(12) United States Patent
`KalogerOpoulos et al.
`
`USOO633756OB1
`(10) Patent No.:
`US 6,337,560 B1
`(45) Date of Patent:
`Jan. 8, 2002
`
`(*) Notice:
`
`(21) Appl. No.: 09/722,884
`(22) Filed:
`Nov. 28, 2000
`7
`(51) Int. Cl.' ................................................... H02. 7700
`(52) U.S. Cl. ........................................ 320/160; 320/125
`(58) Field of Search ................................. 320/160, 125,
`320/130, 162
`
`OTHER PUBLICATIONS
`(54) LIFE CYCLE CHARGING FOR BATTERIES
`E. of U.S. application No. 09/397,001, filed Sep. 15,
`(75) Inventors: Sarandis Kalogeropoulos, Malmö;
`European Standard Search Report, Date of Completion. Jun.
`late? Svensson, Landskrona, both
`19, 2001; Date of Mailing. Jun. 25, 2001.
`O
`* cited by examiner
`(73) Assignee: Telefonaktiebolaget LM Ericsson
`Primary Examiner-Gregory J. Toatley, Jr.
`(publ), Stockholm (SE)
`Subject to any disclaimer, the term of this RTÉ Agent, or Firm-Burns, Doane, Swecker &
`patent is extended or adjusted under 35
`s
`•r • - -
`U.S.C. 154(b) by 0 days.
`(57)
`ABSTRACT
`A method and apparatus for extending the cycle life of a
`battery by using different charge Voltages or different current
`cut-offs to recharge the battery during the course of the
`battery's life. The capacity of a battery is gradually depleted
`as the battery is repeatedly charged and discharged. The rate
`at which the battery capacity is depleted varies in accor
`dance to the charge Voltage, or the current cut-off, used to
`recharge the battery. For example, higher charge Voltages
`cause the capacity to be more rapidly depleted than lower
`charge Voltages. A battery may be charged using a high
`charge Voltage, e.g., its rated charge Voltage, during periods
`of expected high usage. To extend the battery's cycle life,
`the battery may be charged using a low charge Voltage
`during periods of expected low usage.
`31 Claims, 9 Drawing Sheets
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5,528,121 A 6/1996 Okamura
`5,631,533 A 5/1997 Imaseki
`5,939,864 A 8/1999 Lenhart et al.
`6,011,380 A * 1/2000 Paryani et al. .............. 320/132
`
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`LGE-1012 / Page 1 of 17
`LGE v. Fundamental
`
`

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`U.S. Patent
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`Jan. 8, 2002
`
`Sheet 1 of 9
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`U.S. Patent
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`Jan. 8, 2002
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`Jan. 8, 2002
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`U.S. Patent
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`Jan. 8, 2002
`Jan. 8, 2002
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`LGE-1012 / Page 6 of 17
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`Jan. 8, 2002
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`Jan. 8, 2002
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`LGE-1012 / Page 9 of 17
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`LGE-1012 / Page 9 of 17
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`

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`U.S. Patent
`
`Jan. 8, 2002
`
`Sheet 9 of 9
`
`US 6,337,560 B1
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`LGE-1012 / Page 10 of 17
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`

`

`1
`LIFE CYCLE CHARGING FOR BATTERIES
`
`US 6,337,560 B1
`
`RELATED APPLICATION
`This application is related to U.S. application Ser. No.
`09/397,001 now U.S. Pat. No. 6,194,874 entitled “System
`and Method for Maintenance Charging of Battery Cells'
`filed on Sep. 15, 1999, the disclosure of which is expressly
`incorporated herein by reference, in its entirety.
`
`5
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`15
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`25
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`35
`
`BACKGROUND
`The present invention pertains to battery cell charging
`methods and Systems. More particularly, the present inven
`tion relates to the maintenance charging of lithium-based
`battery cells, including, for instance, lithium-ion battery
`cells and lithium-polymer battery cells.
`The increasing popularity of consumer electronics in
`conjunction with the miniaturization of electronic circuits
`has given rise to a great number of devices which are battery
`operated. Portable electrical devices including mobile
`phones, laptop computers, Video cameras, and the like,
`typically rely upon one or more battery cells for electrical
`power.
`Since batteries have a limited capacity, they must peri
`odically be connected to an external charger to be recharged.
`The conventional units for measuring the capacity of a
`battery cell are Milliamp Hours (mAh). That is, the dimen
`Sional units of mAh are used as a Standard measurement for
`defining the potential power rating, or capacity, of a battery
`cell. Higher mAh ratings for a battery correlate to longer
`usage periods for the electronic device being powered by the
`battery, e.g., a longer talk time and/or Standby time for a
`cellular phone or usage time for a notebook computer.
`A number of different types of battery cells are presently
`in use as power Supplies in portable electronic devices.
`Among the most widespread type of batteries are, nickel
`cadmium (Ni-Cd), sealed lead acid (SLA), nickel-metal
`hydride (NiMH), and, more recently, lithium-ion (Li-ion)
`and lithium-polymer (Li-polymer). Each of these battery
`technologies may be characterized by relative advantages
`and disadvantages.
`Ni-Cd batteries are a commonly used type of battery
`cells which are often found in devices requiring relatively
`large amounts of current (e.g., as required by an electric
`drill). A primary advantage of Ni-Cd battery technology is
`cost, since Ni-Cd batteries tend to be the least expensive
`type of battery cells. A major disadvantage of Ni-Cd
`battery cells is the “memory effect,” also known as “voltage
`depression,” which effectively reduces the cells capacity if
`50
`the battery cell is not fully discharged before re-charging. A
`battery cell impaired by the memory effect appears to be
`fully charged but lasts only a short time. The memory effect
`may Sometimes be ameliorated by Subjecting the battery to
`a repetitive Series of charges and discharges.
`55
`Ni-MH batteries, a more recent technology than Ni-Cd
`batteries, have attained recent popularity for use in cellular
`telephones, laptop computers, camcorders, camera flash
`devices, and like portable consumer electronics devices.
`Ni-MH batteries have a higher charge capacity per unit of
`weight than Ni-Cd batteries, but tend to be more expen
`sive. A major advantage of Ni-MH batteries is that they are
`virtually free of the memory effect which plagues Ni-Cd
`batteries. It is not necessary to fully discharge a Ni-MH
`battery prior to recharging it. However, Ni-MH batteries
`65
`can be damaged by overheating which may occur if the
`battery is overcharged.
`
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`Li-ion batteries have recently gained commercial popu
`larity. Li-ion battery cells typically have a lithium-metal
`oxide compound for the positive electrode (the cathode) and
`a carbon-based compound for the negative electrode (the
`anode). The Li-ion battery cell becomes charged and dis
`charged as lithium ions migrate between the cathode and the
`anode, exchanging electrons through doping and de-doping.
`In short, the migration of electrons produces an electrical
`current. Li-ion batteries are advantageous over nickel-based
`batteries in certain respects. For example, Li-ion battery
`Systems have a much higher energy density, as a function of
`maSS. Therefore, for battery Systems of equal charge
`capacities, Li-ion battery Systems tend to be lighter and
`longer lasting than nickel-based battery Systems. Another
`Significant advantage of Li-ion battery technology is that
`Li-ion battery cells do not have the memory effect that exists
`in other types of nickel-based battery cells, particularly
`Ni-Cd cells.
`Li-polymer batteries are another recent entrant in the
`rechargeable battery market. Li-polymer batteries may be
`designed to be very thin, and even exhibit some flexibility.
`Li-polymer batteries are fairly high cost, relative to non
`lithium battery technologies.
`Another battery technology worth noting in addition to
`the aforementioned types of batteries are the Sealed lead
`acid (SLA) batteries. SLAbatteries, which are based on well
`known lead-acid battery technology, are fairly low cost as
`compared to the other types of batteries mentioned. SLA
`batteries tend to be heavy, and are often too cumberSome for
`portable applications.
`A conventional mode of charging a Li-ion battery cell
`involves a two-phase charging process. The charger begins
`with a charging phase of constant current, and then com
`pletes the charging proceSS at a constant Voltage. Such a
`two-phased charging process is termed “constant-current,
`constant-voltage” (“CC-CV") charging. Conventionally, in
`the first phase of the charging process, a constant current is
`applied to the Li-ion battery until the cell approaches its
`maximum voltage. In the Second phase, a constant Voltage
`equal to the maximum cell Voltage is applied to the battery
`until the charge current has decreased to a current cut-off
`value (e.g., 50 mA, 75 mA, 30 mA). The current cut-off
`value is an indication of a fully charged battery.
`Li-ion batteries have a useful life that typically lasts
`anywhere from 200 to 1000 charge cycles. Each time a
`battery is fully charged to its maximum Voltage, the useful
`life of the battery is reduced. It would be useful if the cycle
`life, that is, the number of charge/recharge cycles, of a
`battery could be increased. This would extend the useful life
`of the battery.
`
`SUMMARY
`Due to the aforementioned drawbacks of conventional
`charging Systems, there is a need in the art for a System and
`method of charging battery cells So as to extend the useful
`life of the battery. The present invention utilizes novel
`charging techniques to address this need by reducing the rate
`at which the battery capacity diminishes due to repeated
`charging.
`There is also a need for a Li-ion battery charging System
`and method which does not require additional charging
`circuitry or logic hardware (i.e., specialized charger logic)
`which would result in additional cost, weight and Volume
`within a portable battery operated device.
`It should be emphasized that the terms “comprises” and
`“comprising”, when used in this Specification, are taken to
`
`LGE-1012 / Page 11 of 17
`
`

`

`3
`Specify the presence of Stated features, integers, Steps or
`components, but the use of these terms does not preclude the
`presence or addition of one or more other features, integers,
`Steps, components or groups thereof.
`An exemplary embodiment of the present invention
`involves a method of charging a battery in which the battery
`is repeatedly charged using a first charge Voltage during a
`first time period, and then repeatedly charged using a Second
`charge Voltage during a Second time period. The Second
`charge Voltage is different from the first charge Voltage and
`the Second charge Voltage diminishes the battery's capacity
`at a different rate than the first charge Voltage.
`Another exemplary embodiment of the present invention
`involves a method of charging a battery. A determination is
`made of a first number of cycles during which the battery
`capacity diminishes by a first amount, a first charge Voltage
`being used to charge the battery. A determination is then
`made of a Second number of cycles during which the
`capacity of the battery diminishes by a Second amount, a
`Second charge Voltage being used to charge the battery. The
`battery is repeatedly charged using Said first charge Voltage
`until the battery capacity has diminished by Said first
`amount, and then the battery is repeatedly charged using the
`Second charge Voltage. In accordance with a preferred
`embodiment, the first charge Voltage is lower than the
`Second charge Voltage. In an alternative embodiment, the
`first charge Voltage could be higher than the Second charge
`Voltage.
`Another exemplary embodiment of the present invention
`involves a method of charging a rechargeable battery of a
`device. In accordance with this exemplary method an
`expected usage profile for the device is accessed which
`identifies periods of expected high usage and periods of
`expected low usage for the device. A charge Voltage adjust
`ment Scheme is developed based upon the expected usage
`profile. The battery is then charged to a higher first charge
`Voltage during periods of expected high usage, and charged
`to a lower Second charge Voltage during periods of expected
`low usage.
`An exemplary embodiment of the present invention
`involves a battery charging System which has a variable
`charge Voltage battery charger and a charge Voltage decision
`logic that determines whether a battery is to be charged
`using a first charge Voltage or a Second charge Voltage. The
`charge Voltage decision logic controls the battery charger to
`repeatedly charge the battery using the first charge Voltage
`during a first period and using the Second charge Voltage
`during a Second period. The Second charge Voltage dimin
`ishes the battery capacity at a different rate than the first
`charge Voltage.
`Another exemplary embodiment of the present invention
`involves a battery charging System which has a charge
`controller, a battery charger, a capacity measurement
`Section, and a logic Section that repeatedly charges the
`battery using a first charge Voltage until Said capacity
`measurement Section determines that a battery capacity has
`diminished by a first amount, and uses a Second charge
`Voltage. The first charge Voltage is higher than the Second
`charge Voltage in accordance with this embodiment.
`Another exemplary embodiment of the present invention
`involves a battery charging apparatus which has battery
`charging circuitry and a controller. The controller controls
`the battery charger Such that is repeatedly charges the battery
`using a first charge Voltage during a first period and using a
`Second charge Voltage during a Second period of the bat
`tery's cycle life. The Second charge Voltage is different from
`the first charge Voltage, and the Second charge Voltage
`diminishes the battery capacity at a different rate than the
`first charge Voltage.
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`US 6,337,560 B1
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`4
`Another exemplary embodiment of the present invention
`involves a battery charging apparatus for charging a battery
`which is repeatedly charged and discharged. In accordance
`with this exemplary embodiment the battery charger has
`battery charging circuitry and a controller. The controller
`causes the battery charging circuitry to charge the battery
`using a first charge Voltage until the controller determines
`that the battery capacity has diminished by a first amount.
`Then the controller causes the battery charging circuitry to
`charge the battery using a Second charge Voltage until the
`controller determines that the capacity has diminished by a
`Second amount. In accordance with a preferred embodiment,
`the first charge Voltage is lower than the Second charge
`Voltage. In an alternative embodiment, the first charge
`Voltage could be higher than the Second charge Voltage.
`Another exemplary embodiment of the present invention
`involves a battery charging apparatus which has battery
`charging circuitry, a controller; and a battery usage database
`which contains an expected usage profile for the device. The
`controller accesses the expected usage profile for the device,
`develops a charge Voltage adjustment Scheme based upon
`the expected usage profile, and causes the battery charging
`circuitry to charge the battery to a first charge Voltage during
`periods of expected high usage and charge the battery to a
`Second charge Voltage during periods of expected low usage.
`In accordance with this embodiment, the first charge Voltage
`is higher than the Second charge Voltage.
`In alternative embodiments of the present invention the
`current cut-off is adjusted instead of, or in addition to,
`adjusting the charge Voltage.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`These, and other objects, features and advantages of the
`present invention will become more readily apparent to
`those skilled in the art upon reading the following detailed
`description, in conjunction with the appended drawings, in
`which:
`FIG. 1 depicts the cycle life capacity of a rechargeable
`battery repeatedly charged to full capacity at its rated charge
`Voltage;
`FIG. 2 depicts the effect on cycle life of using various
`charging Voltages for a given battery;
`FIG. 3 depicts an exemplary cycle life capacity relation
`ship for a first embodiment of the present invention, wherein
`the life cycle of a battery is extended by changing the
`charging Voltage during the life of the battery;
`FIG. 4 depicts an exemplary cycle life capacity relation
`ship for a Second embodiment of the present invention in
`which the initial charging Voltage is chosen to be lower than
`the rated charge Voltage of the battery;
`FIG. 5 depicts a variant of the second embodiment in
`which a battery is charged for more cycles at lower charge
`Voltages than at higher charge Voltages,
`FIG. 6 depicts another variant of the second embodiment
`wherein the charge Voltage at which a battery is charged is
`adjusted upward by increasing increments during the course
`of the battery's cycle life;
`FIG. 7 depicts a third exemplary embodiment of the
`present invention in which the choice of the charge Voltage
`of the battery is based upon the usage habits or expected
`usage needs of the device in which the battery resides,
`FIG. 8 depicts a System for implementing the present
`invention; and
`FIG. 9 depicts a method according to the present inven
`tion.
`
`LGE-1012 / Page 12 of 17
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`

`

`S
`DETAILED DESCRIPTION
`These and other aspects of the invention will now be
`described in greater detail in connection with a number of
`exemplary embodiments. To facilitate an understanding of
`the invention, many aspects of the invention are described in
`terms of Sequences of actions to be performed by elements
`of a computer System. It will be recognized that in each of
`the embodiments, the various actions could be performed by
`Specialized circuits (e.g., discrete logic gates interconnected
`to perform a specialized function), by program instructions
`being executed by one or more processors, or by a combi
`nation of both. Moreover, the invention can additionally be
`considered to be embodied entirely within any form of
`computer readable Storage medium having Stored therein an
`appropriate Set of computer instructions that would cause a
`processor to carry out the techniques described herein. Thus,
`the various aspects of the invention may be embodied in
`many different forms, and all Such forms are contemplated
`to be within the scope of the invention. For each of the
`various aspects of the invention, any Such form of embodi
`ment may be referred to herein as "logic configured to
`perform a described action, or alternatively as "logic that
`performs a described action.
`FIG. 1 depicts the cycle life capacity of a rechargeable
`battery repeatedly charged to full capacity at its rated charge
`Voltage. This figure illustrates an exemplary relationship
`between battery capacity, in milliamp hours (mAh), and the
`number of charge/discharge cycles over the cycle life of the
`battery. AS shown in the figure, the battery's capacity
`diminishes over the life of the battery due to repeated
`charging and discharging.
`AS used herein, the term capacity is defined herein to be
`the amount of energy that can be withdrawn from a filly
`charged cell or battery under Specified conditions, that is, the
`battery's Store of electrical energy. The capacity of a battery
`may be determined during the course of the battery's life by
`measuring the amount of energy discharged from a fully
`charged battery to its discharged State, by measuring the
`amount of energy that a battery accepts while it is being
`charged, or by other like means of determining the capacity
`known to those of skill in the art.
`A battery may be characterized by a rated charge Voltage
`which is generally defined in the Specifications provided by
`the manufacturer of the battery. The rated charge Voltage,
`which may also be referred to as the Specified maximum
`charging Voltage, is the maximum recommended Voltage for
`charging the battery. The rated charge Voltage depends upon
`the battery chemistry and other design parameters of the
`battery. The exemplary relationship of FIG. 1 reflects a
`battery being repeatedly charged to that rated charge Volt
`age. For the example depicted in FIG. 1, the battery is
`designed to have a rated charge Voltage of 4.2 volts, and a
`cut-off current of, for instance, 50 mA.
`As a battery is repeatedly charged and discharged over the
`life of the battery, the battery's capacity gradually dimin
`ishes. In general, the rate at which a battery's capacity
`diminishes varies over the cycle life of a battery. The
`capacity diminishes at a faster rate when the battery is new.
`When a battery is repeatedly charged to full capacity at its
`rated charge Voltage, the battery's capacity typically dimin
`ishes at a gradual exponential rate during the first portion of
`its life. This can be seen in FIG. 1 at the beginning of the
`battery's life, e.g., for the first 50 cycles. After an initial
`number of cycles, the rate at which capacity diminishes for
`each cycle levels off to be more or less linear. That is, the
`capacity does not decrease as fast after having gone through
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`US 6,337,560 B1
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`6
`a number of charge/discharge cycles at the beginning of the
`battery's life. At Some point after quite a number of cycles
`(e.g., 300 cycles), the battery capacity falls below an accept
`able capacity level. At this point, the battery is Said to be at
`the end of its useable life.
`In regard to the useable life of a battery, it will be
`appreciated that the length of the usable life, or cycle life,
`depends largely upon the level of capacity that is acceptable
`for a given application. AS Such, the definition of usable life
`is Somewhat arbitrary in that it depends upon the purpose for
`which the battery is to be used. For Some purposes (e.g., a
`cellular telephone), a battery may be acceptable So long as
`the capacity is above a certain amount. Other purposes (e.g.,
`for use in an aircraft's black box) may be such that a larger
`value of capacity is required for the battery to be considered
`acceptable.
`FIG. 2 depicts the effect on cycle life of using various
`charging Voltages for a given battery. The exemplary rela
`tionship illustrated in FIG. 2 involves a battery designed to
`have a rated charge Voltage of 4.2 volts and a rated cut-off
`current of 50 mA. For illustrative purposes, the cycle life
`curves are also shown for the same battery being charged to
`4.1 volts and 4.0 volts. The present invention may be used
`with batteries and battery technologies which have various
`other charge Voltages and current cut-off values.
`AS Shown in the figure, a battery which is repeatedly
`recharged at its rated charge Voltage has a shorter cycle life
`than the battery would have if repeatedly charged to a lesser
`Voltage. In other words, using a reduced charge Voltage
`increases the cycle life of the battery. This is because the
`capacity of a battery diminishes more rapidly over the cycle
`life of the battery when the battery is charged to its full rated
`charge Voltage, than for the same battery charged to a lesser
`Voltage. AS the charging Voltage decreases, the cycle life of
`a given battery tends to increase.
`It should be noted, however, that a battery charged using
`a reduced charging Voltage initially has a Smaller capacity
`than a battery charged to a relatively higher Voltage. This can
`be seen for the first few cycles of the three cycle life curves
`of FIG. 2. After a number of charge/discharge cycles, the
`capacity associated with the lower charge Voltage is actually
`greater than the capacity associated with the higher charge
`voltage. For example, FIG.2 shows that after 300 cycles, the
`capacity associated with the 4.0 volt charge Voltage is
`greater than the capacity associated with the 4.1 volt charge
`Voltage, which in turn, is greater than the capacity associated
`with the 4.2 volt charge Voltage. This occurs because the
`capacity diminishes more rapidly over the cycle life of a
`battery for higher charge Voltages than for lower charge
`Voltages.
`FIG. 3 depicts an exemplary cycle life capacity relation
`ship for a first embodiment of the present invention, wherein
`the life cycle of a battery is extended by changing the
`charging Voltage during the life of the battery. In accordance
`with this first embodiment, the initial charging Voltage is
`chosen to be the rated charge Voltage of the battery. At
`certain points during the life of the battery, the charge
`Voltage is adjusted downward to a lesser charge Voltage.
`In the exemplary first embodiment depicted in FIG. 3, the
`battery is initially charged to its rated charge Voltage of 4.2
`Volts. Later in the cycle life of the battery, the charge Voltage
`is altered Such that the battery is charged to a lesser Voltage
`of 4.1 volts. Finally, the charge voltage is set to be 4.0 volts.
`The value of the various charge Voltages and the number of
`times the charge Voltages are changed (e.g., from 4.2 to 4.1
`and finally to 4.0 volts) have been chosen in this disclosure
`
`LGE-1012 / Page 13 of 17
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`

`US 6,337,560 B1
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`15
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`25
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`7
`for the sake of illustration. The invention is not limited to
`these particular voltages, nor is the invention limited to
`having three different charge Voltages. In accordance with
`the present invention, the number of times the charge
`Voltage is changed and the amount of the charge Voltage may
`be chosen to conform to a particular Situation or to meet an
`objective. Furthermore, the charge Voltages may be adjusted
`upward, downward, or varied up and down in combination,
`as discussed below for alternative embodiments.
`In accordance with alternative embodiments, charging
`parameters other than the charge Voltage may be varied
`during different periods in order to extend the battery cycle
`life. For example, the current cut-off may be adjusted instead
`of, or in addition to, adjusting the charge Voltage. Using a
`lower cut-off current tends to diminish the battery capacity
`more over a given number of cycles than using a higher a
`value for cut-off current.
`As shown in FIG. 3, the points at which the charge voltage
`is changed from the initial charge Voltage of 4.2 volts to 4.1
`volts, and then from 4.1 volts to 4.0 volts, are labeled as
`points A and B, respectively. These points at which the
`charge Voltage is changed may be determined in a number
`of different ways, in accordance with the present invention.
`The charge Voltage may be changed after a predetermined
`number of charge/discharge cycles has taken place. For
`example, the charge Voltage could be changed from 4.2 volts
`to 4.1 volts after 150 cycles, and then from 4.1 volts to 4.0
`Volts after 230 cycles have taken place. In accordance with
`the present invention, an identification circuit (ID ckt),
`which may be inside the battery pack or the battery-powered
`device itself, may be used to Store information pertaining to
`the number of charge (or discharge) cycles that the battery
`has undergone.
`In alternative embodiments of the present invention,
`instead of relying upon the number of cycles completed, the
`points A and B could be set to take effect when the capacity
`of the battery falls to certain predetermined values. For
`example, the charge Voltage could be changed from 4.2 volts
`to 4.1 volts when the capacity becomes 430 mAh, and then
`from 4.1 volts to 4.0 volts when the capacity falls to 380
`mAh. In Some Situations, it may be Sufficient to simply
`Schedule the points A and B for a particular time and date.
`For example, if the battery usage conditions are known (e.g.,
`the battery is depleted each day and charged each night) the
`time for altering the charge Voltage for recharging the
`battery could simply be prescheduled for a certain time and
`date.
`Alternatively, a combination of a measurable quantity
`(e.g., number of cycles or battery capacity) could be used in
`conjunction with a predetermined Schedule. That is, point A
`for changing the charging Voltage could be set for either
`50
`when the capacity falls to 430 mAh or a predetermined date,
`whichever occurs earlier.
`Furthermore, it should be appreciated that the points. A
`and B at which the charge Voltage is changed need not
`coincide with a particular fraction of the expected cycle life.
`For example, the points A and B could be chosen Such chat
`the charge Voltage is changed from 4.2 volts to 4.1 volts after
`50 cycles, and then from 4.1 volts to 4.0 volts after 500
`cycles have taken place, or any other points appropriate for
`the usage requirements of the battery.
`FIG. 4 depicts an exemplary cycle life capacity relation
`ship for a Second embodiment of the present invention. In
`accordance with this Second embodiment, the initial charg
`ing Voltage is chosen to be lower than the rated charge
`voltage of the battery. During the cycle life of the battery, the
`charge Voltage is adjusted upward until the rated charge
`Voltage.
`
`8
`As shown in FIG. 4 which depicts capacity over the cycle
`life of a rechargeable battery, the battery charge Voltage is
`increased from 4.0 volts to 4.1 volts at point C (e.g., 200
`cycles). The battery charge Voltage is again increased from
`4.1 volts to 4.2 volts at point D (e.g., 400 cycles). By
`incrementally increasing the charge Voltage over the cycle
`life of the battery, the battery capacity is effectively
`increased to compensate for the diminishing battery capacity
`due to repeated charging. In this way, the battery capacity
`remains closer to a constant value over the cycle life of the
`battery.
`In the exemplary second embodiment depicted in FIG. 4,
`the charge Voltage of the battery is initially Set to 4.0 volts,
`and later is adjusted to 4.1 volts, and finally, the charge
`Voltage is adjusted to be 4.2 volts. These charge Voltage
`values and the number of times the charge Voltage is
`changed have been chosen for illustrative purposes. In
`accordance with the present invention, the charge Voltage
`may be changed many more times in Smaller increments. For
`example, instead of adjusting the charge Voltage upward
`twice by 0.1 volts at points C and D, the charge Voltage
`could be adjusted upward twenty times in 0.01 volt incre
`ments. The use of more adjustment increments having
`Smaller values produces a Smoother capacity curve over the
`cycle life of the battery. That is, the “sawtooth' capacity
`curve of FIG. 4 would be finer in that there would be more
`adjustments with each upward adjustment (or "Sawtooth)
`being of a Smaller increment.
`Furthermore, the charge Voltage may be adjusted by more
`than the 0.2 volts (i.e., 4.0 V to 4.2 v) chosen to illustrate this
`embodiment. The charge Voltage could be adjusted acroSS a
`wider range of charge Voltages, Such as an initial value of 3.5
`volts to a final v