1. Field of the Invention
The present invention relates to an assembled battery system provided with a plurality of secondary batteries and, more particularly, to an assembled battery system having an improved safety circuit configuration which protects the system even if an abnormality such as internal short circuit is occurred in the secondary batteries.
2. Description of the Related Art
Recently, a secondary battery having high-energy density is developed and is utilized as a power source for a small-sized information apparatus such as a cellular phone, notebook-sized personal computer, and the like. Such a secondary battery is utilized in some cases as an assembled battery or a packed battery in which secondary batteries are connected in series or in parallel in accordance with a voltage and current necessary for the apparatus to which the battery is applied. Heretofore, in most of small-sized information apparatuses, a battery is provided with one secondary battery or not more than three secondary batteries which are connected in series.
However, in recent years, the use of the secondary battery is not limited to the information apparatus, and is now rapidly extending in an application of high power output or high voltage such as household electrical appliance, power tool, power-assisted bicycle, hybrid vehicle, and the like. Concomitantly with the use of the secondary battery in the wide field described above, the amount of energy required of the whole assembled battery is also increased.
Basically, the battery capacity of the assembled battery using secondary batteries can be arbitrarily designed for such a requirement. Hence, it is possible to cope with an increase in the required voltage by increasing the number of secondary batteries to be connected in series to secure the required voltage. It is also possible to cope with an increase in the required capacity by increasing the battery capacity of the secondary battery itself to meet the requirement. However, there is a limit to the increasing of the battery capacity of the secondary battery, and thus there is a case where it is necessary to connect the secondary batteries in parallel, or a case where it is desirable that the secondary batteries be connected in parallel. The former case corresponds to a case where the required capacity is enormous, and a single battery can hardly realize the required capacity as in the case of electric power storage. Further, in the latter case, it is difficult to secure the safety by using large capacity batteries in some cases.
In general, as the capacity of the battery increases, it becomes difficult for the battery to radiate heat, whereby the battery becomes subject to a temperature rise. Further, it is known that when a failure that causes the energy of the battery to be concentrated at a part such as an internal short circuit occurs, the safety is lowered. Thus, it is thought desirable that a plurality of battery units for which necessary safety is secured are connected in series, in parallel, or in series-parallel to obtain a necessary capacity.
Here, in an assembled battery including both the parallel connection and series connection, there are basically two types of connection methods. One is a parallel/series connection circuit configuration in which a plurality of battery units are first connected in parallel to form a block, and then a plurality of such blocks are connected in series to constitute an assembled battery. The other is a series/parallel connection circuit configuration in which a plurality of battery units are first connected in series to form a block, and then a plurality of such blocks are connected in parallel to constitute an assembled battery. Further, there is a case where a plurality of series connection blocks and parallel connection blocks are combined with each other in multiple stages such as a connection of series/parallel/series . . . , and the like. However, in such a case, it is seen that the connection includes one of the above two types or both by paying attention to a part of the connection.
Further, in the former parallel/series connection circuit configuration, the parallel battery block can be considered as one battery in view of a battery voltage, and hence battery voltage measurement can be achieved by only measuring the voltage for the number of series connections, thereby offering an advantage that the protection circuit can be made relatively simple. In the parallel/series connection circuit configuration, on the other side, when a fault such as an internal short circuit or the like occurs in one of batteries constituting the parallel battery block, not only energy of the battery unit, but also energy of the other batteries in the same parallel battery block is concentrated at the short-circuited part, and generation of heat is enhanced, whereby there is the strong possibility of the generation of heat being led to combustion or explosion. That is, in the parallel/series connection circuit configuration, there is a state of a configuration similar to the configuration in which larger battery units each of which corresponds to the number of the parallel-connected batteries are used. On the other hand, in the latter series/parallel connection circuit configuration, even when a fault such as an internal short circuit or the like occurs in one of battery units of the series block, the current to be supplied to the series block from outside the series block is supplied to the defective battery while the other batteries in the same series block to which the defective battery belongs are charged by the current, and hence the degree of concentration of energy is less than the case of the parallel/series connection, and the degree of safety is also higher. However, in the series/parallel connection circuit configuration, each of all the battery units constituting the assembled battery can take an independent voltage, and hence in order to prevent overcharging or over-discharging resulting from variation in the unit-battery voltage, it is necessary to monitor voltages of all the battery units, thereby posing a problem that the configuration of the protection circuit is made complicated. Further, in the series/parallel connection circuit configuration, the number of voltage measurement lines is increased, which is not desirable in view of the reliability of the overall assembled battery.
As a method for avoiding both the problems, there is a method disclosed in JP-A 2001-352666 (KOKAI), in which a fuse is used as a protection device of the secondary battery. In the assembled battery system using such a fuse, in the above-mentioned former parallel/series connection circuit configuration, a parallel battery block in which each battery unit and a fuse are connected in series is formed, and the parallel battery blocks are connected in series to constitute an assembled battery. According to the circuit configuration, even when a fault such as an internal short circuit occurs in one battery unit constituting the parallel battery block, the fuse blows out at a time point at which a short-circuit current flows from the other battery unit in the same parallel battery block to the short-circuit part. Accordingly, in the short-circuit battery itself, the same safety as the single internal short circuit can be secured.
When the fuse blows out as described above, regarding the overall assembled battery, when the assembled battery is constituted of only the parallel connections, the short-circuit battery is isolated, and only the capacity of the assembled battery is reduced, whereby the possibility of a serious problem being caused is eliminated.
However, in an assembled battery in which parallel battery blocks are further connected in series, the parallel battery block in which a fuse blows out, and the capacity is largely lowered is liable to be brought into an overcharged or over-discharged state that is a dangerous state. Accordingly, in an assembled battery having the circuit configuration described above, it is important to detect the blowout of the fuse, and, when the fuse blows out, stop or limit the charging or discharging of the assembled battery. However, even when the short-circuit battery is isolated from the assembled battery by the blowout of the fuse, the protection circuit controlling the assembled battery measures voltages of the other normal battery units in the block including the short-circuit battery, and hence there is the problem that it becomes difficult to detect the abnormality of the fuse blowout.