Application Note July 2005 AN126.0 Author: Carlos Martinez, Yossi
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Smart Battery Primer
Application Note July 2005 AN126.0
Author: Carlos Martinez, Yossi Drori Ciancio
Introduction
Rapid increases product performance features along with demands longer operating life have driven designers battery based electronics consider significant changes product design. These include using lower voltage components, turning unused subsystems, managing applications software developing "smarter" batteries battery management. Designers battery systems need knowledge many different, diverse and, many cases, fields. These fields range from battery chemistry knowledge battery operates; system engineering knowledge various components interact; product design knowledge user operates particular device. This application note introduction overview battery operation terminology, provides initial discussions portable system user design considerations. intended provide some building blocks needed understand complex issues involved with battery portable system design.
Cadmium (Ni-Cd,) Nickel Metal Hydride (NiMH) Lithium (Li+ Li-Ion). Lead acid batteries typically used automotive applications fixed installations because their large size weight. Newer Lithium technologies (such Lithium Polymer) emerging, they commercially available quantity this time. Lithium Polymer cells expected appear starting year 2000.
Terminology
Before getting into battery specifics, important understand some terms used defining characterizing batteries. ELECTRODE Electrodes positive (cathode) negative (anode) terminals cell. These made different materials, depending cell chemistry. farther apart these materials Standard Potentials Table, higher electronic potential oxidation reduction chemical reactions higher voltage produced cell. ELECTROLYTE electrolyte chemical that separates electrodes provides medium conduction ions intermediate compounds between electrodes. intermediate compounds ions, result from chemical reactions anode cathode carry current through battery. electrolyte usually some type liquid paste. VOLTAGE voltage cell based chemical composition electrodes electronic potential that results form oxidation reduction potential chemical reactions electrodes. There number terms used when discussing battery voltage. Open circuit voltage voltage cell with load applied. This usually good approximation theoretical cell voltage. Nominal voltage typical operating voltage rated voltage cell. example, nominal voltage NiCd cell 1.25V. Discharge Cut-off voltage Under Voltage Limit) specified voltage discharge also called voltage. This term ambiguous. Sometimes refers minimum operating voltage system, voltage dependent application. Cut-off voltage also apply voltage which there damage cell brought irreversible processes, voltage determined cell. diagram Figure uses term voltage describe
Battery Basics
battery made more cells. These cells chemically store energy system. There types cells. primary cells chemical reaction reversible, they must discarded (thrown away) when chemicals consumed. Secondary rechargeable) cells provide energy transforming chemical compound into another. These cells recharged putting energy into cell order convert chemicals back their original state. Primary cells typically have higher capacity size weight lower self-discharge rates1 than rechargeable cells. However, using primary cells systems with high energy demands constant economically practical. need replace these cells often both inconvenient expensive. Primary cells best used power intermittent applications, such flashlights, radios, calculators some PDAs. Primary cells today need built intelligence. Their "throw away" nature means there need fuel gauging, safety monitoring recharge control circuits. such, primary cells will discussed further this application note. Rechargeable cells have been around long time variety chemistries. different characteristics each chemistry make more suited than others particular application. Currently most common types rechargeable batteries Sealed Lead-acid (SLA), Nickel1. Terminology section.
CAUTION: These devices sensitive electrostatic discharge; follow proper Handling Procedures. 1-888-INTERSIL 1-888-468-3774 Intersil (and design) registered trademark Intersil Americas Inc. Copyright Intersil Americas Inc. 2005. Rights Reserved other trademarks mentioned property their respective owners.
Application Note
voltage which cell longer supply voltage system term cut-off voltage describe voltage where damage cell occurs. Ideally system should designed with voltage close possible cut-off voltage. This allows maximum usable capacity battery. Working voltage term that refers operating voltage range cell under load. This voltage between open circuit voltage voltage. Charge Cut-off voltage Over Voltage Limit) specified voltage charge cycle. charge cut-off voltage level where electrode chemicals have been converted with minimum gassing overcharge reactions. This voltage typically higher than open circuit voltage fully charged cell. Li-Ion battery chemistries, charge discharge cut-off voltages very important have very narrow tolerances between acceptable unacceptable levels. Violating charge discharge cut-off voltage limits result cell damage explosion. Since movement ions affects internal resistance, temperature have impact. example, cold conditions, ions move more slowly, raising internal resistance. Since higher internal resistance results lower peak current, some devices that have intermittent high current demands, such cell phone, operate perfectly well room temperature, fails when making call outside cold morning. Internal resistance also increases battery discharged. This explains nearly empty battery operate radio (with peak current requirements), fails when used radio controlled (with high peak requirements). Finally, battery ages, internal resistance increases, failures possible with battery, even when fully charged. CURRENT Current produced electron flow electrode materials, flow electrolyte, charge/transfer reactions interface electrode/electrolyte. Current flow affected internal resistance, temperature size electrodes cell. CURRENT CAPABILITY Regardless chemistry, current capability function surface area electrodes. more surface area, higher current that obtained. This means that, using same battery technology, system needs physically larger batteries higher current applications. CAPACITY Cell capacity essentially amount energy available specific application until cell reaches (cutoff) voltage. Capacity usually expressed milliamperehours (mAh). battery with capacity 1500mAh hour rate) pass electric current 1.5A hour. However, available capacity battery decreases when discharged higher current. battery compared drum water with hole bottom. water drum comparable capacity size hole equivalent surface area electrodes. application, total cell capacity always same available cell capacity. cell fully charged system, there less available capacity, even when total capacity remains same. cell discharges, system often shuts down prior voltage insure "graceful" system power shut down. This also decreases available capacity. Systems that utilize more total cell capacity increase their time.
separator porous layer that placed between electrolyte electrodes. addition providing separation, pores some newer film materials designed smaller temperatures above 120°C restrict current flow increase safety. Protection devices intended "open circuit" when temperature rises above certain level.
Charge cut-off voltage
Open circuit voltage
Voltage
Working voltage
voltage Unused Capacity Discharge cut-off voltage Time FIGURE VOLTAGE DEFINITIONS
INTERNAL RESISTANCE Internal resistance internal load within cell. This load limits maximum current that flow through battery. ideal battery, internal resistance zero. Internal resistance made many small losses. These largely related restriction migration ions. electrolyte material, through which ions travel, thickness electrolyte layer, restrict movement. choice electrode materials, battery assembled, increase internal resistance. Assembly factors include method connecting electrodes battery terminals, type thickness separator2 layers, connection various passive protection devices attached cell3.
AN126.0 July 2005
Application Note
RUN-TIME This function long battery sustain current needed application before reaching cut-off voltage. Regardless chemistry, greater quantity material anode cathode longer battery sustain required current. Cells designed longer time increasing thickness electrodes, however cell designed longer time will have lower current capability, unless battery size increases. ENERGY DENSITY Energy density defines amount energy available battery. measurement energy density based volume (Watt-Hrs/Liter) weight (Watt-Hrs/Kilogram). portable applications, both volume weight critical. different types batteries about same size same energy density, then that less weight generally prevails. This seen recent migration from NiMH Lithium cellphones PCs. Lithium NiMH have about same volumetric energy density, Lithium higher gravimetric energy density. DISCHARGE This conversion chemical energy cell electrical energy, with electrical energy being dissipated into load. DISCHARGE RATE discharge rate amount current taken from battery over period time. discharge rate often defined terms battery's capacity called Rate Specification". following expression refer discharge rate:
Discharge manufacturers rate Discharge rate
Voltage
Discharge rate
FIGURE DISCHARGE DIFFERENT RATES
Cells operating very high discharge rates, such 10C, behave very differently from nominally similar cells because design construction difference. example, cells that were designed long life current drain designed with thick anodes cathodes, whereas cells intended bursts high current designed maximize electrode surface area. DEPTH DISCHARGE degree discharge expressed percentage. When battery reaches voltage condition, battery reached 100% depth discharge. term "deep discharge" generally refers withdrawal least rated capacity cell. Increasing depth discharge decreases maximum number charge/discharge cycles possible from cell. Figure
Where discharge current, rating cell specified C-rate (from manufacturer) Cell discharge rate application. Specified multiple fraction battery rated 5000mAh rate, when discharged rate (M=0.2) will provide 1000mA hours. Since capacity changes based discharge rate, battery above, when discharged C/10 rate 500mA,
Number cycles NiCd Li-Ion
500mA 5000mAh
will more than hours provide more than rated 5000mAh capacity. When discharged rate 5000mAh, battery will less than hour deliver less than rated 5000mAh capacity. Figure
Depth discharge
FIGURE CYCLE LIFE DEPTH DISCHARGE
AN126.0 July 2005
Application Note
SELF DISCHARGE battery sits shelf, connected host device extended periods without being charged discharged, certain amount electric energy lost internally, reducing capacity battery. This loss accelerates with heat. This very important consideration when choosing battery technology. alkaline primary cell good flashlights, because very little self discharge. NiCd battery, however, very poor this application (without constant charging), because self discharge month. CYCLE cycle refers process discharging, then charging cell. There different measures cycle. When manufacturer rates cell cycle life, cycle refers fully discharging cell voltage then fully charging charge cut-off voltage. fuel gauging applications, during typical usage, however, cycle also refer charging discharging battery levels between fully charged fully discharged. COMPARING CHEMISTRIES Today's high performance portable products migrating Lithium type batteries, because their characteristics high capacity weight volume. However, perspective various battery chemistries, discussion below highlights some characteristics main types rechargeable batteries.
Voltage
Discharge Capacity FIGURE TYPICAL NiCd DISCHARGE CURVES DIFFERENT RATES 20°C
NiCd batteries typically recharged using constant current. Most NiCd batteries safely charged without electronic control current rates C/10 C/3. Higher rates charge, required fast recharge applications, must electronics detect when charge cycle complete. Some mechanisms used when charging NiCd batteries detect charge listed below.
Voltage (Volts) Voltage (C/10 rate) Temp (C/3 rate) Temp (C/10 rate) Charge Capacity) Voltage (C/3 rate)
Temperature (°C)
Nickel Cadmium (NiCd)
Nickel Cadmium batteries contain positive electrode made nickel hydroxide, negative terminal made with cadmium compound electrolyte potassium hydroxide. chemical reaction anode NiCd battery
2OH- (discharge goes left right, charge goes right left)
This reaction electric potential -0.81V. chemical reaction cathode NiCd battery
OH(discharge goes left right, charge goes right left)
FIGURE NiCd CHARGE PROFILE
This reaction electric potential 0.52V. This gives theoretical voltage NiCd cell
NiCd Cathode Anode (0.52) (-0.81) 1.33V
Standard method. This technique uses only resistor between power supply battery. uses other electronics. battery then charged C/10 rate. This method limits temperature rise. Circuitry terminates charge when battery charged 140-150% charge input. limitation this method that very slow recharge time. Timer Control. timer used reduce current from higher rate lower "trickle charge" state (see below). This technique used where battery frequently charged without prior deep discharging since this result overcharging battery. Without periodic deep
AN126.0 July 2005
Application Note
discharge, NiCd battery develops reversible condition, known "voltage depression" "memory effect", where delivered capacity gradually decreases. Using timer that fixed recharge time cell overcharge cell experiencing memory effect. such, timer technique would normally also include thermal cut-off control. Temperature detection. This circuit uses sensor detect temperature rise cell terminate charge. This method adversely affected ambient temperature result under charge conditions when charged high ambient temperature environments over charge ambient temperature environments. Negative delta This preferred methods determining charge NiCd batteries, because independent ambient temperature. battery charged voltage rises. battery passes full charge, however, voltage starts decline. This negative charge battery voltage determines charge. negative voltage 10-20mV typically used negative voltage change. Charging rates must greater than this technique useful, since delta otherwise detect. Trickle Float charge. Trickle charging refers very small amount current applied battery. This technique used when battery continuously connected charger supplementary charge fast charge cycle. Trickle charge rates range from 0.02 0.05C, depending frequency depth discharge. three main rechargeable chemistries hand-held applications, NiCd oldest least capacity size weight. avoid memory conditions, NiCd batteries often fully discharged prior charging. These factors make NiCd unattractive small, high performance light weight portable products. Because their very internal resistance, however, they still best choice power tools other high peak current applications. NiCd batteries need great deal electronics built-in, though there increasing electronics NiCd battery packs better measurements remaining energy. NICKEL METAL HYDRIDE (NIMH) Using chemistry similar NiCd, Nickel Metal Hydride uses nickel hydroxide positive terminal, uses hydrogen absorbing alloy negative terminal. Like NiCd battery, NiMH uses potassium hydroxide electrolyte. chemical reaction anode NiMH battery
(discharge goes left right, charge goes right left)
This reaction electric potential -0.83V. chemical reaction cathode NiCd battery (discharge goes left right):
OH(discharge goes left right, charge goes right left)
This reaction electric potential 0.52V.
NiMH Cathode Anode 0.52 0.83 1.35V
Voltage
Discharge Capacity FIGURE TYPICAL NiMH DISCHARGE CURVES DIFFERENT RATES 20°C
NiMH batteries charged using techniques similar NiCd batteries. NiMH, however, requires more monitoring greater sensitivity overcharging. NiMH battery often charged with constant current, with current limited some rate avoid excessive temperature rise. Mechanisms terminating charge techniques allow faster charging shown below.
Voltage (Volts) Voltage (C/10 rate) Temp rate) Temp (C/10 rate) Charge Capacity) Voltage (C/3 rate)
Temperature (°C)
FIGURE NiMH CHARGE PROFILE
AN126.0 July 2005
Application Note
Timer Control. This technique same NiCd type cells typically used only charge rates, because charge state battery cannot determined prior charging. most often used only complete recharge after using some other charge technique. Negative delta While this often used NiCd batteries effective NiMH types. With NiMH, voltage peak prominent exist less than charge rates, particularly higher temperatures. voltage monitor circuit must detect 10mV decrease voltage determine charge, must sensitive that noise other factors causes premature charge. Voltage Plateau. This method similar negative delta circuitry looks when voltage peaks slope voltage curve zero. risk overcharge less than with negative delta trickle charge complete full charge operation. Temperature detection. This circuit uses sensor detect temperature rise cell terminate charge. This method affected ambient temperature, result undercharged battery when charged warm environment. This technique often used with other methods. Delta temperature cut-off. This method measures battery rise above ambient temperature cuts when exceeds threshold. This threshold level varies based cell size, number cells pack, heat capacity battery. cut-off level determined separately each battery pack design. Rate temperature increase. This technique eliminates effects ambient temperature. this method, change temperature over time monitored charge terminated when particular incremental temperature rise reached. This preferred method detecting charge NiMH, since provides long cycle life battery. Using NiMH cells results battery pack with more capacity smaller package with less weight than NiCd, making them better suited portable applications. They also more environmentally friendly, since they contain Cadmium. However NiMH batteries supply peak current that NiCd offers, they provide gravimetric energy density Lithium batteries. NiMH displace NiCd because environmental concerns. This already happening Europe. However, NiMH batteries must overcome their peak current limitations they will totally displace NiCd power tools other high current applications. many light weight portable applications NiMH already being replaced Lithium Ion. future NiMH certain. present time, NiMH offers more environmentally friendly battery than NiCd lower cost than Lithium Ion. Electronics being added NiMH batteries better monitor charge state give user more information about remaining capacity.
Lithium
There rapid move Lithium (Li-Ion Li+) batteries portable products need increased performance along with smaller size less weight. Li-on batteries today provide highest volumetric gravimetric capacity other production rechargeable battery, though cost higher. Lithium batteries physically constructed with three main layers. These positive electrode plate, negative electrode plate separator. basic operation Lithium-Ion battery consists movement only Lithium from electrode other. anode contains compound that receives ions during discharge gives ions during charge. cathode operates reverse, giving ions during discharge receiving them during charge. Lithium battery chemistries relatively evolving rapidly. compounds being developed improve capacity reduce size cell. gain market share improve performance their cells over competition, different manufacturers have developed their combinations chemical compounds anode cathode. more learned compounds developed more changes Li-Ion chemistries expected. These changes will also impacted required capacity increases declining costs. Since each manufacturer strives develop better battery they different combinations chemicals electrodes. positive electrode uses compounds such lithium cobalt oxide, lithium nickel oxide lithium manganese oxide. negative electrode typically uses graphite amorphous carbon (coke). However other, more exotic compounds have been proposed. Since each combination compounds different electrical potentials, operating voltages charge/discharge characteristics differ from battery battery. Further unknowns arise with Lithium-Ion Polymer batteries, which substitute polymer compound liquid electrolyte normal LiIon cell. overall chemical reaction Li-ion battery quite complex. Using Lithium Cobalt oxide overall reaction (including both anode cathode)
(discharge goes left right, charge goes right left)
This reaction electric potential 4.2-4.3V.
AN126.0 July 2005
Application Note
Li-ion batteries require circuitry carefully monitor over voltage under voltage conditions, since these lead battery failure capacity reductions. circuits must also monitor over current short current conditions since these lead temperature increases cell that also result catastrophic failures. backup, most Li-ion battery packs monitor internal temperature shut pack down event significant temperature increase. wide range electrode materials complicates pack design. Since Lithium batteries more volatile than either NiCd NiMH, electronics required each pack insure that operation does exceed specified limits. electronics must able handle varieties Li-Ion batteries provide maximum safety, battery cycle life time. Table
TABLE BATTERY CHEMISTRY COMPARISON BATTERY PARAMETER Energy Density (Wh/Kg) Energy Density (Wh/L) Operating Voltage Open Circuit Voltage Voltage Average self discharge (per mo.) Internal Resistance Fast charge current Charge method NiCd 50-85 150-180 15-20% 3.5300mW constant current NiMH 75-100 220-300 20-30% 19800mW Li-Ion 110-130 270-320 4.1-4.3 2.0-2.3 6-10% 300500mW
Voltage
Discharge Capacity FIGURE TYPICAL C/LiCoO2 Li-ion DISCHARGE CURVES DIFFERENT RATES 20°C
mechanisms charging Li-ion batteries different than those charging Nickel based batteries. Li-ion uses constant current, constant voltage charge mechanism. charger applies constant current battery until voltage reaches desired voltage, then holds voltage constant current declines. Figure pulse charge techniques being explored. These chargers apply pulses fixed current variable rate duty cycle. When current shut circuits monitor voltage cells pack. cell voltages close charge cutoff voltage, pulse rate duty cycle) declines, though charge current remains constant. Pulse charging circuits promise reduce charge time, increase safety, improve cell balancing techniques increase amount charge cells. However, more battery research required before this charging technique approved battery manufacturers.
constant constant current current/con stant voltage DT/Dt 60°C Minimum Current -20°C 60°C High
charge Voltage Charge/Discharge cycles Battery Voltage Charge Current Capacity Operating Temp Range Cost
Peak Volt Detect (PVD) 1500 -20°C 60°C
Current Capacity
Notebook Application Example
portable computer today exemplifies need higher capacity batteries demonstrates trend increased intelligence power management. 1996, typical portable computer consumed about watts generated internal temperatures 34°C. Through 1996 1997 performance increased, screens became larger, disk drives packed more bits. result, power consumption temperature increased. obvious 1997 that batteries would capable providing necessary power system heat generated would result customer discomfort injury. Without system
Charge Time FIGURE Li-ion CHARGE CHARACTERISTICS
AN126.0 July 2005
Application Note
power management, Intel estimated that 1999 power consumption would more than double, 35W, generate case temperatures over 50°C (122°F). result, Intel, Microsoft others created portable power standards, which defined design hardware, operating system software applications better manage power consumption. Additionally, goal maintain increase battery life reduce physical dimensions while keeping power dissipation below specified maximum watts. There several ways address increased system performance smaller packages with lower operating temperature longer battery life. Some these are: Turning sub-systems that being used. This takes intelligence operating system well hardware architecture peripherals. Turning software that needed. This takes intelligence application software well understanding user preferences habits. example, text editing programs need operate maximum processor performance unless graphics added document gets larger. Reducing operating voltage: power used system drops square voltage system components. Battery Selection Intelligence: Using small, lightweight battery with more capacity coupled with electronics increases overall energy system. electronics provides more charge battery (while maintaining safe conditions) with more accurate voltage current sensing. electronics also provide better monitoring remaining capacity system operate closer battery charge condition. Many systems today stop operating with more battery capacity remaining, because uncertainty fuel gauge desire "graceful" system power down. Electronics battery pack also speed charge cycle allowing fast charge techniques while maintaining battery safety. Faster charging means more operating time day. configuration. Depending available space system power consumption, designer then specifies level power management system. typical notebook might four series combinations parallel Li-Ion cells. This termed 4S-2P pack. each cell nominally 3.6V with 1350mAh capacity, then overall pack provides 2700mAh 14.4V.
FIGURE 4S-2P ARRANGEMENT CELLS BATTERY
Lithium-Ion Cell Limitations
discussed earlier, Lithium batteries achieving performance portable devices near future. They have excellent volumetric gravimetric energy density. However, they also present some serious design safety issues that must solved. Lithium batteries need special safety circuits every battery pack monitor over-charge, under-charge short circuit conditions. these conditions occur, battery pack must "shut down." worst case, over charging lithium batteries result sudden, automatic, rapid disassembly (explosion). best case, overcharging lithium batteries result damage cells, reducing capacity cycle life. battery pack safety circuit limits maximum charge voltage prevent unsafe conditions. However, overvoltage limit significantly reduces time capacity given over-voltage limit high result damage cells. Similarly, under voltage limit must accurately, since over discharging Li-Ion battery results chemical changes that irreversible, reducing capacity cycle life, while stopping discharge soon leaves usable capacity battery. safety unit must, therefore, have correct overvoltage under-voltage limits these limits must accurate very narrow range. Because critical nature safety circuits battery pack, some systems employ redundant backup mechanisms shutting down battery pack switching load from cells. Lithium batteries need very precise electronic monitors safety circuits built into battery pack, pack also needs other electronic content. pack system designer must implement cost effective hardware software provide user with greatest possible time most accurate information possible status battery. this requires ability monitor
AN126.0 July 2005
Battery Pack Configurations
Battery packs consist more cells. Serial connections provide higher voltage, while parallel connections provide higher current capacity4. Some packs utilize combinations serial parallel cells. Typically there trade-off between available space, capacity (run-time) required voltage. battery powered design should actually start with consideration battery space, weight capacity. Knowing this having knowledge operating voltage various system components plus total system power requirements, designer determines cell
Higher capacity might available single cell instead putting devices parallel, however thickness cell would increase, resulting increased height pack. some applications, such there more limitations pack height than length width.
Application Note
current flowing into taken battery over wide dynamic range5. circuit must factor temperature, battery cycle history, battery chemistry, charge/discharge state, charge discharge rates, battery age, perhaps other conditions (such violations temperature minimum/maximum, over current conditions over/under voltage excursions) achieve highest accuracy gauge remaining capacity battery life. Finally, battery pack electronics must very small, within ever smaller battery pack geometries. Space requirements depend desired level pack functionality level electronic integration.
VCELL1 balance VCELL2 balance VCELL3 balance VCELL4 balance
Cell Balancing
State Lithium batteries from mainstream manufacturers have high standard quality, with voltages variations less than 50mV cell cell. careful examination operation protection circuits, however, reveals obstacle that pack designer needs consider. Since, with most protection circuits, pack stops charging when cells reaches overvoltage limit, there other cells that below limit fully charged. Additionally, pack will shut down when cells reaches minimum voltage, even though other cells minimum value. mismatch voltage between cells effects. First, reduces overall capacity pack. cells fully charged discharged, even though electronics sense that pack fully charged discharged. This leads reduced time. Second, having cells charged discharged different values leads increasing pack imbalances reduced cell life. Another factor that leads cell imbalances temperature. This especially true newer portable that have high performance that generates more heat than surrounding circuits. Placing battery pack close proximity that result temperature gradient across battery pack 20°C. This cause cells pack charge discharge vastly different rates, accentuating cell disparities. typical cell balancing circuit provides mechanism dealing with these potential actual cell imbalances. resistor across each cell parallel cell combination) either discharges specific cell diverts charging current from particular cell. Through software algorithms and/or hardware, cell balancing scheme strives keep battery cell voltages equal each other (with specified tolerance).
FIGURE CELL BALANCING CIRCUITS
There several ways implement cell balancing selection resistor depends implementation method, application, cell chemistry. Some cell balancing options are: When cell reaches overvoltage condition, charging stops over-voltage cells partially discharged through resistor during next discharge cycle. When cell reaches over-voltage condition, charging stops. Marking over-voltage cells this time allows controller partially bypass over-voltage cells while other cells continue charge normally. When pack recovers from under voltage condition, even during normal operation, voltages between cells compared overvoltage cells partially bypassed necessary during charging sequence equalize cells. Based history pack, some cells might partially bypassed during charge discharge based specific conditions. Each these techniques, combinations variations improve capacity life cells pack. When designing cell balancing circuits, important keep mind that Li-ion batteries have large capacity (1500 mA/h typical, with capacities 2000mAh near future), balancing cell voltages either takes long time (tens hundreds seconds) requires that handle great deal current (amps) balance within seconds. either case, best time cell balancing most appropriately conducted charge condition, when voltage higher drops faster under load than does cell working voltage. Note: voltage also drops quickly charge condition, typically this unacceptable time balance pack draining extra charge from cell, since leads premature system shut down.
AN126.0 July 2005
system idle state might consume little milliamps, while pulses several amps uncommon portable computer graphic subsystems during spin-up disk drives.
Application Note
Smart Battery Standards
Adding intelligence battery pack very recent development. such, there wide disparity smart batteries implemented. Some standards emerging, however these currently being driven laptop computer manufacturers. systems with only cell batteries, such cellular telephones, cost constraints limit number operations possible within pack, complicating development standards. Communication between battery pack system important consideration. This interface should have signal lines reduce number connections pack. Many single cell battery packs, such those found cellular telephones, single wire interface. single wire interface, zero determined duration line. This type interface provides asynchronous link. There currently three different single wire standards, each slightly different from others. intrinsic nature operation single wire interface transmission speeds very low. This speed acceptable cost systems, because there typically minimal information passed host. battery packs with multiple cells, however, more information needed host, requiring higher interface speeds. two-wire communication link provides synchronous communication between system host battery pack. typical interface based specification from Philips. this case, single wire provides data signal other provides clock signal. Embedded start stop bits provide command synchronization. 1996, Intel (along with number other companies) developed variation called System Management (SMBus). This uses same wires standard bus, adds additional voltage options restrictions long device control before releasing other devices. part development SMBus, Intel others created Smart Battery Standard. This specification adds protocol SMBus interface defines commands communication between battery pack, battery charger, battery selector circuit, host. While commands communication protocols necessary, they provide common framework from which system designers battery pack providers work. Using SMBus standard non-PC applications also gives designer starting point developing communication procedure. more information about Smart Battery System SMBus, please look world wide http://www.sbs-forum.org.
Intersil Corporation reserves right make changes circuit design, software and/or specifications time without notice. Accordingly, reader cautioned verify that Application Note Technical Brief current before proceeding.
Summary
This application note explored some concepts batteries, they work some design considerations associated with battery pack development. idea smart batteries emerging intriguing area development. Because sudden rapid changes this area, however, there many concepts new, evolving, standards. Battery chemistry, improving rapidly. With these improvements come greater capacity longer system time, also greater risks. These batteries need tight tolerance design safety, programmability thresholds flexibility, techniques monitoring current pack achieve maximum available cell capacity least abuse cells.
More Information
This application note condensed from source material originating from many sources, including those listed below. Additional details battery chemistry, recent industry trends smart battery standards available through these sources. Huret, Barry, Huret Associates, Inc., Yardley, Linden, David, "Handbook Batteries," McGraw-Hill, Inc., 1995. Panasonic, "Lithium Batteries Technical Handbook", 1998 Vincent, Colin Scrosati, Bruno, "Modern Batteries, Introduction Electrochemical Power Sources," John Wiley Sons, Inc. 1997. Intel Corporation, al., "System Management Specification, Revision 1.1," December 1998. (http://www.sbs-forum.org) Intel Corporation, al., "Smart Battery Data Specification, Revision 1.1," December 1998. (http://www.sbsforum.org)
information regarding Intersil Corporation products, www.intersil.com
AN126.0 July 2005
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