Mobile Computing

By Robin Sarah Tichy, Micro Power

Mobile computing, whether for the consumer or for industrial applications, has two fundamental requirements, wireless communication and portable power, but device manufacturers continue to add more power-hungry features to their portable devices beyond the Bluetooth or WiFi interfaces, such as color displays, MP3 playback capabilities, and digital cameras. Additionally, these manufacturers exacerbate the portable power issues by reducing the size and weight of the portable devices.

To extend run-time, manufacturers are optimizing their power management schemes (i.e. screen dimming, processor shutdown when inactive), using custom processors
Figure 1. Li-ion batteries are available in a variety of shapes and sizes.
catered to mobile devices, and using authentication technology to eliminate the use of low-quality counterfeit batteries. Battery pack manufacturers are doing their part to meet the challenge of packing more energy into smaller and smaller battery configurations. The power management electronics are also improving so that every bit of energy is used efficiently. This article on the latest in battery pack technology will enable the designers of wireless devices to make an informed decision on which kind of battery pack to choose.
Li-ion Batteries Offer Many Advantages
Lithium ion (Li-ion) battery technology has largely replaced the older NiMH and NiCd chemistries. Li-ion batteries offer many attractive advantages over other rechargeable chemistries, including a much higher energy density, lighter weight, longer cycle-life, superior capacity retention, broader ambient-temperature endurance, and higher current tolerance. Li-ion is more environmentally friendly than the other chemistries and modern designs are very safe. Over the last 10 years the fundamental materials on which Li-ion is based have not changed much. New safety schemes have been developed and energy density has increased by stuffing more and more material in the same size can. Battery cell producers have provided minor improvements in energy/density performance, typically an annual 10 % improvement.  Li-ion has decreased in cost because of the economies of scale driven by consumer products, such as laptop and cell phones. Often a Li-ion solution is at cost parity with a Ni-metal hydride solution because the higher operating voltage, 3.6V vs. 1.2V, allows for a lower cell count. In addition, new cathode chemistries are being introduced which have the potential for lower cost than the current LiCoO2 based cells. Li(CoMnNi)O2 is a safer and less expensive material that will be offered by a number of the tier 1 cell suppliers. The new cells are now offered in the common, 18mm in diameter and 65mm long, size, but eventually they will be offered in a both the cylindrical and prismatic shapes as depicted in figure 1. The variety of shapes and sizes that Li-ion is available in is an added bonus; Li-polymer cells, a subset of Li-ion technology, are as thin as 2.6 mm.
Smart Battery Packs Communicate
Design engineers are making great strides in power management technologies which complement the novel battery chemistries. A schematic of a battery pack is shown in figure 2. The pack consists of the cells, which are the primary energy source, the printed circuit board, a plastic enclosure, and LED’s for status indicators. The stable condition is normally maintained with a safety circuit or Battery Management Unit (BMU). All Li-ion batteries must be protected against over- and under-voltage, as well as short-circuiting.

A smart battery pack employs a BMU with features that go beyond the minimum safety requirements. A smart pack has the ability to monitor its status, accurately predict its remaining run time, and communicate its operational status to the host device when the electronics include fuel gauging and communication via a serial data communications bus. Traditional battery fuel gauges either monitored the voltage or the capacity, and the accuracy was quite limited. A new gas gauge monitors the number of coulombs being transferred and opportunistically calibrates with the open circuit voltage of the Li-ion pack. Texas Instruments claims an accuracy of 99% for its Impedance TrackTM technology. Accurate fuel gauging, combined with smart charging algorithms, enables Li-ion to be charged in the inconsistent manner previously only accepted by large, heavy sealed lead acid batteries.
Battery Management Units Help Ensure Optimum Performance
Proper choices, with respect to the BMU and battery chemistry, will determine the reliability of a wireless device. Manufacturers typically specify cell performance at an
Figure 2. A smart Li-ion battery pack has a fuel gauge and communications to monitor the cells.
ideal C/5 constant current and +20 C degrees external temperature. However, many portable devices, especially industrial handhelds, are expected to operate in a range from 㪬 C to +40 C degrees with higher, pulsating currents, so testing of the performance profile of cells and assembled battery pack in simulated use is necessary to ensure proper function. The first step in ensuring a proper battery choice is to fully describe the “real world usage profile” of the device. The usage profile includes temperature ranges, discharge profiles, charging regimens, expected shelf life, and transportation requirements and should account for foreseeable misuse as well as intended use. Extremes in temperature or current pulses can cause similarly rated cells, from different manufacturers, to demonstrate widely varying a performance results.
It is crucial that battery system engineers avoid design problems and device failures by effectively planning, developing and implementing smart battery system solutions into their portable applications. Designing a power-management system for high-performance portable electronics applications can be difficult, even for the most experienced design engineer. Underestimating the complexity of the battery system and the interrelationship between battery and device circuitries can lead to setbacks during product development. Worse, the entire system may fail in the field. These kinds of problems indicate that many original equipment manufacturers (OEMs) are facing battery-system design issues that they may not have the tools or internal expertise to solve. Fortunately, planning for the battery system early enough in the design process, along with proper implementation, can minimize or eliminate the possibility of battery problems. Micro Power Electronics not only offers the best quality manufacturing in the industry but has the engineering and test expertise to tackle the toughest design challenges.

About the Author
Dr. Robn Sarah Tichy is technical marketing manager for Micro Power;; (503) 530-4901.