Battery pack with a lithium ion secondary battery

Abstract: A battery pack of the present invention and using a lithium ion secondary battery is capable of correcting, when temperature around the battery pack is low and causes the voltage of the battery to drop due to the characteristic of the battery, the voltage to a voltage expected to appear at normal temperatures. Therefore, the battery pack outputs a constant voltage on its output terminal without regard to the ambient atmospheric temperature.

Description: BACKGROUND OF THE INVENTION The present invention relates to a battery pack for use with a portable apparatus and, more particularly, to a battery pack using a lithium ion secondary battery and operable with a constant discharge characteristic without regard to the ambient atmospheric temperature. Many of portable apparatuses are powered by Ni-Cd (nickel-cadmium) secondary battery packs. An Ni-Cd battery has a discharge characteristic, i.e., battery capacity and terminal voltage little susceptible to the ambient atmospheric temperature. However, for the small size, light weight configuration of a portable apparatus, it is necessary to reduce the size and weight of a battery pack while increasing the capacity of the same. Particularly, for handy phones, a 3 V power source voltage is replacing a 5 V power source voltage. In this respect, a lithium ion secondary battery is advantageous over the Ni-Cd battery or an Ni-Mn (nickel-manganese) battery. However, a lithium ion secondary battery is susceptible to the ambient atmospheric temperature, i.e., its terminal voltage noticeably drops at low temperature. Therefore, the output voltage of a battery pack using a lithium ion battery drops at low temperatures. As a result, a portable apparatus loaded with such a battery pack cannot be powered by a constant voltage. Technologies relating to a battery for use in a portable apparatus are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 4-127620, 2-133038, 58-225368, 62-223680, and 3-180784. None of them, however, teaches an implementation for coping with the drop of the output voltage of a lithium ion secondary battery at low temperatures. Japanese laid-open patent publications 4-33271 and 4-75430 show a battery pack for the use with a lithium ion secondary battery corresponding to the preamble part of claim 1. These battery packs are described in connection with figure 2. GB-A-2 282 924 pertains to a set of batteries and teaches an anti-overcharge circuit that monitors the voltage of the individual battery. By contrast, the present invention pertains to a lithium ion secondary battery and senses the temperature of the battery so as to control the boosting ratio of a boosting circuit and the protection voltage of an overcharge protecting circuit. US-A-5 109 151 teaches a prepatory apparatus for a starter associated with an engine, particularly a Diesel engine. The apparatus controls a current to flow to a heater by sensing the temperature of the heater. US-A-4 687 956 proposes a device for driving a liquid crystal element mounted on a motor vehicle. The device controls a voltage to be applied to the liquid crystal element, i. e., corrects the temperature characteristic of the element by sensing the temperature of the element. The present invention corrects the temperature characteristic of a lithium ion secondary battery and has nothing to do with an on-board battery. JP-54-085767 discloses an electronic watch with an implementation for preventing oscillation from stopping. The implementation consists in sensing a battery voltage and controlling a voltage to be applied to an IC in accordance with the sensed battery voltage. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a battery pack using a lithium ion secondary battery and capable of outputting a constant voltage even when the ambient atmospheric temperature is low. A battery pack including a lithium ion secondary battery of the present invention has a protection circuit consisting of an overdischarge protecting circuit, an overcharge protecting circuit, and an overcurrent protecting circuit. A temperature sensing circuit consists of a temperature non-linear element, a current detector, and a voltage detector. A controller controls a boosting ratio in response to the output of the temperature sensing circuit. A boosting circuit boosts the output voltage of the lithium ion secondary battery in response to a control signal output from the controller. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings in which: FIG. 1 is a block diagram schematically showing a conventional battery pack using an Ni-Cd secondary battery; FIG. 2 is a block diagram schematically showing another conventional battery pack using a lithium ion secondary battery; FIG. 3 is graph showing a discharge temperature characteristic particular to a lithium ion secondary battery; and FIG. 4 is a block diagram schematically showing a battery pack embodying the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT To better understand the present invention, a brief reference will be made to a conventional battery pack for a portable apparatus, particularly a handy phone. As shown, the battery pack has an Ni-Cd secondary battery 71 by way of example. A PCT (Positive Temperature Coefficient) device 72 is a protective device against overcurrent. A thermistor 73 is responsive to temperature elevation to occur when the battery 71 is charged. A diode 74 has a charge terminal and a discharge terminal separate from each other in order to obviate discharge toward the charge terminal. The Ni-Cd battery 71 has a discharge characteristic, i.e., battery capacity and terminal voltage little susceptible to the ambient atmospheric temperature. However, a lithium ion secondary battery is advantageous over the Ni-Cd battery 71 in respect of the small size, light weight configuration of a portable apparatus, as stated earlier. FIG. 2 shows another conventional battery pack using a lithium ion secondary battery. As shown, the battery pack has a lithium ion secondary battery 81 and a protection 82 consisting of an overdischarge protecting circuit, an overcharge protecting circuit, and an overcurrent protecting circuit. However, the lithium ion battery 81 is susceptible to the ambient atmospheric temperature, i.e., its terminal voltage noticeably drops at low temperatures, as also stated earlier. FIG. 3 shows the discharge temperature characteristic of a lithium ion secondary battery specifically. As shown, the voltage of the battery is far lower at -20°C than at 21°C, 40°C and 60°C. Therefore, the output voltage of the above battery pack 81 drops at low temperatures. As a result, a portable apparatus loaded with the battery pack 81 cannot be powered by a constant voltage. Referring to FIG. 4, a battery pack embodying the present invention will be described. As shown, the battery pack has a lithium ion secondary battery 1. A temperature sensing circuit 2 is responsive to temperature around the battery pack. A controller 3 controls the boosting ratio in response to a signal output from the temperature sensing circuit 2. A boosting circuit 4 boosts a voltage in response to a control signal fed from the controller 3, as will be described specifically later. There are also shown in FIG. 4 a protection 5 consisting of an overdischarge protecting circuit, an overcharge protecting circuit, and an overcurrent protecting circuit, and an output terminal 6. The temperature sensing circuit 2 has a thermistor 21, a current detector 22 for detecting a current flowing through the thermistor 21, and a voltage detector 23 for detecting a voltage acting on the thermistor 21. The discharge temperature characteristic of the secondary battery 1 is stored in the controller 3 beforehand. The controller 3 computes a temperature based on the outputs of the current detector 22 and voltage detector 23, computes a voltage based on the computed temperature, and then computes a boosting ratio setting up a preselected voltage. The boosting circuit 4 is a PWM (Pulse Width Modulation) control type switching regulator consisting of a PWM controller 41, a semiconductor switching device 42, an inductor 43, a diode 44, and a capacitor 45. In operation, the resistance of the thermistor 21 varies with the varying ambient atmospheric temperature. As a result, the current and voltage of the thermistor 21 vary. The current detector 22 and voltage detector 23 send their outputs respectively representative of the above current and voltage to the controller 3. In response, the controller 3 computes a temperature around the battery pack and then determines a boosting ratio by referencing the discharge temperature characteristic of the lithium ion battery 1 stored therein. Specifically, assume that the lithium ion battery 1 has the discharge temperature characteristic shown in FIG. 3, and that the temperature is about -20°C. Then, the controller 3 feeds a control signal to the boosting circuit 4 to cause it to raise the voltage by about 0.4 V which is the difference between the voltage at, e.g., 21°C and the voltage at -20°C. In response, the boosting circuit 41 causes its PWM controller 41 to control the ON/OFF time ratio of the switching device 42, thereby raising the voltage. At the same time, the controller 3 delivers a control signal to the protection 5. In response, the protection 5 lowers its overdischarge protection voltage by the value raised by the boosting circuit 4. As a result, a preselected voltage expected to appear a t normal temperatures successfully appears on the output terminal despite the low temperature around the battery pack. In summary, it will be seen that the present invention provides a battery pack using a lithium ion secondary battery and capable of correcting, when temperature around the battery pack is low and causes the voltage of the battery to drop due to the characteristic of the battery, the voltage to a voltage expected to appear at normal temperatures. Therefore, the battery pack outputs a constant voltage on its output terminal without regard to the ambient atmospheric temperature. Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

Claims: A battery pack including a lithium ion secondary battery (1), comprising: a protection circuit (5) consisting of an overdischarge protecting circuit, an overcharge protecting circuit, and an overcurrent protecting circuit; characterized by a temperature sensing circuit (2) consisting of a temperature non-linear element, a current detector, and a voltage detector; a controller (3) for controlling a boosting ratio in response to an output of said temperature sensing circuit (2); and a boosting circuit (4) for boosting an output voltage of said lithium ion secondary battery (1) in response to a control signal output from said controller (3). A battery pack as claimed in claim 1, wherein said temperature non-linear element comprises a thermistor (21). A battery pack as claimed in claim 1, wherein said control circuit (3) feeds a control signal to said overdischarge protection circuit in order to control an overdischarge protection voltage of said overdischarge protection circuit.

Inventors: TAKAMORO KENJI, TAKAMORO, KENJI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: H02J7
IPC codes: H01M10, H02J7, H02H7