Choosing the right battery for wireless communications
(BU37)
Research has brought about a variety of battery chemistries, each offering distinct
advantages but none providing a fully satisfactory solution. With today's variety
of battery types, better choices can be made to suit specific user applications.
This paper talks about the recommended battery chemistry for cell phones and two-way
radios in terms of energy density, durability and price.
What's the best
battery for cell phones?
Early cell phones were powered with nickel-based
batteries but most newer phones are now equipped with lithium-ion. This chemistry
is lightweight, offers high energy density and lasts long enough to span the typical
life of the product. Lithium-ion contains no toxic metals.
To obtain thin
geometry, some cell phone manufacturers switched to lithium-ion-polymer.
This satisfied consumer requests for slim designs. In the meantime, technological
advancements also made low profile lithium-ion possible. lithium-ion packs are
now available in 3 mm, a profile that suits most designs. lithium-ion has the
advantage of lower manufacturing cost, better performance and longer cycle life
than the polymer version.
Lithium-ion is a low maintenance battery. No
periodic discharge is needed and charging can be done at random. A random charge
means that the battery does not need to be fully depleted before recharge. In
fact, it is better to recharge before the battery gets too low. Full discharges
put an unnecessary strain on the battery. A recharge on a partially charged battery
does not cause memory because there is none.
Charging lithium-ion is
simpler and cleaner than nickel-based batteries but the chargers require tighter
tolerances. lithium-ion cannot absorb overcharge and no trickle charge is applied
on full charge. This allows lithium-ion to be kept in the chargers until used.
Some chargers apply a topping charge every week or so to replenish the capacity
lost through self-discharge while the battery sits idle in the charger. Repeated
insertion into the charger or cradle does not damage the battery though overcharge.
If the battery is full, no charge is applied. The battery voltage determines the
need to charge.
On the negative side, lithium?ion gradually loses charge
acceptance as part of aging, even if not used. lithium?ion batteries should not
be stored for long periods but be rotated like perishable food. The buyer should
be aware of the manufacturing date when purchasing a replacement battery. Aging
affects battery chemistries at different degrees.
Counterfeit
cell phone batteries (clone batteries)
In the search for low-cost battery replacements, consumers
may inadvertently purchase clone cell phone batteries that do
not include an approved protection circuit. Lithium-ion packs
require a protection circuit to shut off the power source if the
charger malfunctions and keep on charging, or if the pack is put
under undue stress (electrical short). Overheating and 'venting
with flame' can be the result of such strain. (See photos of an
exploded cell phone with clone battery on charge.)
|
|
Photos
of a cell phone with a clone battery
that exploded while left on charge in a car |
|
|
 |
Cell
phone manufacturers strongly advise customers to replace the battery
with an approved brand. Failing to do so may void the warranty.
Counterfeit cell phone batteries have become visible since the
beginning of 2003 when the world was being flooded with cheap
replacement batteries from Asia.
Cell phone manufacturers act out of genuine concern for safety
rather than using scare tactics to persuade customers to buy their
own accessories. They do not object to third party suppliers in
offering batteries and chargers as long as the products are well
built, safe and functioning. The buyer can often not distinguish
between an original and a counterfeit battery because the label
may appear bona fide.
Caution should also be exercised in purchasing counterfeit chargers.
Some units do not terminate the battery correctly and rely on
the battery's internal protection circuit to cut off the power
when fully charged. Precise full-charge termination and a working
protection circuit are needed for the safe use of the lithium-ion
battery.
What's the best battery for two-way radios?
Most two-way radios use nickel-cadmium. These batteries are durable
and forgiving if abused. But nickel-cadmium batteries have only
moderate energy density and are environmentally unfriendly. Environmental
agencies have been discouraging its use, especially in Europe.
The recommended alternative is nickel-metal-hydride, a battery
that has higher energy density and contains no toxic metals. nickel-metal-hydride
has been tested in two-way radios for a number of years but the
results are mixed. Shorter than expected service life is the major
drawback.
For two-way radios, nickel-metal-hydride has a cycle life, which
is half that of standard nickel-cadmium. nickel-metal-hydride
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 and sharp
pulse currents shorten battery life.
To compare the longevity of nickel-metal-hydride under different
load condition, a test was carried out in which batteries of the
same type were discharged with a DC and digital load. In both
tests, the batteries were discharged to 1.04 volts per cell. The
DC load was a steady 500mA; the digital load simulated the Global
System for Mobile Communications (GSM) at 1.65 ampere peak for
12 ms every 100 ms with 270 mA standby. (Note that the GSM pulse
for voice is about 550 ms every 4.5 ms).
With the DC discharge, nickel-metal-hydride wore out gradually,
providing an above average service life. At 700 cycles, the battery
still provided 80% capacity. By contrast, the same battery type
faded more rapidly with a digital discharge and the 80% capacity
threshold was reached after only 300 cycles. This phenomenon indicates
that the kinetic characteristics for nickel-metal-hydride deteriorate
more rapidly with a digital than analog load. Although the test
was simulating a GSM cell phone, Tetra and other digital two-way
radios have similar loading.
Let's briefly compare the characteristics of nickel-cadmium and
nickel-metal-hydride. nickel-cadmium has the advantage of maintaining
steady high capacity and low internal resistance through most
of its service life. nickel-metal-hydride, on the other hand,
starts with good capacity and low internal resistance but the
resistance increases after a few hundred cycles, causing the voltage
to drop on a load. Even though the energy may still be present,
the battery cannot deliver the high current during transmit and
the message cuts off. The radio becomes unreliable.
Nickel-based batteries are high in maintenance. Periodic discharge
cycles are needed to prevent crystalline formation on the cell
plates, also known as memory. nickel-cadmium is more receptive
to memory than nickel-metal-hydride because both nickel and cadmium
plates are affected by memory.
Nickel-cadmium should be exercised once ever 1 to 2 months, whereas
nickel-metal-hydride can get by with a deliberate full discharge
once every 3 months. Without proper maintenance, the advantage
of nickel-cadmium over nickel-metal-hydride in terms of cycle
life cannot be realized.
Lithium-ion has been tested for two-way radios and the results
are positive. Substituting lithium-ion with nickel-based will
require chargers specifically suited for this chemistry. While
nickel-cadmium and nickel-metal-hydride can often share the same
charger, lithium-ion uses a different charge algorithm. There
is also a cost premium for lithium-ion. Future two-way radios
will undoubtedly be fitted with lithium-ion.
_________________________
Created: May 2003, Last edited: September 2005
