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.
Conclusions
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; [email protected];
(503) 530-4901.
Filed Under: M2M (machine to machine)
