Digital's Assault on Batteries

Ask which digital cellular technology provides more advantages to the consumer, and you would have to hold a run-off to decide the majority. CDMA enables many subscribers to use the same frequency channel at the same time. TDMA enables many subscribers to use the same frequency channel, but in time increments.

Ask what digital cellular technology does to battery technology, and the story is a little different. Digital affects batteries and their performance in a number of ways. One of the effects, also prevalent in analog technology, is a change in the input power requirement. Digital phones can be powered by input voltage as low as 3.6V. This can be attributed to advancements in the integrated chip technology that accommodates lower input voltages without affecting the performance of the phone.

As these voltage levels have decreased, the importance of available current (provided by the battery) has increased. A cellular battery must have the ability to maintain a stable voltage level for its specified rate. Typically, phones are designed with a cutoff point -- the point at which a cellular phone does not have an input voltage level high enough to maintain power to the phone. This is measured at 1V per cell for the number of cells incorporated in the particular battery pack. If a battery incorporates five cells -- in terms of a 6V nickel cadmium (NiCd) or nickel metal hydride (NiMH) battery -- the cutoff point would be 5V. Therefore, as the number of cells has decreased, and the cutoff voltage is reduced, the importance of dependable cells increases.

Digital cellular also drains current from the battery at a different rate. Both TDMA and CDMA technologies alter the current required by cellular phones. The typical continuous drain currents of digital cellular phones are less than the typical continuous drain currents of analog cellular phones. However, digital cellular phones will draw instantaneous peak currents that can be two to three times as high as the typical continuous current draws of an analog cellular phone.

A typical battery will supply approximately 25% more talk time when used with a digital phone. However, because of peak current draws, a battery manufacturer must be concerned with the cell chosen for the configuration of the particular battery pack. Although it would not be apparent when using a AA-size cell at 1,200mAh in a battery pack, a similar pack using AAA-size cells at 550mAh can encounter a peak current draw at three times its rated capacity. A battery manufacturer must choose a cell with the lowest internal impedance possible. A cell with a lower internal impedance will result in a higher output voltage. If the voltage level decreases below the predetermined level, the phone will power off.

To achieve the power needs, handset manufacturers must work with cell and battery manufacturers to design the power systems. To date, handset vendors have been able to consider battery chemistries that include NiCd, NiMH and lithium ion (Li-Ion). Each of these systems has the benefits of acceptable performance, rechargeability that reduces overall costs and the availability of production.

NiCd and NiMH are similar systems. NiCd has been the rechargeable system of choice for many years but now is considered the general-purpose rechargeable system. NiMH has the advantage of 30% to 50% greater energy (30% to 50% more talk time) with a comparable 30% to 50% increase in cost. However, as power supplies continue to shrink, the volumetric advantage will prove that NiMH is the superior system.

Li-Ion technology is the latest chemistry being used in consumer applications. The significant advantages include higher operating voltage (3:1 voltage ratio over NiCd and NiMH); energy density providing greater talk time; and low self-discharge, allowing longer periods of storage without charging. However, these can be offset by the cost of both cell and charger, cycle life that varies based on daily usage, and the need for additional controls to ensure consumer safety.

Several generalizations can be made about the impact of battery technology and its use in wireless handsets, whether digital or analog. First, it is possible to use all three chemistries in the same system. Second, handsets will allow flexibility to use the various systems. Third, the charger and charging systems are the critical factors for the vendor and the aftermarket suppliers. Finally, the handset sophistication, target market and desired cost from the vendor will determine which chemistry should be used.