With the remarkable development of small, thin, and high-functionality portable electronic equipment such as mobile phones and PDAs, there is a growing demand for smaller, thinner, and high-capacity batteries as their power source. Lithium ion rechargeable batteries can be designed small and high-capacity, and in particular, flat prismatic types are suitable in making the equipment thinner; they have therefore been increasingly used as the repeatedly usable rechargeable battery for portable electronic equipment.
Because lithium ion rechargeable batteries have high energy density and contain a flammable organic solvent as electrolyte, it is essential to take account of safety measures. They must have such safety features as to ensure that no damage is caused to the equipment or injury to the user in the event that an abnormality arises for some reason. For example, if the positive and negative terminals of the battery are short-circuited for some reason, a large short-circuit current flows in high energy density batteries, whereupon the inner resistance generates Joule heat and the battery temperature rises. A temperature rise in the battery causes a rapid build-up of gas pressure inside resulting from reactions between positive electrode active materials and electrolyte, or from evaporation or decomposition of electrolyte, which may result in fire or explosion of the battery. Batteries may fall into a high-temperature state not only because of external short-circuiting but also of overcharge; the same applies if the portable electronic equipment loaded with the battery is placed near a heater or left inside a car parked in a hot weather environment.
A battery abnormality can be induced by any of electrical, mechanical, or thermal factors; thus non-aqueous electrolyte batteries represented by lithium ion rechargeable batteries are provided with safety features for preventing batteries from falling into an abnormal state and for evading a further dangerous state even if an abnormality should arise. Such features are usually incorporated in batteries as their own natures; for example, active materials on the electrodes and electrolyte may be made not to be excessively reactive, or, a polyolefin porous film may be employed for the separator because of its “shutdown function,” in which minute pores are softened and close under an abnormally high temperature. Cylindrical lithium ion rechargeable batteries are usually provided with a protective feature such as a Positive Temperature Coefficient (PTC) element connected in series to the input/output circuit at the sealing end, which limits current flow in the event of external short-circuiting. Batteries that do not include the PTC element inside are normally provided with a PTC element or temperature fuse as outside circuit components. Further, a circuit for protecting the battery from overcharge and over discharge is an absolute requirement. In general, these constituent elements are all packed with the battery inside a pack case to form a battery pack.
However, battery packs using pack cases are not suited to portable electronic equipment that are re-modeled in short cycles, because the manufacturing cost of molding dies used in the resin molding of pack cases tends to be high, and the time required for designing new molding dies is relatively long. Moreover, battery packs with resin-molded outer cases also have limitations in making portable electronic equipment smaller and thinner because of the limitations on the moldable thickness in the resin molding process.
Furthermore, in order to prevent the user from disassembling a battery pack for wrong use or for satisfying curiosity, it must have a design that is hardly disassemblable, or a design that alerts the user that it has been disassembled. Taking account that the battery packs are used for portable electronic equipment, they also need to have a rigid structure that can withstand vibration or shocks in a falling accident, and a moisture resistance, particularly for the electronic circuit parts. In achieving the structure having a disassemblablity, a certain rigidity, and a moisture resistant, the idea has emerged that a battery may be united with a circuit substrate including a battery protective circuit by resin molding.
The applicants of the present invention have disclosed resin-molded battery packs in Japanese Laid-Open Patent Publications Nos. 2002-134077 and 2002-166447, in which a battery and a circuit substrate are connected by a connecting member to form an intermediate product, which is placed inside a die, and resin is filled around the intermediate product such as to expose external terminals formed on the substrate to the outside, to unite the battery with the substrate.
Japanese Laid-Open Patent Publication No. 2000-315483 discloses a structure in which a battery and a circuit substrate are connected by a connecting member and placed inside a die, and the circuit substrate is resin-sealed and fixed on the battery or its pack case (battery lid), or both the circuit substrate and the battery are resin-sealed.
According to the structure shown in the Japanese Laid-Open Patent Publication No. 2000-315483, connectors are provided to the tips of leads extended from the resin-sealed circuit substrate for male-female connection coupling with the equipment side connectors. This external connection structure will present no problem if the equipment is relatively large and offers enough space for accommodating the battery, but the battery packs to which the invention is directed are primarily designed for small equipment where battery accommodation space is scarce, and therefore cannot adopt such connection structure. The battery pack according to the present invention requires a connection structure with the equipment side in which, when the battery pack is accommodated in the battery accommodation space provided on the equipment side, contact terminals or probes in the equipment make pressure contact with external terminals exposed on the outside at preset locations of the battery pack.
In the case with a resin-molded battery pack containing a battery and a circuit substrate with external terminals, the outer dimensions of the battery pack and the locations of the external terminals must be precisely controlled, in order that the connection terminals on the equipment side in the battery accommodation space and the external terminals make pressure contact with each other with low contact resistance. If the dimensional precision is low, the contact resistance between the equipment-side connection terminals and external terminals becomes high, leading to malfunctions such as contact failure and voltage drop.
Referring now to FIG. 26, in a structure in which external terminals are formed on the circuit substrate that is arranged parallel to a sealing plate of the battery, it is particularly necessary to precisely control the dimension L from the bottom face of the battery pack 100 to the surface of the circuit substrate 102 on which the external terminals 103 are formed. However, there tends to be variations in the height h of the battery 101, and because the circuit substrate 102 is connected with the battery 101 merely by a connecting member 104, its position and angle tend to vary. In a common battery pack structure, the battery 101 and the circuit substrate 102 are fixed in position by the pack case, so that the dimension L is controlled. However, in a structure in which the battery 101 and the circuit substrate 102 are united by resin molding, measures must be taken to absorb the variations in the height h of the battery 101 and to fix the circuit substrate 102 in position.
Because of the difficulty in achieving precise control of the height, the prior art resin-molded battery packs employ one of the aforementioned connection structure using connectors or other alternative structures including, for example, a structure in which biasing means such as a spring is provided in the battery accommodation space in the equipment to apply a bias to the battery pack toward the equipment-side connection terminals so as to absorb the variations in dimensions; and a structure in which the battery pack has external terminals exposed at one end on the longer side, which will make sliding contact with resilient connection terminals on the equipment side when the battery pack is inserted into the battery accommodation space of the equipment, so as to avoid contact failure. These structures, however, require an increased accommodation space for the battery pack on the equipment side and a larger number of components for the connection of the battery pack, prohibiting the miniaturization of such equipment. Namely, these structures are hardly applicable to small electronic equipment such as mobile phones that have limited space.
Furthermore, the resin molding for uniting a circuit substrate with a rechargeable battery must be performed under a low melting temperature of the filled resin and a low temperature of the die used, and a possibly small molding pressure so that the rechargeable battery and the electronic components are not thermally or mechanically affected. For example, favorable resin molding can be performed using hot melt resins that melt at low temperatures and are moldable with low temperature molding dies and low pressure.
Hot melt resin, however, has a low hardness after curing and is not suitable as an outer member exposed to the outside because it is susceptible to scratches and contaminant adhesions. It is suited for uniting the rechargeable battery and the circuit substrate, but for the outer material, a resin that has a relatively high hardness is more desirable for good appearance.
It is an object of the present invention to provide a battery pack of a rechargeable battery and a circuit substrate that are united by filling with resin, the battery pack being covered with a resin mold made of a resin material that is suited as an outer member at a given location exposed to the outside of the battery pack, and to provide a method for manufacturing a battery pack of a battery and a circuit substrate that are united by resin molding, with which the outer dimensions of the battery pack and the locations of external terminals are precisely controlled.