Lithium-based rechargeable batteries are dominating the consumer portable-equipment market. Their benefits versus older battery technologies are many, and their continuous improvement in safety and energy capacity per weight and volume increase their market potential.
While Li-Ion (lithium ion) is the leader in this area, Li-polymer (lithium polymer) is growing fast in popularity and use, given its flexibility in terms of battery-pack form factor and, therefore, its benefits for many industrial designs. A few new Lithium-based battery technologies have been introduced to the marketplace recently. They are trying to address the need for increased safety, longer usable battery life and higher discharge rates. This, in turn, is further expanding the potential market. While the benefits of adapting these new battery technologies in new portable-system designs are many and varied, so are the challenges. However, these challenges need to be addressed, as new battery-charging and power-management implementations will be necessary for end users to cope with their idiosyncrasies and to take advantage of their strengths.
Rechargeable benefits
Rechargeable Li-Ion batteries have been the energy source of choice in the portable consumer-electronics market for some time now. Their performance and environmental benefits versus other technologies are many and significant. These include higher energy density, higher operating voltage, low self-discharge rate, no memory effect, and (relatively) environmental friendliness. Li-Ion was successful in quickly replacing older technologies, such as NiMH and NiCd, in most applications. In addition to the consumer hand-held market, Li-Ion has been slowly penetrating other market segments, such as hybrid electric vehicles and power tools, primarily due to significant technology improvements over the last few years.
As with any other battery technology, Li-Ion also has some weaknesses, which have been partially addressed via incremental improvements in the materials employed, more specifically the ones used for the battery anode, cathode and electrolytes (Figure 1). Most of the new developments attempt to address three key points: thermal stability and operation over a wider temperature range; higher discharge rates; and overall safety under stress conditions, in other terms overvoltage and overcurrent.
One of the outcomes of such improvements has been the introduction and adoption of Li-polymer rechargeable batteries. These batteries use a polymer electrolyte foil that allows them to be more flexible in terms of form, shape and size—a very attractive benefit for many space-constrained portable devices, such as cellular phones, Bluetooth headsets and MP3 players. Furthermore, the dry polymer design offers simplifications with respect to fabrication, ruggedness, and safety. Electrically, Li-Polymer batteries are identical to their Li-Ion counterparts; therefore, charging implementations for both these technologies are identical. The form-factor flexibility and robustness do come at the cost of lower energy density and decreased cycle count compared to Li-ion; however, improvements in this area are continuous.
Environmental alternatives
Two more lithium-based battery technologies have also gained momentum over the last few years and are very close to wide commercialisation. These are LiFePO4 (Lithium Phosphate) and Li-S (Lithium Sulphur). Phosphate-based batteries offer superior thermal and chemical stability; they are incombustible, thereby withstanding high temperature levels, and their 3D olivine crystal structure allows for high physical integrity. This technology is very promising for addressing safety concerns associated with today's Li-Ion batteries at a very competitive cost. While maintaining the environmental friendliness of their counterparts, these batteries also provide stable operation under overcharge and shortcurrent conditions. Another potential benefit is their lower internal resistance, which allows a lower voltage drop at the terminals under load. This in turn increases the maximum current that can be drawn from the battery—in other words, achieve a higher discharge rate). Nominal cell voltage for Li-Phosphate batteries is around 3.2V. One of the downsides of this technology is the lower energy density versus existing battery types.
The main advantage of using sulphur in the battery package is its ability to reach a specific energy of 400Wh/kg, more than twice that of Li-Ion, and an energy density of 425Wh/l, comparable to that of Li-Ion batteries. The theoretical upper limit of Li-S chemistry is as high as 2500 Wh/kg, five times the upper limit of Li-Ion chemistry. This attribute allows for a significantly lower battery weight for a given capacity, a very critical decision factor in new battery-powered electronic designs. In addition to energy density, this chemistry uses component elements that are less expensive than those of Li-Ion batteries, yet the same manufacturing tools and processes can be used to fabricate the units. Nominal cell voltage for Li-S batteries is around 2.1V, significantly lower than the ones for Li-Ion and Li-Polymer; this may introduce significant challenges to portable-system designs. The major concern with this technology, however, is its ability to provide stable and safe operation, given its very high energy density and its potential for very rapid discharge.
Future demands
These and other, emerging battery technologies will be continuously evaluated for mass adoption in new portable-system designs with a goal of addressing the following, increasing market needs. These needs are the requirement for higher energy density, allowing longer usable battery life in new, feature-rich applications without compromising industrial design. Another requirement is for higher discharge rates to enable a battery technology's adoption in applications that are particularly demanding, such as power tools and electric cars. Additionally, designs are constantly demanding improved chemical and thermal stability for improved safety, and customers are also requesting reduced costs associated with secondary-system and battery protection. Finally, there is a need for a wider voltage operating range to be available that may allow a lower-cost system design due to a lesser need for multiple DC/DC conversions.
While the benefits of using new battery technologies for new and emerging portable applications are clear, there are many challenges associated with developing the battery technologies. Unlike Li-Ion batteries that require a constant voltage regulation of 4.2V and offer a wide operating range at 3.7V, LiFePO4 batteries, as an example, require a constant voltage regulation of 3.5 or 3.6V, and their nominal voltage is approximately 3.2V. Charging algorithms, including charging rates and voltage-regulation levels, for new batteries may deviate significantly from today's, commonly used Li-Ion ones (Figure 2).
Bringing flexibility
The typical application diagram shown in Figure 3 demonstrates a flexible battery-charging solution that can adapt to many of the new battery technologies. Companies such as Summit are developing programmable battery-charger ICs, such as the SMB329, which has the ability to regulate battery voltage from 3.46 to 4.72V, thereby addressing new float-voltage requirements.
The device is based on a 3MHz, switch-mode architecture, with minimal external components, which makes it compact in size. High-efficiency operation enables fast charging due to higher output/charge currents, while reducing thermal problems of conventional linear solutions. The company's proprietary TurboCharge technology enables high charge current, even from relatively low-power sources. For example, it can provide up to 750mA output from a 500mA USB source. The high-efficiency switch-mode operation and TurboCharge mode allow for 30 to 50% shorter charging times, while reducing thermal dissipation and wasted power by 80%, compared to competing solutions. The shorter charge times are particularly designed to appeal to the consumer market. The programmable IC supports all relevant interface standards—USB2.0, USB On The Go and USB charging—as well as safety standards such as IEEE1725 and JICS8714, resulting in reduced development time.
Profile changes
For designers, there are additional charging parameters—such as pre-charge, fast-charge and termination-current levels—that have to be adjusted to accommodate a variety of charging profiles. In some devices, such as the SMB329, these parameters are adjustable. As the commercialisation of the new battery technologies finds place, battery-pack datasheets and "handling" recommendations may vary significantly due to the lack of adequate data. Using adaptive battery-charging and power-management solutions will allow design flexibility and significantly reduce the risks associated with design safety/robustness and time-to-market.
In addition to battery-charging design challenges, new power-delivery and control implementations will also be necessary to address the increased battery operating ranges. In a typical portable design, for example, as the battery voltage reaches around 3.4V—the system's cut-off voltage—the system powers off primarily due to a faster battery-discharge rate at those voltage levels, and there is an inability to regulate common 3.3V rails without additional components and corresponding cost penalties. To overcome these hurdles, new batteries will require new system-design architectures that take full advantage of the battery operating profile to achieve longer and safer system operation. This is the ultimate goal for user satisfaction.
Figure 1: The simplified structure of a Li-Ionbased battery.
Figure 2: The programmable charging profile allows design flexibility for new battery technologies.
Figure 3: An example of a typical programmable battery-charging solution.
