Miscellaneous/Ask Isidor/ Wireless Application 040611

Q: How is the right battery chosen for wireless applications?
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 through overcharge. If the battery is full, no charge is applied. The battery Voltage determines the need to charge.

On the negative side, Lithiumion gradually loses charge acceptance as part of aging, even if not used. Lithiumion 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

The number of substandard batteries being sold by street vendors or on the Web is growing. Cell phone manufacturers have issued warnings, saying that these batteries are unsafe. Some Lithium-ion packs offered do not include a safety circuit to shut off the power source when reaching full charge and the battery overheats. Enough heat is generated to melt the phone’s plastic casing and destroy the phone’s internal circuits.
Cell phone manufacturers advise customers to replace the battery with a recommended brand. Failing to do so may void the warranty. The problem with counterfeit cell phone batteries has become acute since the beginning of 2003, especially in Asia, Africa and Europe.
Most 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 batteries and chargers being offered by third party suppliers as long as the products are well built and are functioning properly.
The buyer often cannot distinguish between an original and a counterfeit cell phone battery because the labeling may hint to a bona fide brand. Some packs are labeled to contain Lithium-ion but contain lower cost nickel-based cells. Battery analyzers are able to identify most counterfeit batteries.
Caution should also be exercised in purchasing counterfeit chargers. Some units do not terminate the battery correctly or they rely on the battery’s internal safety circuit to cut off the power on full charge. Counterfeit batteries, and those with damaged safety circuits, may not terminate the fast-charge. The battery may heat to the point of venting with flame.    

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 for nickel-based batteries 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.