Battery Options and System Design Considerations: A Comparison of Primary and Secondary Battery Systems for CDMA-Based PCS Phones

Transcription

1 Battery Options and System Design Considerations: A Comparison of Primary and Secondary Battery Systems for CDMA-Based PCS Phones Jim Pilarzyk Senior Engineer OEM/Technical Products Rayovac Corporation Introduction The usability of a portable device is heavily dependent on the battery chemistry chosen to power the product as well as the battery management techniques applied to the battery technology. Determining which type of battery is best for a given product depends on which performance parameters are the most important for the application. Even with the wide array of rechargeable battery options available to product design teams, primary alkaline batteries are still a popular power system for portable, handheld devices, be it an electronic entertainment product or a connectivity tool such as a PCS phone. This paper serves as a guideline for identifying the important performance characteristics of the battery system, relative to the requirements of CDMA-based PCS phone. The performance characteristics discussed include: system self discharge, thermal capabilities, internal operating impedance over discharge, cycle life, and product safety. Along with the advantages and disadvantages of primary [Alkaline] and secondary [Rechargeable Alkaline, Lithium-Ion, and Nickel-Metal Hydride], chemistries in a PCS phone, contact theory and circuit design options that can further increase the reliability of any batterypowered product are also discussed. Performance Characteristics of Batteries Many different parameters can be used to judge the performance of battery systems. Different battery technologies have been developed which optimize one or more of these parameters, but each technology has limitations as well. Some criteria of interest for low-power and portable products are: Battery Capacity, which affects the product s continuous run-time Self-discharge, which affects how long the product can remain idle between uses Cell size and weight, which have a significant impact on portability Cycle Life, which is related to the overall cost-ofuse for the device Cell Cost and Availability, which affects the device cost and the ease of battery pack replacement by the end-user Battery selection is also based on an understanding of the thermal capabilities, effects of the operating environment, and the battery life requirements of the powered device. This requires the designer to consider two interacting paths: the consumption of the active electrochemical components and the effects of thermal wear out. It is very important to hold the paths of self-discharge and thermal wear out as separate issues. The reason for this is that self-discharge can sometimes be compensated for by increasing the specified battery capacity, while the effects of thermal wear out can only be addressed by selecting a more thermally capable battery. Consumption of Active Battery Components The first path a designer needs to consider is the consumption of the active battery components. Batteries generate an electrical current by producing oxidation and reduction reactions of their active components. Once these components have been consumed, the battery ceases to generate an electrical current. The sum of the energy consumed by the circuit over the expected life plus the inherent loss of energy due to self-discharge equals the battery capacity needed. Self-discharge of the PILARZYK 1

2 chemical system is defined as the loss of capacity, typically referenced as a percentage rate over a given period of time, regardless if the battery is installed into a circuit. Temperature acts as a catalyst to the self-discharge rate. Thermal Wear Out Effects On Performance The second path in determining battery life is thermal wear out. Thermal wear out is defined as the loss of capacity activated by thermal mechanisms. Generally, thermal wear out rates accelerate as temperatures in the operating environment rise. Under conditions of high thermal stress, the seal area of the cell may begin to oxidize, become brittle, and crack. This reduces the compression of the crimp seal, which results in electrical degradation of the cell. Also, under extended storage conditions at high temperatures, a breakdown of the separator material takes place. This breakdown results in an increase in internal impedance within the cell and a loss of operating voltage. High temperatures also cause an accelerated loss of electrolyte from within the cell. The electrolyte diffuses from the cell through and around the seal area. This loss of solvent results again in an increase in cell impedance and electrical degradation. Battery Safety and Environmental Concerns When designing a battery into an application, the chemical stability and toxicity of the battery system are primary concerns. One of the desirable traits of a battery s chemical makeup is a solid cathode material rather than a liquid. Compared to a liquid cathode system, a solid cathode is less volatile and much less rate capable in the event that the battery is subjected to an abuse condition. Battery products are being subjected to an ever growing number of agency and legislative guidelines for environmental and safety, during both their active life cycle in a product and at the time of disposal. Among the safety agencies, Underwriter s Laboratory, UL, is one of the most sought after approvals for signifying a productís safety and fitness for use. At some point in time, every battery needs to be disposed of. There are two major European regulations that impose elemental content restrictions and disposal regulations of many types of products, including batteries. The European Community Directive published in 1992, [also know as the EC 92 Directive] imposes restrictions on the amount of Cadmium, Mercury, and Lead in batteries and other products. More recently, Blue Angel, a regulatory group in Germany, published their guidelines for electronic equipment. The 1996 Blue Angel regulations address an electronic deviceís energy consumption, elemental content of its components, and product disposal. Competitive Battery Technologies To satisfy the power requirements of a CDMA-based PCS phone, there are three vanguard cell chemistries that are available to the designer, Nickel-based systems such as Nickel-Metal Hydride, Lithium-based, Lithium-Ion, and Alkaline cells in both primary and secondary [rechargeable] formats. Nickel-Metal Hydride [NiMH] With an energy density nearly twice that of its Nickel Cadmium [NiCd] counterpart, NiMH cells offer designers the ability to double the usage time of a device over a NiCd powered product. Like NiCd, the NiMH cells are a 1.2 Volt per cell system with a very similar internal impedance profile. With its stable voltage profile, decent pulse-rate current drain capability, and its acceptance of quick recharge regimes [up to a 3C rate], NiMH cells are one of the leading choices for portable and handheld products. Unlike NiCd batteries, the NiMH chemistry does not suffer from the memory effect condition. This nemesis of NiCd cells is brought about by repetitive shallow discharge cycles of cells in an application, rather than full, complete discharges to the cell s 1.0 Volt endpoint. NiMH cells perform well in applications where the use and duty cycle of the device is well understood and predictable, combined with a medium to high frequency of use. For example, a cellular phone powered by NiMH and used regularly as a business tool, extends the user s talk and standby time significantly over a comparable NiCd battery pack. However, because of its high self-discharge rate, about three percent per day, an infrequent user of a cellular phone with a NiMH battery can be plagued with short battery life unless they remember to recharge the batteries prior to use. PILARZYK 2

3 Lithium-Ion The Lithium-Ion [Li-Ion] battery is a relatively high energy density rechargeable system which allows designers to reduce the form factors of their product. Depending on the amount of battery capacity required for the application, the size of the device can be reduced dramatically using this chemistry as the power source. Unlike metallic Lithium cells, Li-Ion keeps the Lithium in an Ionic state. Metallic lithium is very volatile and reactive, to the point of burning in the presence of water. Lithium Ion cells retain the electrochemical properties of a Lithium cell [i.e. Voltage profile] while eliminating the inherent safety issues associated with metallic Lithium. Li-Ion cells, also known as rocking chair technology [RCT] because of the way the ions are shuttled back and forth between the charge and discharge function, relies on intercalation of the Lithium Ions into the active material on charge and de-intercalation during discharge. From a design standpoint, the nominal 3.6 Volts of this Lithium system can reduce the overall size and weight of the battery portion of the device. Figure ma Discharge Curves Two key benefits of the rechargeable alkaline system are low self-discharge and excellent shallow-discharge performance (Figures 2 and 3). Primary Alkaline Given the wide array of battery options available to product design teams, primary alkaline batteries are still one of the dominant power systems chosen for portable, handheld devices, be it an electronic entertainment product or a productivity tool like a handheld PC. The clear advantage of using primary alkaline cells is reliability the power is there when you need it no concerns about shelf-life; no concerns about retail availability of replacement cells. The big drawback can be cost, especially if you re a heavy user of a power hungry device. Rechargeable Alkaline A comparison of discharge curves between rechargeable alkaline, primary alkaline, and NiCd AA-cells is shown in Figure 1. The discharge capacity of rechargeable alkaline cells fades with each cycle, with the majority of the fade occurring in early cycles. Figure 2. Estimated Charge Retention (20 C) These characteristics make the rechargeable alkaline system ideal for intermittent-use and/or frequentrecharge applications. Figure 3. Partial Discharge Response (Voltage at the end of 200 ma / 90 min. discharge) PILARZYK 3

4 Ideal applications for rechargeable alkalines include lowand moderate-power portable applications such as HPCs. cordless phones, CDMA phones, or portable terminals. Power Requirements for CDMA-Based PCS Phones The present generation of CDMA, known as cdmaone, has been standardized and is in widespread use for cellular and personal communications in many countries worldwide. CDMA is a proven technology, providing the highest voice quality of any system to date, and is evolving to support efficient medium rate data. As part of the evoluation of this technology, the wireless industry is actively addressing the system requirements needed to provide even higher voice quality and extended battery life. Among the third generation IMT-2000 systems are Qualcomm s cdma2000 and Ericsson./Siemens W-CDMA. However, before these proposals can converge into a single, global third generation system, there are several principles dealing with single chip rate, codec techniques, and waveforms that need to be addressed. Regardless of how these principles are incorporated, the resulting third generation CDMA will deliver higher voice quality, greater battery to the end user, and, ultimately, introduce a larger and more diversified group of users to CDMA technology products. The third generation IMT-2000 system delivers better battery life in a number of ways. One of the ways is during calls. The transmitter is on continuously, but transmits and receives only in allocated time slots. Here, the mobile unit uses slotted reception when it is not on a dedicated traffic channel. Most of the circuits are turned off during the slots in a cycle which are not assigned to that particular mobile station. They are powered-on only in time to receive the assigned slot. Likewise, a variable rate transmission is utilized whenever possible to reduce the required transmission power. This, combined with power control methods, also reduces the transmitted power. From a battery-standpoint, this means longer life for the medium to heavy users of the system. From an emerging market standpoint, these lower drains can now be levereged to develop a lower cost phone for the infrequent or low call volume user. Because of the lower power requirements of the phone, discrete alkaline cells, especially rechargeable alkaline cells, are now a cost effective and viable alternative for this new market. Below is an example of what a CDMA phone, incorporating the third generation system, may require, Figure 4. Figure 4. Approx. Drain Rate for Third Generation CDMA Phone using IMT-2000 system standards The reduced power requirements could yield up to six hours of talk time or 90 hours of stand by from a set of Alkaline AA-size cells. Alkaline AAA-size cells would yield about half of those values, respectively. Figure 5. AA Rechargeable Alkaline Cell Capacities at 300 ma continuous discharge [talk time]. Using AA-size rechargeable alkaline cells in this example, Figure 5, outlines the number complete talk time cycles from a single set of cells. Under partial discharge conditions, however, rechargeable alkaline cells can last hundreds of cycles. Figures 6-8 illustrate the response of a rechargeable alkaline cell subjected to short, repetitive discharge, simulating a 10 minute phone call. PILARZYK 4

5 This also illustrates that the memory effect phenomenon, prevalent in some Nickel-based chemistries is not an issue for the rechargeable alkaline chemistry. Figure 6. AA Rechargeable Alkaline Talk Time Simulation [10 minute call] Cycles 1-4 After each discharge, the cell is recharged. After several hundred of these short cycles, the cell response was not significantly different than on the first few cycles. Figure 7. AA Rechargeable Alkaline Talk Time Simulation [10 minute call] Cycles After every 200 of these short discharges, the cell was fully discharged to demonstrate that the capacity of the cell was still available for long calls or use patterns. Contact Reliability The design criteria for a device which uses discrete cells has additional contact reliability concerns not associated with a battery pack design. When designing a battery contact system for discrete cells, both the mechanical and electrical attributes of the contact design must considered. The mechanical characteristics of the design are somewhat more intuitive than the electrical attributes. For example, the normal or axial force exerted on the cells must be sufficient to not only make physical contact but also be high enough to hold the cells in place during the use of the product. The use pattern of the device includes shock, vibration and drop testing parameters. For AA form-factor cells, the amount of normal force to reliably hold the cell in place is approximately 200 grams. The mechanical integrity of the contact material must be robust enough to last the life of the product. This mandates that the contact material maintain its permanent set and any degree of stress relaxation kept to a minimum. The base metals which best meet this criteria include Beryllium Copper, Phosphor-Bronze, and basic spring steel. Electrically, oxidation and corrosion of the contacts are the paramount concerns. As discussed above, it is the magnitude of this axial force which establishes and maintains a gas-tight interface between the contact surfaces. Contact theory states that when two flat surfaces are brought together, they appear to form a large continuous surface. In reality, these smooth surfaces are made up of peaks and valleys and it is only these asperities or Aspots which make contact. The A-spots constitute as little as one percent of the apparent surface area. Any micro-movement of these surfaces can cause surface oxidation at the A-spots which ultimately results in a loss of intimate metal-to-metal contact between the cell and the contact material. Unless the integrity of this surfaceto-surface connection is maintained, corrosive contaminants and oxides can form at the battery-to-contact interface, resulting in electrical instability. Figure 8. AA Rechargeable Alkaline Talk Time Reserve Capacity every 200 short cycles [300 ma] PILARZYK 5

6 Dual Use Contact System For Primary and Secondary Alkaline Cells By designing a contact system that exploits the mechanical differences between Renewal cells and other cell chemistries of the same cell size, a contact scheme can be created which allows for the discharge of any voltage-compatible cell chemistry but discriminately charges only Renewal cells. This duality offers the user the option of secondary battery cost savings without giving up the convenience typically associated with primary alkaline cells, should the need arise. Mechanical Uniqueness of Rayovac Rechargeable Alkaline AA & AAA Size Cells The plastic label overwrap of the primary and secondary batteries available in the consumer market extends over the top edge of the positive can onto the face or nubbin end of the cell. With Rayovac s Rechargeable Alkaline AA & AAA Figure 9. Rechargeable Alkaline AA & AAA cells have a patented label design which leaves the an exposed are of the positive electrode available for the charge contact whereas the insulating plastic overwrap of a primary AA or AAA prohibits electrical contact from being made. cells, this area is left exposed, Figure 9, whereby this metal surface can used as a mechanical means of identifying a rechargeable alkaline cell. Discriminating Charge Contacts As illustrated in Figure 10, this patented label overwrap design of the Rechargeable Alkaline cell allows the cell to be charged at the exposed area along the edge of the can of the cell. The nubbin contact permits discharge of any size-compatible cell, regardless of chemistry. Memory Protection Devices, Incorporated [MPD] of Farmingdale, New York, has developed a complete set of low-profile contacts which includes a discriminating charge contact for use with Rayovac Renewal AA & AAA cells. This high quality, spring steel contact system is currently available to OEMs and value-added assemblers world-wide. Design Option: Rechargeable Alkaline Charge Regime The life-cycle performance of rechargeable alkaline battery packs can be optimized by the use of appropriate battery management techniques. The battery management requirements for rechargeable alkaline chemistry differ from those of other systems in a few key areas. The preferred charging method for these batteries is a voltage-controlled pulse-charge, as described in references 1 and 3. As with any rechargeable system, discharge should be terminated on a low-battery condition to prevent over-discharge of the cells. Rechargeable alkaline systems place a few additional requirements on the Figure 10. Rechargeable Alkaline discriminating charge contact system in AA form-factor. battery management circuitry. These are: End-of-discharge termination threshold should be adjusted for typical load conditions- high currents allow lower termination voltage, low currents require earlier cutoff Individual cell monitoring should be used to avoid overcharge or overdischarge of any cell in the pack, maintaining cell matching. The circuit also needs to implement power-management features to maximize operating and storage life of the battery pack. Further discussion concerning battery management for rechargeable alkaline systems can be found in references 2 and 3. Conclusion With the inclusion of the Rechargeable Alkaline charge contact, the addition of A/D lines to the contact points between the cells, and the rechargeable alkaline charge regime incorporated into the battery option choices, rechargeable alkaline cells can add significant value to a low cost, third generation CDMA phone by combining the intuitive use of alkaline cells with the economic benefits of a rechargeable cell. References and Sources 1. Charging Characteristics of Lithium-Ion Batteries, Anthony Wang, National Semiconductor Corp. 2. In-System Charging of Rechargeable Alkaline Batteries, Paul Nossaman and Jehangir Parvareshi, Benchmarq Microelectronics Inc. 3. Reusable Alkaline Technology, Upal Sengupta, Rayovac Corp. 4. Enhancing the Usability of Portable Products: Rechargeable Alkaline Technology. Jim Pilarzyk, Rayovac Corp. 5. Intelligent Battery Management For Rechargeable Alkaline Battery Packs. Upal Sengupta, Rayovac Corp. 6. The Technical Case For Convergence of Third Generation Wireless Systems Based on CDMA. Qualcomm. 7. The cdma2000 RTT Candidate Submission. International Telecommunications Union Study Group. 8. WCDMA/NA RTT. Stephen Hayes, Ericsson; Tony Chu, Siemens. PILARZYK 6