(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property
`Organization
`International Bureau
`
`
`
`OEARE
`
`(43) International Publication Date
`6 May 2004 (06.05.2004)
`
`(10) International Publication Number
`WO 2004/038831 A2
`
`(51) International Patent Classification’:
`
`HOIM
`
`(21) International Application Number:
`PCT/VS2003/034363
`
`(22) International Filing Date: 27 October 2003 (27.10.2003)
`
`(25) Filing Language:
`
`(26) Publication Language:
`
`English
`
`English
`
`(30) Priority Data:
`60/421 ,624
`
`25 October 2002 (25.10.2002)
`
`US
`
`(71) Applicant: RAYOVAC CORPORATION[US/US]; 601
`Rayovac Drive, Madison, WI 53744-4960 (US).
`
`(72) Inventors: BUSHONG, Wiliam, C.; 6306 Keelseon
`Drive, Madison, WI 53705 (US). CHEESEMAN,Paul,
`3779 Swoboda Road, Verona, WI 53593 (US). DAVID-
`SON, Greg, 323 South Main Street, Oregon, WI 53575
`(US). KAUFMAN, Tom, 4420 Stone Wood Drive, Mid-
`dleton, WI 53562 (US). MANK, Richard; P.O. Box
`
`(74)
`
`(81)
`
`(84)
`
`45546, Madison, WI 53744 (US). ROOT, Michael,
`165 E. County Rd. B2, Little canada, MN 55117 (US).
`ROSITCH, Aaron; 1709 East Road 4, Edgerton, WI
`53534 (US). VU, Viet, H.; 3926 Meridian Circle, Verona,
`WI 53593 (US).
`
`Agent: FORMAN, Adam, J.; Quarles & Brady LLP, 411
`East Wisconsin Avenue, Milwaukee, WI 53202 (US).
`
`Designated States (national): AB, AG, AL, AM, AT, AU,
`AZ, BA, BB, BG, BR, BY, BZ, CA, CH, CN, CO, CR, CU,
`CZ, DE, DK, DM, DZ, EC, EE, ES, FL, GB, GD, GE, GH,
`GM, HR, HU, ID, 1, IN, 1S, JP, KE, KG, KP, KR, KZ, LC,
`LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, MN, MW,
`MX, MZ, NO, NZ, OM, PH, PL, PT, RO, RU, SD, SE, SG,
`SK, SL, TJ, TM, TN, TR, TT, TZ, UA, UG, UZ, VN, YU,
`ZA, ZM, ZW.
`
`Designated States (regional): ARIPO patent (GH, GM,
`KE, LS, MW, MZ, SD, SL, SZ, PZ, UG, ZM, ZW),
`Eurasian patent (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM),
`European patent (AT, BE, BG, CH, CY, CZ, DE, DK, EE,
`ES, FI, FR, GB, GR, HU, TE, IT, LU, MC, NL, PT, RO,
`SE, SI, SK, TR), OAPI patent (BF, BJ, CE, CG, CI, CM,
`GA, GN, GQ, GW, ML, MR, NE, SN, TD, TG).
`
`{Continued on next page]
`
`
`
`
`
` CAPACITY 2004/038831A2TLUTENIMRNAIANTIGENAURARSCTA
`
`
`
`
`
`(54) Tiles METHOD AND APPARATUS FOR REGULATING CHARGING OF ELECTROCHEMICAL CELLS
`
`RECOMBINATION COMBINES
`OXYGEN GAS WITH ANODE
`MATERIAL
`
`CATHODE
`REVERSIBLE
`CAPACITY
`
`CATHODE
`RESIDUAL
`
`
`
`
`
`ANODE EXCESS
`CAPACITY TO PREVENT
`ANODE FROM GASSING
`WHICH INTERFERES
`WITH RECOMBINATION
`
`ANODE
`REVERSIBLE
`CAPACITY
`
`ANODE
`RESIDUAL
`CAPACITY
`
`(57) Abstract: A rechargeable electrochemical cell is provided having a pressure-responsive apparatus for determining a charge
`termination point. In particular, a reversible pressure-responsive switch may be disposed in a cap at the open end of a rechargeable
`metal hydride cell. The reversible pressure-responsive switch may also contain a vent system for releasing the cel] internal pressure.
`Additionally, a rechargeable cell is used combination with a charging source that can supply constant voltage, constant current,
`
`2 alternating current, or voltage that varies between a minimum threshold and a maximumthreshold. Components of the switch are
`
`preferably made of a material that facilitates predictable switch activity.
`
`

`

`WO 2004/038831 A2 MCCAIN
`
`For two-letter codes and other abbreviations, referto the "Guid-
`Published:
`— without international search report and to be republished—ance Notes on Codes and Abbreviations” appearing at the begin-
`upon receipt of that report
`ning of each regular issue of the PCT Gazette.
`
`

`

`WO 2004/038831
`
`PCT/US2003/034363
`
`EXPRESS MAIL LABEL NO.
`
`METHOD AND APPARATUS FOR REGULATING CHARGING OF
`ELECTROCHEMICAL CELLS
`
`CROSS-REFERENCE TO RELATED APPLICATIONS
`
`This application claimspriority to provisional USSN 60/421,624 filed
`
`October 25, 2002, the disclosure of which is hereby incorporated by reference as if
`
`set forth in its entirety herein.
`
`STATEMENT REGARDING FEDERALLY
`SPONSORED RESEARCH OR DEVELOPMENT
`
`BACKGROUND OF THE INVENTION
`
`[0001] The present invention relates generally to nickel rechargeable cells, such as
`
`nickel metal hydride (NiMH)cells, and more specifically to a method and apparatus
`
`for automatically reversibly terminating a cell charging process. This invention may
`
`also be employed in nickel cadmium (NiCd)cells.
`
`[0002] For greater convenience and portability, many modem electrical appliances
`
`and consumer products may be operated to draw electric current from batteries of
`
`standard size and electrical performance. For convenience and economy, various
`
`rechargeable batteries have been developed, such as nickel metal hydride cells and
`
`the like.
`
`[0003] Metal hydride cell technology provides excellent high-rate performanceat
`
`reasonable cost when compared to nickel cadmium and lithium ion technology.
`
`Moreover, metal hydride cells have about a 50% higher volumetric energy density
`than NiCd cells and about equalto lithium ion cells. The internal chemistry ofmetal
`hydride rechargeable cells has an impact on the ability to charge such cells. Issues
`
`affecting the ability to charge nickel rechargeable cells arise as a result of the
`
`internal chemistry of such cells. When a nickel rechargeable cell approachesa full
`
`charge state, oxygen is generated at the positive electrode as follows:
`
`40H —> O, (gas) + 2H,O + 4e°
`
`

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`WO 2004/038831
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`PCT/US2003/034363
`
`{0004] The oxygen gas diffuses across a gas-permeable separator to the negative
`
`electrode whereit is recombined into cadmium hydroxide or water as follows:
`
`1/20, (gas) + HyO + Cd —» Cd(OH), + Heat @ Cadmium negative electrode
`
`1/20, (gas) + H, + H,O + Heat @ Hydride negative electrode
`
`[0005] When recharging suchcells, it is important to ascertain when the cell has
`
`becomefully charged. For example, if a cell were to become overcharged for an
`
`extended period of time, the pressure buildup within the cell could cause the cell to
`
`fail as well as electrolyte to leak, thereby further subjecting the chargerto potential
`
`damage.
`
`[0006] Metal hydride rechargeable cells are typically recharged by applying a
`
`constant current rather than constant voltage to the cells. In this scheme, cell voltage
`
`increases gradually until the cell approaches full charge whereuponthe cell voltage
`peaks. Asthecells reach the overcharge state, the released heat causes the cell
`temperature to increase dramatically, which in turn causesthe cell voltage to
`decrease. Cell pressure also rises dramatically during overcharge as oxygen gas is
`generated in quantities larger than the cell can recombine. Unfortunately,it is
`
`knownthat the rate of pressure changeis several orders of magnitude faster than the
`
`rate of voltage or temperature change. Thus, conventional constant current charge
`
`interruption methods cannot support a very fast charge rate without risking internal
`
`pressure buildup, rupture, and electrolyte leakage. For this reason, metal hydride
`
`cells may be provided with safety vents.
`
`{0007] One common way to reduce pressure buildupat the full-charge state is to
`
`provide a negative electrode having an excess capacity of greater by 40-50% more
`
`than the positive electrode, a gas-permeable separator, and limited electrolyte to
`
`accommodate effective diffusion of gasses. This avoids the production of hydrogen
`
`gas at the negative electrode while permitting the oxygen to recombine with the
`
`negative electrode material. When a cell reaches full charge, oxygen gas continues
`
`to be producedat the positive electrode, but hydrogenis not produced from the
`negative electrode. Ifhydrogen were produced, the cell could rupture from excess
`pressure. The oxygen recombination reaction therefore controls the cell pressure, as
`
`is illustrated in Fig. 1. The oxygen gas then crosses the separator and reacts with the
`
`

`

`WO 2004/038831
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`PCT/US2003/034363
`
`negative electrode material. Detrimental aspects of this arrangement include
`
`reduced cell capacity and corresponding shorter cell cycle life due to degradation of
`the negative electrode from overcharge with oxidation and heat.
`[0008] It is important to stop charging a cell or plurality of cells when a full charge
`
`state is reached to avoid possible cell rupture or leakage due to the increasing
`
`internal gas pressure. Conventional metalhydride rechargeable cells cannot
`
`themselves signal a suitable charge termination point. One must instead rely upon
`
`expensive and sophisticated detection circuitry in an associated charger device to
`
`determine when charging should end. Charge termination is typically determined by
`
`the detection circuitry based on (1) peakcell voltage, (2) peak cell temperature
`
`(TCO), (3) duration of charging time, (4) -dV, and (5) dT/dt. Each known method
`
`for terminating a constant current charge has disadvantages. For example, time-
`
`based termination can be unreliable except at very low charge rates becausethe cell
`
`can become overcharged before termination.
`
`[0009] Charge termination based on peak voltage can be unreliable at the end of the
`charging period because an over-voltage condition can exist before termination.
`Termination based on a voltage decline (-dV) is necessarily associated with oxygen
`
`recombination and the accompanying detrimental temperature rise. In practice, this
`means that voltage detection must be accurate and fast. Unless the ambient
`
`temperature is steady, it can be difficult to accurately measure a change in voltage.
`
`Moreover, when the charge rate is slower than 0.3 C, the voltage drop measurement
`
`is too small to be detected accurately. By definition, a charge rate of 1C draws in
`
`one hour of constant charge a current substantially equal (e.g., within 80%) to the
`
`rated discharge capacity of the electrochemical cell or battery. Termination based
`
`only on peak temperatureis also easily affected by ambient temperature changes.
`
`[0010] Termination based upon the rate of change in temperature over time (dT/dt)
`
`is somewhat morereliable than detecting an absolute temperature change becauseit
`
`is less subject to effects caused by ambient temperature change and becausethere is
`
`less negative effect on cycle life, but it is still based on heat which is detrimental to
`cell performance and cycle life. This is because temperature increases faster, and, in
`fact, precedes, the drop in voltage. Accordingly, there is somewhatless risk of
`
`

`

`WO 2004/038831
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`PCT/US2003/034363
`
`rupture and leakage than in the other methods noted above. This makes it the most
`
`common charge termination method inuse today.
`
`{0011] Others in the art have sought pressure-based mechanismsfor breaking the
`
`connection between the electrode and the cell terminal when pressure exceeds a
`
`predetermined level. For example, U.S. Patent No. 5,026,615 discloses a pressure-
`
`sensitive switch in an end cap assembly that comprises a conductive spring member,
`a nonconductive fulcrum member and a moveable conductive member. The
`
`conductive spring memberis in electrical connection with a terminal on one end and
`
`with the moveable conductive memberon the other end. The moveable conductive
`
`memberis in turn in electrical connection with an electrode. Asthe internalcell
`
`pressure increases, the moveable conductive memberexerts force on the spring
`
`member, which pivots on the nonconductive fulcrum member and disconnects from
`
`the terminal. This patent therefore requires a first and second contact, one of which
`
`being movable with respect to the other and rotatable about a fulcrum inorder to
`
`pivot with respect to the other contact. This arrangement requires more essential
`
`parts than necessary, and further requires that the assembly be constructed with tight
`
`tolerances, thereby increasing complexity as well as the cost of production.
`
`{0012] Other examples of these technologies include US Patent Numbers 5,747,187,
`
`5,405,715, 5,741,606, 5,609,972, 6,018,286, 6,078,244, and 6,069,551, all of which
`
`are incorporated herein by reference as if set forth in their entirety. Some such
`
`mechanisms prevent a pressure-induced rupture of the cell but in doing so
`
`permanently disable the cell. In other cases, reversible switch devices preventcell
`
`rupture, but do not detect an early charge termination state to avoid heat build up
`
`and to ensure superior cell performance and cyclelife.
`
`{0013] With constant voltage charge, on the other hand, the charging current is high
`
`at the beginning of the charge, when the cell can accept higher currents, and then
`
`decreases to lowerlevels as the cell approaches full charge. When constant voltage
`
`charging, the above-noted signals for the end of a constant current charge process
`are not useful because as the cell approaches the full charge state, the cell voltageis
`
`constant and the cell temperature is leveling. Like a constant current charge
`
`approach, charging time cannotbe used for the constant voltage charge when the
`
`

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`WO 2004/038831
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`PCT/US2003/034363
`
`chargerate is higher than 0.3C due to run away ofpressure that can damagethecell
`
`and the charger. As a result of these shortcomingsit has been difficult to identify an
`
`effective termination signaling means and constant voltage charging for metal
`
`hydroxide cells has therefore been generally considered to be impractical.
`
`[0014] With alternating current charge, the charging current may be modulated at a
`
`defined frequency or combination of frequencies to producea net positive current
`
`that enables the cell to become charged. Analternating current charge can provide a
`
`fast charge with less pressure buildup and lower temperature increase than constant
`
`current or constant voltage charge. However, whenusing an alternating current
`
`charge, the above-noted signals for the end of a constant current charge process are
`
`not useful because as the cell approachesthe full charge state, changes in thecell
`
`voltage are difficult to detect above the voltage response to the applied alternating
`
`current. As a result it has been difficult to identify an effective termination
`signaling means and alternating current charging for metal hydroxide cells has also
`therefore been generally considered to be impractical. It should be appreciated that
`
`an alternating current charge 1s used throughoutthe present disclosure to mean a
`
`varying current that produces a net positive charge, such as a modulated alternating
`
`current. For example, an alternating current may be half-waverectified or full-wave
`
`rectified to produce a series of current pulses, or an alternating current may be offset
`
`by a desired DC current.
`
`[0015] Published Australian patent application number 199926971 A1 discloses a
`
`method for fast charging a nickel metal hydride battery in an implant by
`
`transcutaneous transmission of electric power from an external power- transmission
`
`part to a power-receiving part in the implant. The patent application considers the
`
`desirability of an initial rapid high-current charge phase whentheinternalcell
`
`resistance is low, followed by a second lower-current, constant cell voltage charge
`phase to ensure that the cell is charged only with as much energy as the
`electrochemicalstate allows, without excess gassing or heating of the cell. Harmful
`
`effects on the battery are precluded while, at the sametime, the charging rate
`
`remains high. In the method disclosed therein,a first of two charging phases
`
`includes the step of allowing a relatively high constant charging current to flow to
`
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`PCT/US82003/034363
`
`the powerreceiving part while the cell voltage rises until it reaches a predetermined
`limiting charging voltage. In the second charging phase, the charging currentis
`lowerthan the current level at the endofthe first phase while the cell voltage is kept
`
`at least approximately at the predetermined constant voltage value. In the Australian
`patent application, the second charge phase ends when an associated micro-
`electronic controller determines that the rate of change of the charging current over
`
`time does not reach a predetermined slope. This cumbersome two-step constant
`
`current/constant voltage approachis typical of prior approachesin theart.
`
`[0016] In summary,as the metal hydride rechargeable cell reachesits fully charged
`
`state, oxygen is evolved from the positive electrode, thereby increasing the internal
`
`cell pressure and driving the exothermic oxygen recombination reaction. At a very
`
`high constant current charge rate, usually less than one hour, charge currentis still
`
`very highat the end of charge. This results in severe heating of the cell and
`
`shortened cycle life. The available methods of terminating constant current charge
`are not very reliable when cell temperature is high. In addition, cell heatingis
`
`detrimental and it is desirable to terminate the charge before significant cell heating
`
`at the stage where damaging pressure beginsto rise within thecell.
`[0017] Whatis therefore needed is a method and apparatus for more accurately
`determining the charge termination point for a cell that is fully rechargeable under
`
`constant voltage, constant current, and alternating current/voltage charging.
`
`[0018] What would be desirable is a reversible regulating switch that is responsive
`
`to a stimulus for determining a charge termination pointthat is less complex andless
`
`destructive than those currently available.
`
`{0019] Whatis also desirable is a more cost-efficient and reliable charge termination
`
`detection apparatus thanthat currently achieved, and that is compatible with
`
`conventional rechargeable batteries.
`
`BRIEF SUMMARY OF THE INVENTION
`
`[0020] In one aspect the invention provides an axially extending rechargeable
`
`electrochemical cell including an outer can that defines an internal cavity with an
`
`open end, a positive and negative electrode disposed in the internal cavity, and a
`
`terminal end cap enclosing the open end. The cell has an end cap assembly that
`
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`

`WO 2064/038831
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`PCT/US2003/034363
`
`includes a flexible member formed from a material having a heat deflection
`
`temperature greater than 100 C at 264 PSI and a tensile strength greater than 75Mpa.
`
`The flexible member extends radially inwardly from the can and flexes from a first
`
`position towards a second position in response to internal cell pressure. The end cap
`assembly further includesa first conductive elementin electrical communication
`with the terminal end cap. The end cap assembly also includes a second conductive
`
`element in electrical communication with the positive electrode, and in removable
`
`electrical communication with the first conductive element. The second conductive
`
`element is in mechanical communication with the flexible member. The first and
`
`second conductive elements are removed from electrical communication when the
`
`flexible member flexes towards the second position in response to an intemal
`
`pressure exceeding a predetermined threshold.
`
`{0021] The foregoing and other aspects of the invention will appear from the
`
`following description. In the description, reference is made to the accompanying
`
`drawings which forma part hereof, and in whichthere is shown by way of
`
`illustration, and not limitation, a preferred embodiment of the invention. Such
`
`embodiment does not necessarily represent the full scope of the invention, however,
`and reference must therefore be madeto the claims herein for interpreting the scope
`of the invention.
`
`BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
`
`[0022] Fig. 1 is a schematicillustration of the oxygen recombinationreaction
`
`controlling cell pressure;
`
`[0023] Fig. 2A is a cross-sectional view of an end cap assembly containing a
`
`pressure-responsive switch and a pressure-release vent constructed in accordance
`
`with a preferred embodimentof the invention,illustrated in a low pressure position;
`
`{0024] Fig. 2B is a cross-sectional view of the end cap assembly illustrated in Fig.
`
`2A in a high pressure position;
`
`[0025] Fig. 3 is a cross-sectional isometric view of an end cap assembly containing
`
`a pressure-responsive switch and a pressure-release vent constructed in accordance
`
`with an alternate embodimentof the invention, depicted in a low pressure position;
`
`[0026] Fig. 4 is a cross-sectional elevation view of the end cap assembly of Fig. 3;
`
`

`

`WO 2004/038831
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`PCT/US2003/034363
`
`[6027] Fig. 5 depicts an exploded view of the components of the end cap assembly
`
`of Fig. 3;
`
`[0028] Fig. 6A is a sectional side elevation view ofthe positive terminal of a cell
`
`incorporating a switch constructed in accordance with an alternate embodiment of
`
`the invention;
`
`[0029] Fig. 6B is a view similar to Fig. 6A, but constructed in accordance with an
`
`alternate embodimentof the invention.
`
`[0030] Fig. 7 is a sectional side elevation view of the positive terminal of a cell
`
`incorporating a switch constructed in accordance with an alternate embodimentof
`
`the invention;
`
`{0031} Fig. 8 is a graph plotting capacity (Ah) vs. AP (psig) for a nickel metal
`
`hydride cell during alternating current and constant current charge;
`
`[0032] Fig. 9 is a graph plotting capacity (Ah) vs. AP (psig) for a nickel metal
`
`hydride cell during alternating current and constant voltage charge;
`
`[0033] Fig. 10 is a graph plotting internal cell pressure (psig) vs. time (min) for a
`
`plurality of cells constructed in accordance with the preferred embodiment,
`
`[0034] Fig. 11 is a graph plotting pressure, temperature, and voltage vs. time (min)
`
`for a cell during charging using a constant current charge, and subsequent
`
`discharging;
`
`[0035] Fig. 12 is a graph plotting internal pressure (psig) vs. time (min) for various
`
`cycles during charging using a constant current charge, and subsequent discharging;
`
`[0036] Fig. 13 is a graph plotting the pressure rise forthe cell illustrated in Fig. 12
`
`during charging;
`
`[0037] Fig. 14 is a graph plotting pressure fall for the cell illustrated in Fig. 12
`
`during discharging;
`
`{0038] Fig. 15 is a graph plotting pressure and temperature vs. time for cells at
`
`different cycles under a constant current charge;
`
`[0039] Fig. 16 is a graph plotting pressure vs. time for a plurality of cells at different
`
`cycles under a constant current charge;
`
`[0040] Fig. 17 is a graph plotting pressure, temperature, and current vs. time for
`
`plurality of cells under a constant voltage charge.
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`
`[0041] Fig. 18 is a graph plotting and comparing internal pressure vs. applied charge
`
`capacity during constant current charging versus constant voltage charging;
`
`[0042] Fig. 19 is a graphillustrating and comparing the current profile of two cells
`
`during charging under constant voltage versus constant current.
`
`[0043] Fig. 20 is a graph plotting and comparing cell temperature vs. capacity for
`
`two cells charged under constant current versus constant voltage, respectively;
`
`[0044] Fig. 21 is a graph plotting and comparing the voltage profile vs. time for the
`
`twocells illustrated in Fig. 20;
`
`[0045] Fig. 22 is a graph plotting and comparing temperature and capacity vs. time
`
`during charging under varying constant voltages
`
`[0046] Fig. 23 is a sectional side elevation view of an end cap assembly containing a
`
`pressure-responsive switch and a pressure-release vent constructed in accordance
`
`with an alternate embodimentof the invention, illustrated in a low pressure position;
`
`[0047] Fig. 24 is a sectional side elevation view of an end cap assembly containing a
`
`pressure-responsive switch and a pressure-release vent constructed in accordance
`
`with another alternate embodimentof the invention, illustrated in a low pressure
`
`position;
`[0048] Fig. 25 is a sectional side elevation view of an end cap assembly containing a
`pressure-responsive switch and a pressure-release vent constructed in accordance
`
`with yet another alternate embodimentof the invention, illustrated in a low pressure
`
`position;
`
`{0049} Fig. 26A is a schematic view of a battery pack constructed in accordance
`
`with one embodiment of the present invention;
`
`[0050] Fig. 26B is a schematic view of a battery pack constructed in accordance
`
`with an alternate embodimentof the present invention;
`
`[0051] Fig. 26C is a schematic view of a battery pack constructed in accordance
`
`with anotheralternate embodimentof the present invention;
`
`{0052} Fig. 27 is a graph illustrating the charge and discharge capacity for battery
`
`packs having matched and mismatched cells;
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`WO 2004/038831
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`
`[0053] Fig. 28A is a graph illustrating %elongation at break vs. tensile strength for
`
`polymers usable in rechargeable cells in accordance with a preferred embodiment of
`
`the present invention;
`
`{0054} Fig. 28B is a graph illustrating heat deflection temperaturevs. tensile
`
`strength for polymers usable in rechargeable cells in accordance with a preferred
`
`embodiment of the present invention;
`[0055] Fig. 29 is a graph illustrating charge capacity vs. charge time for
`
`rechargeable NiMH cells having a reduced active vohimein accordance with an
`
`alternate embodiment of the present invention;
`
`[0056] Fig. 30 is a chart comparing characteristics of a NIMH size AA cell
`
`constructed in accordance with the embodiment described with reference to Fig. 29
`
`compared to supercapacitors having similar volume;
`
`[0057] Figs. 31A-B illustrate an assembly of a battery pack constructed in
`
`accordance with one embodimentof the present invention;
`
`[0058] Figs. 32A-B illustrate an assembly of a battery pack constructed in
`
`accordance with an alternate embodiment of the present invention; and
`
`[0059] Figs. 33A-C illustrate various embodiments that produce cell electrodes with
`
`reduced electrode volumes.
`
`DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
`
`[0060] Referring now to Fig. 2A, an axially extending cell includes a can 12 having
`
`closed end (not shown) and an open end 13 disposed opposite the open end and
`
`axially downstream therefrom. A cap assembly 10 includesa positive terminal end
`
`cap 18 that is secured in the open end of the negative can 12 to provide closure to
`
`the cell. In particular, the end cap assembly 10 and the open end of the can 12 are
`
`adapted in size and shape suchthat the end cap assembly 10 is sealingly
`
`accommodated in the open end by crimping the negative can 12 during assembly of
`
`a cylindrical rechargeable metal hydride cell. The closed end of the can is
`
`conventional and is not shown.
`
`[0061] A positive (e.g., nickel hydroxide) electrode 14 is in removable electrical
`connection with the positive terminal cap 18, as will become more apparent from the
`
`description below. Thecell further contains a negative electrode 21 (e.g., hydride
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`WO 2004/038831
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`YH
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`
`electrode) that is in electrical connection with the can 12, and an alkaline electrolyte
`
`(e.g., potassium hydroxide) alone or in combination withother alkali metal
`
`hydroxides. The electrodes are disposed in an internal cavity 15, and are separated
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`by a separator 16. A cell comprising the can 12 and the end cap assembly 10 ofthe
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`invention can further comprise conventional positive 14 and negative 21 wound
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`electrodes in its interior, although the relative size of these electrodes can be
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`adjusted to meet the physical andelectrical specifications of the cell.
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`{0062} The positive terminal cap 18 has a nubbin 20 that is sized and shaped to
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`provide a positive terminalto the cell having a pressure-responsive switch 11
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`constructed in accordance with the present invention. The pressure-responsive
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`switch 11 comprises a flexible non-conductive mono-stable grommet 22 adapted in
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`size and shape to fit securely in the open end 13. Grommetincludesa radially outer
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`seal 25, an inner hub 27, and an arm 29 that extends substantially radially and
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`connects the seal to the hub. It should be appreciated that arm 29 extends radially
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`throughoutthe cell and, accordingly, the terms “arm” and “disc” are to be used
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`interchangeably throughout this disclosure. Grommet 22 further includes has a
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`centrally disposed opening 19 extending axially through the hub 27 in whichis
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`seated a conductive spool-shaped connector 24 having a pair of oppositely disposed
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`radially extending outer flanges 23. The space between the outer surface of
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`grommet 22 and innersurface of terminal end cap 18 defines a cavity 17 in the end
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`cap assembly 10.
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`[0063] Connector 24 is securely fixed in the opening 19 of grommet 22 suchthat the
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`conductive connector moves in concert with the grommet. A first annular
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`conductive contact 26, which is a metal washer in accordance with the illustrated
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`embodiment, surrounds the hub of connector 24 and has an upper surface in
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`electrical contact with the upper flange 23. A second annular conductive contact 28
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`(which can also be a metal washer) surrounds the grommetandis positioned axially
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`upstream and adjacent the first contact 26. The first and second contacts 26, 28 are
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`circular plates in Fig. 2A but they can be provided in other shapes, as illustrated, for
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`example, in Figs. 3-5. Contact 28 has an upper surface 29 that is in electrical
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`connection with the terminal cap, and in removable mechanical (and therefore
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`electrical) connection with the bottom surface ofthe first contact 26, as will become
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`more apparent from the description below.
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`[0064] The grommet 22 can be formedof any sufficiently flexible, nonconductive
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`inert material that does not adversely impact the cell chemistry. Suitable materials
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`include but are not limited to polypropylene, polyolefin, and nylon, including glass
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`filled nylon and other glass filled polymers, as will be described in more detail
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`below.
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`[0065] The outer seal 25 of grommet 22 includes an upwardly and radially inwardly
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`extending peripherallip 38 that is shaped and sized to formatight seal with the open
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`end of the can to provide a barrier between the interior and the exterior ofthecell.
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`The lip 38 also partially defines a cavity in the outer seal 25 in whichthe outer end
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`of terminal end cap 18 and second contact 28 are disposed. The lip 38 presents a
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`radially outer convex surface to permit the can 12 to be crimped overthe grommet
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`22 during assembly of the cell. When the axially downstream end of can 12 is
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`crimped over the grommet 22 during assembly, a tight seal is provided between the
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`grommet 22, second contact 28, and terminal end cap 18 to isolate the interior of the
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`cell from the ambient environment. An optional sealant such as asphalt or tar can
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`also be employed between the end cap assembly 10 and the can 12 to strengthen the
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`seal.
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`[0066] A flexible conductive tab 30 electrically connects the conductive connector
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`24 to the positive electrode 14 in the interior of the cell. The conductive connector
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`24 can be aneyelet orrivet that is secured in the central opening 19 by crimpingat
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`its ends to provide flanges 23 that secure the hub 27 of grommet 22 andthefirst
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`contact 26. The conductive connector 24is in electrical and physical contact with
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`the first contact 26 thereby helping to secure the conductive connector 24 into
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`position.
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`{0067} Fig. 2A illustrates the end cap assembly in a low pressure state, such that the
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`grommet 22 is in its stable position. In this low pressure state, the positive
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`electrodes 14 are in electrical connection with the positive terminal cap 18 via the
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`conductive tab 30, connector 24, first contact 26, and second contact 28.
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`Accordingly, the cell may be charged by introducing a recharging current or voltage
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`to the cell. Advantageously, wheninternal pressure within the cell accumulates
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`beyond a predetermined threshold, the grommet 22 flexes (reversibly) axially
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`downstream along the direction of arrow A to bias the pressure-responsive from the
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`first position illustrated in Fig. 2A to a second positionillustrated in Fig. 2B. It
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`should be appreciated that the predetermined threshold may depend on the intended
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`type of charge being used (e.g. constant current, constant voltage, etc...), and may
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`be determined by the material selected for the grommet, and thickness and flexibility
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`of the arm 29.
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`[0068] Referring now to Fig. 2B, whenthe internal pressure within the cell exceeds
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`the predetermined threshold sufficient to flex the grommet 22, the hub 27 is
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`translated axially downstream, thereby also translating the first contact axially
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`downstream with respect from the second contact 28, and removingthe electrical
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`connection therebetween. As a result, an electrical connection at the nubbin 20 will
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`not transfer to the electrodes 14 within the cell, and further charging is prevented
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`until the overpressure situation subsides.
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`[0069] Optionally, an insulating overpressure stop 32 can also be provided in an
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`interior cavity defined by the nubbin 20. The overpressure stop 32 can also be used
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`to pre-load the contact pres