White Paper: Battery availability - a true measure of mobile productivity

While lightness and battery life are useful measures of a notebook’s appeal, the battery recharge time also needs considering

While lightness and battery life are useful measures of a notebook’s appeal, the battery recharge time also needs considering

Over the past few years, portable computers have evolved from technology "gadgets" with few users to mainstream, full-featured computing platforms with a rapidly increasing market. Industry analysts predict that the growth rate of portable computers will far outpace that of desktops in the next several years. A major contributor to this evolution has been a rapid reduction in the performance gap between portable and desktop computers. The following factors have been instrumental in reducing the perceived performance trade-off between portable and desktop computers:

Near equivalence in microprocessor speeds

High-capacity mass storage devices

Larger display sizes

Cost-effective packaging technologies to help satisfy form factor constraints

Docking stations that offer a wide variety of expandability options

Considering these technology trends and the continuing effort by employers to increase employee productivity, it is easy to see why many corporations are considering portable computers as a primary computing platform.

To continue this advance toward mainstream status for their products, manufacturers of portable computers must come to terms with a customer base that is becoming increasingly knowledgeable and more demanding. In particular, battery life, overall system weight and ease of mobility are key factors that customers consider when buying a portable computer. The reasons for customers' concerns about battery life are fairly obvious. While faster processors and graphics controllers, larger, brighter displays and more peripherals have helped bridge the gap between portable and desktop computers, they have also led to a significant increase in the average power consumption of a portable computer.

In fact, in a recent Sherwood Research survey that evaluated the level of satisfaction with various portable computer attributes, 41 per cent of the 600 users polled expressed dissatisfaction with battery life; 30 per cent were dissatisfied with system weight.

In contrast to the rapid increase in portable computer features and performance, battery technology improvements have come slowly. As a result, users have begun to experience decreasing battery life.

Industry efforts to meet the power challenge

Attempts to alleviate the power consumption problem are being made on several fronts. One major thrust is to improve power management to allow more efficient use of available battery power and extend operating time by managing power for optimal length of service between battery recharges.

Intel and several major mobile system and component manufacturers have endorsed the Intel Mobile Power Initiative, which encompasses existing and new initiatives. The Mobile Power Initiative is an industrywide program to assist mobile PC system manufacturers, component suppliers and software vendors in delivering future high-end mobile systems and components within mobile thermal limits without sacrificing battery life.

Intel and Duracell have developed the Smart Battery System (SBS) architecture specification to help enable more accurate battery status information for use by the operating system and the user.

Microsoft, Intel and Toshiba, with participation from other major system manufacturers, have developed the Advanced Configuration and Power Interface (ACPI) specification (shown under the Operating System Power Management Initiative) to help enable operating system-directed power management for better use of system resources and available battery power.

The Mobile Power Guidelines (under the Platform Power Initiative) provide performance, feature and power targets for future mobile PC hardware. One method of achieving lower power is reducing component voltages.

The Mobile Software Power Initiative provides guidelines for developing software application programs that are optimised for low power consumption.

These power consumption improvements will gradually make their way into future portable PCs. Reducing device operating voltages requires that device manufacturers move to next-generation fabrication technologies to maintain yields. Considering the associated costs and advanced planning required, such a transition is likely to take a few years.

While the Operating System Power Management Initiative and the Mobile Software Power Initiative will considerably reduce power consumption, the anticipated savings in power will be offset by the increases in power consumption of next-generation technologies.

Alternatively, several portable computer manufacturers have opted to either increase the number of cells per battery pack or provide computers with reduced feature sets to minimise overall system power consumption. However, these approaches run counter to customer demands. Increasing the number of cells per pack (from eight to 12 cells) or the number of battery packs per computer substantially increases system weight - an area where customers are demanding reductions. Providing systems with reduced feature sets increases the performance gap between desktop and portable computers, stifling the market opportunity that has brought the portable computer to its current level of acceptance.

The need to provide portable computer users no-compromise systems with minimum downtime is clear. And it must be done in a manner that comprehends and balances the exponential increases in power consumption, the evolutionary growth of battery technology and customer demands for features without adding to the overall weight of the portable computer.

To provide a better understanding of the options for obtaining maximum battery power with minimum weight, the three key variables in battery technology - battery life, battery weight and battery charge time - are discussed next.

Battery life

Lithium ion (Li-ion) batteries are currently the technology of choice in the industry because of their significantly higher gravimetric energy. For a given computer with a fixed battery weight, a Li-ion battery pack will last approximately three times as long as an NiMH battery and approximately four times as long as a NiCad battery. However, recent trends indicate that airlines will begin offering in-flight outlets for AC adaptors over the next few years. Battery run-down time will become less important when this service is widely available.

Other battery technologies, such as zinc-air, lithium polymer and lithium metal, have been on the horizon for the last few years. However, manufacturers must overcome major problems before using these technologies to manufacture batteries in volume. In the case of lithium polymer technology, problems include the lack of stable technology that can meet peak system requirements, high cost and lack of infrastructure to support volume requirements. For zinc-air technology, the larger size of the resulting battery pack is an issue. With lithium metal, safety issues and associated governmental regulations regarding maximum lithium content are major concerns.

Concerns with these new technologies and the rapid increase in production capacity of Li-ion cells (which will drive costs lower) make it likely that Li-ion will be the mainstream battery of choice in the foreseeable future.

Battery weight

The second key variable to be considered is battery weight. On average, the battery weight with a single battery pack constitutes 12 to 15 percent of the overall computer weight. Users who carry extra batteries to increase run time on the road add to the weight they must carry.

Battery charge time

The third variable that determines system uptime - one that has been largely overlooked - is battery charge/recharge time. Battery charge time is a function of the number of cells in the battery pack and the overall efficiency of the computer's power subsystem design. Thus, increasing the number of cells in a battery pack to obtain higher-capacity batteries not only increases system weight but also increases battery recharge time.

Battery availability: a new metric

The notion of availability is not new to the computer industry; its roots are in the mainframe/server industry where uptime and productivity are critical. Similar to the portable computer industry's current narrow view on battery life, there was a time when one of the primary product attributes in the server industry was mean time between failures (MTBF), a measure of the inherent reliability of the product. As the server industry matured, it became obvious that MTBF alone was not a good measure of productivity. For example, system manufacturers reached physical limitations of component lifetimes even when using the most expensive components. As standards emerged and components became commodities, only marginal differences in MTBF could be observed from one system to another.

Because MTBF's definition implies that a failure will eventually occur, the mean time to repair (MTTR) metric emerged. MTTR addresses how quickly a system can be repaired and brought back up once the predicted failure occurs. System availability evolved as a metric that expresses the ratio of this uptime (MTBF) to downtime (MTTR), thus helping define overall productivity:

Battery availability defines the effective productivity a portable computer user can realise for every pound of battery weight the user carries. Manufacturers can continue to improve the user experience metric in these ways:

Implement new technologies that provide an increase in cell energy density without adding weight

Implement new technologies that provide the same cell energy density at a lower weight

Implement power subsystem designs that dramatically reduce battery-charging time

Increasing battery availability

The concept of battery availability per pound is thus one of the key criteria in the design and development of new notebook platforms. Given the incremental improvements in cell gravimetric and volumetric energy density, the need for a breakthrough charger design that would dramatically reduce battery recharging time became apparent. As a result of strong collaborative efforts between Dell, major Li-ion cell manufacturers and power subsystem component suppliers, a new ExpressCharge technology was developed that "turbo charges" a Li-ion battery in about one hour with no accompanying safety issues.

Using this patented technology, the battery-charging circuitry monitors the power provided by the AC adaptor and dynamically allocates power between the portable computer and the battery pack. As the power required by the computer decreases, more charging current is routed to the battery pack. Conversely, when the power required by the computer increases, less charging current is routed to the battery pack. This efficient use of the power available from the AC adapter considerably reduces charging time for the battery pack.

Additionally, the Latitude CPi's overall power subsystem and thermal design provide adequate headroom for future increases in system power requirements with full backward and forward compatibility of existing battery packs. These factors help provide customers with the benefits of increased availability and mobile productivity while also providing investment protection and lower total cost of ownership.

This clear understanding of customer requirements, the ways customers use their computers and the role of battery subsystem performance, are the reasons why Dell designed the Latitude CPi with industry-leading battery availability per pound - approximately 121 per cent over the average high-end and mainstream value portable computer.


Expressing battery subsystem performance in a manner that is more representative of the user experience than run-down time alone is going to become increasingly important. Battery availability per pound - a metric derived by considering battery charge times and weight in addition to run-down time - is a highly accurate measure of battery performance and mobile productivity.

Compiled by John Sabine

(c) 1998 Dell Computer Corporation

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