Battery pack and charger assembly

Abstract: In order to enable a simple arrangement wherein only two terminals are provided on a battery pack for recharging a battery and sensing the battery temperature, the charger is arranged to intermittently interrupt a constant current supply used to recharge the battery and to apply, during the interruption, a predetermined low constant voltage to the two terminals so as to sample the voltage of a thermistor which is disposed in immediate proximity of the secondary battery.

Description: The present invention relates to a battery pack and charger assembly, in particular to improvements in monitoring or sampling battery temperature while a battery is being recharged, and more specifically to an improved technique which intermittently senses battery temperature using the terminals which are also used for battery recharging. Secondary patteries have found very extensive application in a variety of industrial fields in that they are rechargeable. A secondary battery is alternatively known as a storage battery or an electric accumulator. Before turning to the present invention it is advantageous to briefly discuss, with reference to Fig. 1, a known secondary battery pack including a battery (or cell) and a thermistor for measuring the battery temperature. As shown in Fig. 1, a secondary battery pack 10 is provided with a secondary battery 12, a thermistor 14, and three terminals 16a, 16b and 16c. The battery 12 is coupled to the terminals 16a and 16c, while the thermistor 14 is coupled to the terminals 16b and 16c for outputting battery temperature information. As is well known, a thermistor is a temperature-sensitive semiconductor device possessing a negative temperature coefficient. A known technique similar to the arrangement of Fig. 1 is disclosed in Japanese Laid-open Patent Applications Nos. 4-210738 and 5-13108. When the battery 12 is to be recharged, the battery pack 10 is attached to a battery charger 18 by electrically coupling the terminals 16a-16c to the counterpart terminals 20a-20c of the battery charger 18, respectively. Although not shown in detail in Fig. 1, the charger 18 is supplied with suitable main electricity applied from a commercial ac (alternating current) line, a vehicle's cigarette lighter, or the like. The electric power applied to the charger 18 is irrelevant to the present invention and thus a further description thereof is omitted for brevity. The battery 12 is recharged by passing a direct current, by way of the terminals 16a and 16c, through the cell in the opposite direction (depicted by an arrow 22) to that which occurs during discharge. In the case where the secondary battery 12 is of the nickel-cadmium (or nicad) cell type, it is difficult to reliably measure when the battery is fully charged by monitoring a terminal voltage of the battery under recharge. This is because the full charge voltage changes with repetitive cycling, environmental temperature, charging rate, etc. In order to determine the time when the nicad cell has already been fully charged, a temperature-sensitive semiconductor device such as a thermistor is used and provided in the immediate vicinity of the cell. A nicad cell has an internal chemistry that causes it to produce heat once it is fully recharged and hence, a fully charged condition of a nicad cell can be detected by monitoring its temperature and detecting a rise exceeding a predetermined value. However, the above mentioned known battery pack 10 inherently requires the third terminal 16b which is dedicated to the battery temperature sensing. Therefore, in order to simplify the battery pack 10 by reducing the number of parts and connection wires, it is highly desirable to omit this third terminal which is only used for the temperature monitoring. Further, even if the secondary battery is a type whose full charge condition can be determined by monitoring the battery's terminal voltage instead of the above mentioned cell temperature, it is frequently necessary to detect battery temperature during the recharging operation for safety reasons. Even in this case, it is preferable to omit the terminal 16b which is solely devoted or dedicated to the battery temperature sensing. Accordingly, what is desired is to simplify the arrangement of the secondary battery pack by eliminating the terminal used solely for the purpose particular to the battery temperature sensing. EP-A-0,198,707 describes a system for measuring battery temperature and voltage during charging. The system is designed to charge a battery using two charging terminals and sense battery voltage through the charging terminals. In measuring the temperature of the battery, the battery is electrically isolated from the sensing circuit during a portion of the charging cycle where the charging voltage is less than the battery voltage. Should the temperature of the battery increase to a predetermined upper limit, the charging cycle is switched to a trickle charge on the battery. It is an object of at least the preferred embodiments of the present invention to provide a secondary battery pack and charger which in combination enable the omission of the battery pack terminal which is dedicated solely to the battery temperature detection. Another such object of the present invention is to provide a method of omitting the battery pack terminal which is dedicated solely to the battery temperature. These objects are fulfilled by a technique wherein to enable a simple arrangement only two terminals are provided on a battery pack for recharging a battery and sensing the battery temperature. The charger is arranged to intermittently interrupt a constant current supply used to recharge the battery and to apply, during the interruption, a predetermined low constant voltage to the two terminals so as to sample the voltage of a thermistor which is disposed in immediate proximity of the secondary battery. More specifically, a first aspect of the present invention provides a battery pack and charger arrangement comprising: a battery pack having only first and second terminals and including a rechargeable battery, a thermistor and a diode for preventing discharge of said battery through the thermistor, said thermistor being connected between said first and second terminals and said diode and said battery being connected in series between said first and second terminals; and a battery charger having only third and fourth terminals, said third and fourth terminals being adapted for connection with said first and second terminals respectively, characterised in that said battery charger includes: a source of constant current; a source of constant voltage for generating a constant voltage lower than a fully discharged voltage of said battery; means for generating a control signal comprising a train of pulses; responsive to said control signal for alternately coupling said source of constant current and said source of constant voltage to said battery pack via said first to fourth terminals, said switch means switch means selecting said source of constant current between said pulses of said control signal so as to charge said battery via said first to fourth terminals, and selecting said source of constant voltage for the duration of each pulse of said control signal so as to apply said constant voltage to said thermistor via said first to fourth terminals; and thermistor voltage detection means for sensing the voltage across said thermistor while said source of constant voltage is selected by said switch means. A second aspect of the present invention provides a method of monitoring a temperature of a secondary battery provided in a battery pack while a charger recharges said secondary battery, said battery pack having only first and second terminals and means having a resistance responsive to a temperature of said secondary battery having opposite ends coupled respectively to said first and second terminals, said method being characterized by comprising the steps of: generating a constant current in said charger; applying said constant current to said secondary battery for recharging said battery via said first and second terminals of said battery pack, said constant current flowing into said battery via means for preventing discharge of said battery through the temperature responsive means; intermittently interrupting the application of said constant current to said battery; generating a constant voltage in said charger, said constant voltage being lower than a fully discharged voltage of said battery; applying said constant voltage to said means having a resistance responsive to a temperature of said secondary battery while application of said constant current to said battery is interrupted; and monitoring at said charger a voltage developed across said means responsive to a temperature of said secondary battery in response to said constant voltage while the application of said constant current to said secondary battery is interrupted. Preferred features of the present invention will now be described, purely by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a diagram showing a known battery pack coupled to a battery charger; Fig. 2 is a diagram showing a first preferred embodiment of the present invention in block diagram form; Fig. 3 is a diagram showing operations of the first embodiment; and Fig. 4 is a diagram showing a second embodiment of the present invention in block diagram form. A first embodiment of the present invention is described with reference to Figs. 2-3. The instant embodiment is directed to the detection of a full charge condition of a secondary battery which has characteristics wherein the cell temperature begins to rise when the cell is fully charged. Merely by way of example, a nickel-cadmium cell is referred to which exhibits such temperature characteristics. A battery pack 30, when it is to be recharged, is detachably coupled to a battery charger 32 by way of their terminals 34a-34b and 36a-36b. The battery pack 30 is comprised of, in addition to the terminals 34a-34b, a secondary battery 38, a diode 40, and a thermistor 42 which is deposited close to the battery 38. On the other hand, the battery charger 32 includes the aforesaid terminals 36a and 36b, a constant current generator 44, a constant voltage generator 46, a resistor 48, a pulee generator 50, a switch 52, a thermistor voltage detector 54, and a full charge indicator 56, all of which are coupled as illustrated. As in the prior art of Fig. 1, the charger 32 is supplied with electricity from a suitable main power source. When the battery pack 30 is initially coupled to the charger 32, the constant current generator 44 begins issuing a predetermined constant current. The pulse generator 50 is responsive to the presence of the current on a line 45 and generates a switch control signal (depicted by Scon) which is a train of pulses as shown in Fig. 3. It is to be noted that the aforesaid detection of the coupling of two devices is merely exemplary and the present invention is not directly concerned with a manner of how to detect the pack 30 being initially coupled to the charger 32. The control signal Scon comprises a train of pulses which occur at predetermined timer intervals. Each pulse width is markedly narrower than the time interval between the pulse occurrences. By way of example, each pulse has a width of few seconds and occurs at the predetermined time interval of few minutes. As will be discussed in detail, the battery 38 is charged during the time intervals between pulses, while the thermistor voltage is measured during the actual pulse. While the thermistor voltage is measured or sampled, the battery charging is temporarily interrupted. Since the battery voltage rises gradually as the battery charges, the short time interval (viz., pulse width) with which the thermistor voltage is repetitively monitored does not adversely affect the overall battery charging. The control signal Scon is applied to both the switch 52 and the thermistor voltage detector 54. The switch 52 is responsive to the low level of the signal Scon and couples the constant current generator 44 to the battery pack 30 by way of the terminals 36a and 34a. on the contrary, the switch 52, in response to the high level (viz., pulse) of the signal Scon, couples the constant voltage generator 46 to the battery pack 30. The generator 46 issues a constant voltage V1 which is lower than a fully discharged battery voltage (depicted by V2). The thermistor 42 has high impedance as compared with the forward resistance of the diode 40 and the internal resistance of the battery 38. Thus, while the switch 52 selects the constant current generator 44, a constant current flows into the battery 38 for recharging. On the other hand, while the switch selects the constant voltage generator 46, the constant voltage V1 is applied to the thermistor 42 in that the voltage V1 is lower than the fully discharged battery voltage V2. A voltage developed across the thermistor 42 is measured at the thermistor voltage detector 54 in synchronism with (or in response to) each pulse applied to the pulse generator 56. The thermistor voltage (depicted by Vth) is given by Vth = V1·Rth/(Rth+R) where R: resistance of resistor 48; and Rth: resistance of thermistor 42. Rewriting equation (1), the resistance Rth is represented as follows: Rth = Vth·R/(V1-Vth) The values of R and V1 are previously known. Therefore, when the voltage detector 54 detects Vth, the resistance Rth of the thermistor 42 is defined using equation (2). It can be previously known to the circuit designer to what battery temperature the thermistor voltage corresponds by referring to the data sheet which is available at a thermistor's manufacturer. Therefore, it is in practice unnecessary to calculate the value of Rth each time the detector 54 receives the thermistor voltage. It is well known in the art to design the circuitry for determining the thermistor's ambient temperature based on the thermistor voltage Vth and hence, further descriptions thereof will be omitted for the sake of brevity. The operations of the first embodiment shown in Fig. 2 are further discussed with reference to Fig. 3. It is assumed that the battery 38 is fully discharged and the battery pack 30 is coupled to the charger 32 for recharging the battery 38. As mentioned above, the constant voltage V1 outputted from the voltage generator 46 is below the fully discharged voltage V2. It is further assumed that the recharging begins at a time point T0. If it takes one hour for the charger 32 to fully recharge the battery 38 and if the pulse period is 30 seconds (=(3 seconds of pulse width)+(27 seconds of pulse interval)), the pulse generator 50 issues about 120 pulses per one hour. However, in Fig. 3, only seven pulses P1-P7 are shown merely for the sake of simplifying the drawing. The pulses P1-P7 are respectively generated at time points T1-T7 and control the switch 52 such as to have the voltage detector 54 measure the thermistor voltage. The thermistor voltage is detected by applying the constant voltage V1 to the thermistor 42. The battery voltage (depicted by Vb in Fig. 3) gradually rises as the battery 38 is charged and eventually reaches the fully charged potential (depicted by Vf) at a time point Tf. Until the battery 38 is fully charged, the battery temperature remains at a constant value TEMP1 which is indirectly detected by the voltage detector 54. On the other hand, after the battery 38 reaches its fully charged state, electrical energy is no longer converted into chemical energy (viz., is not used for raising the battery voltage) and instead raises the battery temperature. The voltage detector 54 determines, while interrupting the application of the constant current to the battery 38, if the thermistor voltage has reached the fully charged state by monitoring the battery temperature. In the illustrated case, the fully charged state is detected at time point T7 when the voltage detector 54 detects the thermistor voltage which indicates a temperature equal to or more than a predetermined value TEMP2. Upon detecting the fully charged state of the battery 38, the voltage detector 54 generates a signal Sf which indicates the full charge and which is applied to the constant current generator 44, the pulse generator 50, and the full charge indicator 56. The pulse generator 50 responds to the signal Sf and controls the switch 52 so as to continue to select the constant current generator 44. Further, the signal Sf induces the full charge indicator 56 to energize a suitable indicator (not shown) installed therein. For example, an indicator such as an LED (light emitting diode). Although no problem may be caused by overcharging the battery (if it is a nicad cell), it is still preferable to switch over to a "trickle" charge upon a full charge being attained. To this end, the signal Sf is used to have the constant current generator 44 switch over from a normal charge mode to a trickle charge mode. A second embodiment of the present invention is discussed with reference to Fig. 4. The second embodiment differs from the first one in that the second embodiment includes an abnormal voltage indicator 56' in place of the full charge indicator 56. The second embodiment is used in the case wherein the secondary battery is of a type whose full charge condition can be determined by monitoring the battery's terminal voltage instead of the above mentioned cell temperature. Even in such a case, it is very important to detect battery temperature during recharging from a safety point of view. The instant embodiment finds extensive use in that it is possible to omit the terminal 16b, thereby simplifying the battery pack with the attendant result of reducing manufacturing costs. In the above discussion, the pulse generator 50 is activated to issue a train of pulses immediately after the charger begins recharging the battery 38. As an alternative, the pulse generator 50 issues a train of pulses after a predetermined time elapses from the recharging. It will be understood that the above disclosure is representative of only two possible embodiments of the present invention and that the concept on which the invention is based is not specifically limited thereto. Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. The text of the abstract filed herewith is repeated here as part of the specification. In order to enable a simple arrangement wherein only two terminals are provided on a battery pack for recharging a battery and sensing the battery temperature, the charger is arranged to intermittently interrupt a constant current supply used to recharge the battery and to apply, during the interruption, a predetermined low constant voltage to the two terminals so as to sample the voltage of a thermistor which is disposed in immediate proximity of the secondary battery. The battery charger comprises means for recharging the battery using a constant current source and intermittently interrupting the constant current and applying a constant voltage across the thermistor via the first and second terminals of the battery charger, the constant current being below a fully discharged voltage of the secondary battery, and a voltage detector for monitoring a voltage developed across the thermistor in response to application of the constant current to said thermistor, the voltage detector detecting a temperature of the secondary battery based on the voltage monitored thereby.

Claims: A battery pack and charger arrangement comprising: a battery pack (30) having only first (34a) and second (34b) terminals and including a rechargeable battery (38), a thermistor (42) and a diode (40) for preventing discharge of said battery through the thermistor, said thermistor being connected between said first and second terminals and said diode and said battery being connected in series between said first and second terminals; and a battery charger (32) having only third (36a) and fourth (36b) terminals, said third and fourth terminals being adapted for connection with said first and second terminals respectively, characterised in that said battery charger includes: a source (44) of constant current; a source (46) of constant voltage for generating a constant voltage lower than a fully discharged voltage of said battery; means (50) for generating a control signal comprising a train of pulses; responsive to said control signal for alternately coupling said source of constant current and said source of constant voltage to said battery pack via said first to fourth terminals, said switch means switch means (52) selecting said source of constant current between said pulses of said control signal so as to charge said battery via said first to fourth terminals, and selecting said source of constant voltage for the duration of each pulse of said control signal so as to apply said constant voltage to said thermistor via said first to fourth terminals; and thermistor voltage detection means (54) for sensing the voltage across said thermistor (42) while said source of constant voltage is selected by said switch means. An arrangement as claimed in Claim 1, wherein the voltage sensed by said thermistor voltage sensing means is used to determine complete recharge of said secondary battery. An arrangement as claimed in 1, wherein the voltage sensed by said thermistor voltage sensing means is used to determine an abnormal temperature rise of said secondary battery. An arrangement as claimed in any preceding claim, wherein said train of pulses comprises a train of pulses which occur at predetermined time intervals. An arrangement as claimed in any preceding claim, wherein the pulse width of each pulse of said train pulses is narrower than the time interval between pulses. A method of monitoring a temperature of a secondary battery (38) provided in a battery pack (30) while a charger (32) recharges said secondary battery, said battery pack having only first (34a) and second (34b) terminals and means (42) having a resistance responsive to a temperature of said secondary battery having opposite ends coupled respectively to said first and second terminals, said method being characterized by comprising the steps of: generating a constant current in said charger; applying said constant current to said secondary battery for recharging said battery via said first and second terminals of said battery pack, said constant current flowing into said battery via means (40) for preventing discharge of said battery through the temperature responsive means (42); intermittently interrupting the application of said constant current to said battery (38); generating a constant voltage in said charger, said constant voltage being lower than a fully discharged voltage of said battery; applying said constant voltage to said means having a resistance responsive to a temperature of said secondary battery while application of said constant current to said battery is interrupted; and monitoring at said charger a voltage developed across said means responsive to a temperature of said secondary battery in response to said constant voltage while the application of said constant current to said secondary battery is interrupted.

Inventors: UCHIDA AKIKO, YAMASHITA OSAMA, UCHIDA, AKIKO, YAMASHITA, OSAMA, Uchida, Akiko, c/o NEC Saitama Ltd., Yamashita, Osama c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: H02J7
IPC codes: H02J7, H01M2, H01M10