High-Capacity Batteries For Handheld Devices

Is the latest technology better than its predecessors?

by Samuel Payne, FAE, Future Electronics (Israel)

READ THIS TO FIND OUT ABOUT: Properties of battery technologies Low-profile batteries Battery selection at product design

The features and performance of mobile computing devices are advancing quickly to satisfy demanding users, thanks to the ingenuity of electronics engineers and computer scientists. The batteries for these devices, however, are constrained by higher powers than circuit design: physics and chemistry. In feature-rich devices, the properties of different battery technologies dictate trade-offs between size andweight, features and usage, and the recharge interval. No amount of clever electronics can alter this fact. Samuel Payne, FAE, Future Electronics (Israel) explains.

Proper consideration of the battery and its operating environment will help product developers to balance the combination of advantages and disadvantages for their application, tomaximise the performance of the end product, and to prepare for future innovations as they become available.

 

Lithium-based battery technologies

For portable computing devices fitted with rechargeable batteries, there are currently two dominant battery technologies. Lithium-ion (Li-ion) technology is themost popular and fastest growing, thanksmainly to its dominance in the laptop computermarket. Its advantages include high energy density and low weight. However, safety-protection circuitrymust be used to restrict voltage and current.

Competing with these, Lithium-ion-Polymer (Li-Po) batteries benefit from slimmer geometry and simple packaging. On the other hand, this technology tends to impose a higher cost per Watt-hour.

 

 

Table 1 summarises the characteristics of these battery technologies, and also includes an interesting comparison with Nickel-Cadmium(NiCd) batteries: NiCd has a shorter charge time, delivers a higher load current and offers a lower overall cost per cycle. This technology has, however, been banned since September 2008 in applications other than emergency lighting, power tools, andmedical andmilitary equipment. This puts it beyond the use of the majority of handheld electronics products.

The properties of lithium make it an ideal base for a battery technology. It is the lightest of all metals, has the greatest electrochemical potential, and provides the highest energy density by weight. Althoughmetallic lithium batteries proved difficult to develop,mainly due to safety problems, lithium ions provide a non-metallic base displaying only slightly lower energy density than lithium metal but offering much greater safety. In fact, the energy density of Li-ion technology is typically twice that of NiCd, while achieving similar discharge performance.

Unlike most other technologies, Li-ion is a low-maintenance material. It has no ‘memory’, so users do not have to fully discharge the battery before charging and no periodic deep discharge is required to prolong the battery’s life. Usefully, a Li-ion battery’s energy level can be monitored and displayed to the consumer in a ‘fuel gauge’ display. In addition, self-discharge is less than half that of NiCd. Another fact, increasingly important as the consumer market more actively questions manufacturers’ environmental credentials, is that Li-ion cells cause little harm when discarded.

Despite these many advantages, Li-ion has some drawbacks. It requires additional protection, for example, to maintain safe operation. Protection measures include limiting the peak voltage of each cell during charging, and preventing the cell voltage from falling too low on discharge. The maximum discharge current on most packs is up to 2C* in continuous current discharge mode, and higher when pulsed.

A key consideration in the design process is operating lifetime. In Li-ion batteries, useful life in normal operating conditions is determined by the number of charge/ discharge cycles. The manufacturer’s technical data will specify the correlation between number of cycles and any deterioration in capacity, to allow the design engineer to calculate the probable lifetime of the battery in any given application.

 

 

Li-ion batteries are available commercially in a number of formats and form factors. For applications which do not require an ultra-thin geometry, the industry-standard 18650 cylindrical format (see Figure 1) offers a low cost-to-energy ratio. Each unit is approximately 18mm in diameter and 65mm in length. An example of an 18650 battery is the Varta LIC18650-(x) family, which offers nominal energy capacities of 2200mAh, 2400mAh and 2600mAh, and can be built economically into battery packs for devices such as laptop PCs, medical and navigation equipment.

To answer designers’ requirements for a lower-profile solution, Prismatic Li-ion cells are available in flat packages. These can be as thin as 4.3mm. The Varta LPP 402025, which has a footprint of 22.5mm x 20.5mm, is one such. These reduced dimensions hurt energy storage, however: the LPP 402025 has nominal capacity of 150mAh. The cost per mAh is also higher.

Li-ion, then, is a mature and proven technology which can be reliably used in consumer applications, but which offers severely limited energy capacity when made in low-profile packages.

 

 

Li-Po for high capacity and low profile

Li-Po is a newer technology which has emerged to compete with Li-ion in low-profile applications. It is much easier to make low-profile Li-Po cells than their Li-ion equivalents. Li-Po batteries are also light in weight (see Figure 2).

The PoLiFlex line of Li-Po cells available from Varta comes in package sizes as thin as 2.2mm. This gives the designer great flexibility to determine the dimensions, shape and enclosure design of a handheld product.

In both practical and marketing terms, the thinness of Li-Po technology is by far its greatest attraction. At the same time, the Li-Po products on the market today offer slightly lower energy density than standard Li-ion cells, and are more expensive. This has limited their appeal, and has resulted in slower market growth than some analysts had predicted when the technology first emerged commercially in the 1970s. The technology is ideal, however, when thin geometries are required, such as in batteries for mobile phones and portable media players.

 

Effective battery selection process

By analysing the chemistry and relative merits of Li-ion and Li-Po admtechnologies, engineers are able to make broad-brush design decisions based on parameters such as size, capacity and cost. However, to fine-tune battery selection to align with more detailed considerations such as discharging conditions, operating/storage temperatures and charger design, close co-operation with a knowledgeable supplier is recommended.

Figure 3 shows a comprehensive checklist that battery specialists will expect to complete when assessing the needs of a particular application. Completing this checklist for a given project will help engineers to communicate their requirements accurately and thereby achieve a superior solution. The size of the checklist shows the scope for a battery expert to contribute to the evaluation process of the non-specialist.

 

 

How battery technology is evolving

In the domain of Li-ion technology, a number of important advances have been achieved in recent years. As an example, using nano-phosphate materials within the cathode has increased the power density in W/kg of commercially available Li-ion batteries. The cell can be continuously discharged to 100% depth-of-discharge at 35C and can endure discharge pulses as high as 100C. In such phosphate-based systems, the nominal voltage of 3.3V/cell and peak charge voltage of 3.6V are lower than for other technologies.

Another cathode technology offering interesting properties is lithium iron-phosphate (LFP), which offers enhanced safety as one of its main benefits, although cost remains relatively high.

 

Conclusion

Rarely, in the world of electronics, does a mature product resist attack from newer technologies. Li-ion is an exception. The appeal of Li-Po looks set to remain limited to niches that require ultra-thin geometries, due to its capacity and cost drawbacks compared to Li-ion. Indeed, as the industry’s success with nano-phosphates shows, the most significant technology advances in batteries for handheld devices seem to be those which build upon, rather than replace, Li-ion.

 

*C is the capacity of the battery, expressed inmAh. The discharge current is expressed as amultiple of C: hence a 1000mAh battery will supply 1000mA for one hour, or 500mA for two hours. Amaximum discharge rating of 2C implies that the 1000mAh battery will safely supply 2000mA continuous current. Short-term, pulsed currents can be higher.

 

©Copyright 2009 Future Electronics Ltd. All trademarks contained herein are the property of their respective owners. All applications for samples, badge boards, demo kits and other advertisedmaterials fromFuture Electronics are subject to qualification.