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The two-way radio market uses mostly NiCd batteries.
In the last few years, environmental agencies have been attempting
to discourage the use of NiCd, especially in Europe. NiMH
have been tried and tested in two-way radios for a number
of years but the results are mixed. Shorter cycle life compared
to NiCd is the major drawback.
The reasons for the relatively short life of
NiMH are multi-fold. NiMH is less robust than NiCd and has
a cycle life expectancy that is half or one third that of
the standard NiCd. In addition, NiMH prefers a moderate discharge
current of 0.5C or less. A two-way radio, on the other hand,
draws a discharge current of about 1.5A when transmitting
at 4W of power. High discharge loads shorten the life of the
NiMH battery considerably.
NiCd has the advantage of maintaining a low and
steady internal resistance throughout most of its service
life. Although low when new, NiMH increases the resistance
with advanced cycle count. A battery with high internal resistance
causes the voltage to drop when a load is applied. Even though
energy may still be present, the battery cannot deliver the
high current flow required during transmit mode. This results
in a drop in voltage, which triggers the ‘low battery’ condition
and the radio cuts off. This happens mostly during transmission.
The Li-ion has been tested for use with
two-way radios but has not been able to provide the ultimate
answer. Higher replacement costs, restrictions posed by the
safety circuit and aging pose limitations on this battery
system.
Batteries for laptops have a unique challenge
because they must be small and lightweight.  In fact, the laptop battery should be invisible to the
user and deliver enough power to last for a five-hour flight
from Toronto to Vancouver. In reality, a typical laptop battery
provides only about 90 minutes of service.
Computer manufacturers are hesitant to add a
larger battery because of increased size and weight. A recent
survey indicated that, given the option of larger size and
more weight to obtain longer runtimes, most users would settle
for what is being offered today. For better or worse, we have
learned to accept the short runtime of a laptop.
During the last few years, batteries have improved
in terms of energy density. Any benefit in better battery
performance, however, is being eaten up by the higher power
requirements of the laptops. It is predicted that the even
more power-hungry PC’s of the future will counteract any improvements
in battery technology, as marginal as they might be. The net
effect will result in the same runtimes but faster and more
powerful computers.
The length of time the battery can be used will
get shorter as the battery ages. A battery residing in a laptop
ages more quickly than when used in other applications. After
a warm-up, the official operating temperature inside a laptop
computer is 45°C (113°F). Such a high ambient temperature
drastically lowers the battery’s life expectation. At a temperature
of 45°C, for example, the life expectancy of a NiMH battery
is less than 50 percent as compared to running it at
the ideal operating temperature of 20°C (68°F).
The Li-ion does not fare much better. At
this high ambient temperature, the wear-down effect of the
battery is primarily governed by temperature as opposed to
cycle count. The situation is worsened by the fact that the
battery resides in a high SoC most of the time. The combination
of heat and high SoC promotes cell oxidation, a condition
that cannot be reversed once afflicted.
A fully charged Li-ion battery that is
stored at 45°C suffers a capacity loss from 100 percent
to about 70 percent in as little as six months. If this
condition persists, the capacity degrades further to 50 percent
in twelve months. In reality, the battery in a laptop is exposed
to elevated temperatures just during use and the battery is
in a full charge state only part of the time. But leaving
the laptop in a parked car under the hot sun will aggravate
the situation.
Some Japanese computer manufacturers have introduced
a number of sub-notebooks in which the battery is mounted
externally, forming part of the hinge. This design improves
battery life because the battery is kept at room temperature.
Some models carry several size batteries to accommodate different
user patterns.
What then is the best battery for a laptop? The
choices are limited. The NiCd has virtually disappeared from
the mobile computer scene and the NiMH is loosing steam, paving
the way for the Li-ion. Eventually, very slim geometry
will also demand thin batteries, and this is possible with
the prismatic Li-ion polymer.
Besides providing reliable performance for general
portable use, the Li-ion battery also offers superior
service for laptop users who must continually switch from
fixed power to battery use, as is the case for many sales
people. Many biomedical and industrial applications follow
this pattern also. Here is the reason why such use can be
hard on some batteries:
On a nickel-based charging system, unless smart,
the charger applies a full charge each time the portable device
is connected to fixed power. In many cases, the battery is
already fully charged and the cells go almost immediately
into overcharge. The battery heats up, only to be detected
by a sluggish thermal charge control, which finally terminates
the fast charge. Permanent capacity loss caused by overcharge
and elevated temperature is the result.
Among the nickel-based batteries, NiMH is least
capable of tolerating a recharge on top of a charge. Adding
elevated ambient temperatures to the charging irregularities,
a NiMH battery can be made inoperable in as little as six
months. In severe cases, the NiMH is known to last only 2
to 3 months.
For mixed battery and utility power use, the
Li-ion system is a better choice. If a fully charged
Li-ion is placed on charge, no charge current is applied.
The battery only receives a recharge once the terminal voltage
has dropped to a set threshold. Neither is there a concern
if the device is connected to fixed power for long periods
of time. No overcharge can occur and there is no memory to
worry about.
NiMH is the preferred choice for a user who runs
the laptop mostly on fixed power and removes the battery when
not needed. This way, the battery is only engaged if the device
is used in portable mode. The NiMH battery can thus be kept
fresh while sitting on the shelf. NiMH ages well if kept cool
and only partially charged.
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