery chemistry behaves differently in terms of aging
and wear through normal use.
 When
buying a replacement battery, the buyer often has the choice
of different battery chemistries. Li-ion and Li-ion polymer
batteries are used on newer phones, whereas the NiMH and NiCd
are found in older models. If the buyer has a choice, the
sales person may advise a customer to go for the highest capacity
rating and to stay away from the NiCd because of the memory
effect. The customer may settle for the slim-line NiMH because
it offers relatively high capacity in a small package and
is reasonably priced.
Seemingly a wise choice, an analysis in this
chapter reveals that other chemistries may have served better.
The NiMH offers good value for the price but falls short in
expected cycle life. Although excellent when new, the performance
trails off quickly after about 300 cycles due to decreased
capacity and rising internal resistance. In comparison, the
Li-ion can be used for about 500 cycles. The best cycle
count is achieved with NiCd. Properly maintained, the NiCd
delivers over 1000 cycles and the internal resistance
remains low. However, the NiCd offers about 30 percent
less capacity compared to the NiMH. In addition, the NiCd
is being removed from the mobile phone market because of environmental
concerns.
Switching to environmentally friendlier batteries
is fitting, especially in the mobile phone market where the
NiMH performs reasonably well and can be economical. The battery
disposal issue is difficult to control, particularly in the
hands of a diverse user group.
 The
NiMH and NiCd are considered high maintenance batteries, which
require regular discharge cycles to prevent what is referred
to as ‘memory’. Although the NiMH was originally advertised
as memory-free, both NiCd and NiMH are affected by the phenomenon.
The capacity loss is caused by crystalline formation that
is generated by the positive nickel plate, a metal shared
by both systems.
Nickel-based batteries, especially NiCd’s, should
be fully discharged once per month. If such maintenance is
omitted for four months or more, the capacity drops by as
much as one third. A full restoration becomes more difficult
the longer service is withheld.
It is not recommended to discharge a battery
before each charge because this wears down the battery unnecessarily
and shortens the life. Neither is it advisable to leave a
battery in the charger for a long period of time. When not
in use, the battery should be put on a shelf and charged before
use. Always store the battery in a cool place.
Is the Li-ion a better choice? Yes, for
many applications. The Li-ion is a low maintenance battery
which offers high energy, is lightweight and does not require
periodic full discharge. No trickle charge is applied once
the battery reaches full charge. The Li-ion battery
can stay in most chargers until used. The charging process
of a Li-ion is, in many ways, simpler and cleaner than
that of nickel-based systems, but requires tighter tolerances.
Repeated insertion into the charger or cradle does not affect
the battery by inducing overcharge.
On the negative side, the Li-ion gradually
loses charge acceptance as part of aging, even if not used.
For this reason, Li-ion batteries should not be stored
for long periods of time but be rotated like perishable food.
The buyer should be aware of the manufacturing date when purchasing
a replacement battery.
The Li-ion is most economical for those
who use a mobile phone daily. Up to 1000 charge/discharge
cycles can be expected if used within the expected service
life of about two to three years. Because of the aging effect,
the Li-ion does not provide an economical solution for
the occasional user. If the Li-ion is the only battery
choice and the equipment is seldom used, the battery should
be removed from the equipment and stored in a cool place,
preferably only partially charged.
So far, little is known about the life expectancy
of the Li-ion polymer. Because of the similarities with
the Li-ion, the long-term performance of both systems
is expected to be similar. Much effort is being made to prolong
the service life of lithium-based systems. New chemical additives
have been effective in retarding the aging process.
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