1. Field of the Invention
The present invention relates to a battery system with a plurality of batteries disposed between a plurality of stacked battery holders, and in particular to a battery system with a plurality of stacked battery holders connected and held in place by bolts.
2. Description of the Related Art
As a battery system housing many batteries capable of being charged, a configuration has been developed that has battery units stacked in a plurality of levels with each battery unit housing many batteries (refer to Japanese Laid-Open Patent Publication 2008-110693).
In this battery system, the number of batteries inside can be increased by increasing the number of levels of stacked battery units. Since this structure can adjust the number of batteries by adjusting the number of battery unit levels, it is suitable for use in applications such as in hybrid cars where the required output is different for different types of vehicles.
The battery system cited in JP 2008-110693 A has connecting bolts that pass through the stacked battery units in the direction of stacking to hold those battery units. The connecting bolts are inserted through the stacked battery units and fastened in place. As shown in FIG. 1, the connecting bolts 95 of this battery system pass through the interior of the battery units 93. Specifically, the connecting bolts 95 pass through a plurality of locations interior to the perimeters of the battery units 93. In this type of battery system, since connecting bolts 95 pass through the interior of the battery units 93, space for connecting bolt 95 insertion must be allocated between adjacent batteries. If many connecting bolts are used to tightly connect the battery units, the battery unit outline (footprint) becomes larger. In addition, a battery system of this structure has the drawback that because connecting bolts are disposed at the interior of the battery units, it is difficult to join the battery units with their peripheries tightly connected. In particular, if connecting bolts are strongly tightened to solidly attach the battery units, battery unit center regions can indent making tight connection at the peripheries even more difficult. This drawback becomes more serious when many batteries are disposed in a single plane and battery unit outline becomes large.
To tightly connect battery unit peripheries, a structure can be adopted that has connecting bolts at the battery unit perimeters. In this structure, connecting cylinders are established in single-piece construction with the battery units in a plurality of locations on battery unit perimeters and extending in the direction of battery unit stacking. Connecting bolts are inserted through the connecting cylinders to connect the battery units. A battery system with this structure has the characteristic that since stacked battery units are connected by connecting bolts inserted through connecting cylinders established at battery unit perimeters, battery units can be reliably joined while tightly connecting their perimeters. However, in a battery system of this structure, if the number of stacking levels becomes large, the connecting bolts that hold the structure together become long. To reliably hold battery units stacked in many levels, it is necessary to increase the diameter of the long connecting bolts. This is because long thin connecting bolts can stretch, and battery units stacked in many levels cannot be tightly retained over a long period without loosening. However, if connecting bolt diameter is increased, connecting cylinders must be made wider and battery unit outline increases. This is because wide connecting cylinders protrude out from the perimeters of the battery units. In addition, in a configuration that disposes bus-bars at battery unit perimeters to connect the batteries in series or parallel, many wide connecting cylinders protruding from bus-bar perimeters has the drawback that those connecting cylinders can interfere with bus-bar locations.
The present invention was developed with the object of correcting the drawbacks described above. Thus, it is an important object of the present invention to provide a battery system that can tightly and reliably hold battery unit perimeters together without increasing battery unit outline.