Patent Application
20120031645
This invention relates to the field of battery packs and more particularly to a system for connecting battery cells in series and/or parallel within a battery pack.
Multiple battery cells are often integrated into a single battery pack. Such battery packs are often used, for example, in portable computers, power tools and other devices. A majority of such battery packs include rechargeable battery cells such as lithium ion, nickel metal hydride or nickel cadmium battery cells.
The battery cells are often arranged in series, parallel or series/parallel arrangements to create battery packs that deliver either a higher voltage (serial), higher current (parallel) or both (series/parallel). To connect the individual cells, often terminal strips are spot welded to terminals of the cells, providing a conductive path between terminals of the battery cells. Spot welders use various technologies to physically and electrically bond the terminal strips to the battery terminals. For example, many spot welders use a capacitive discharge in a technique called fusion welding. This allows welding of highly conductive metals such as copper and brass as well as more resistive metals such as steel and nickel.
Another method of interconnecting battery cells in a battery pack is soldering. In this, the terminal is soldered to the terminal strip using a lower melting point metal such as lead, tin or silver.
All methods of interconnecting battery cells introduce heat into the battery cells. For some battery cells, in particular lithium ion battery cells, excess heat will damage the battery cells. For example, in lithium ion cells, excess heat applied to the battery terminal causes the lithium and cathode separator within the cell to melt. The separator (between the lithium cathode cap and the cathode) becomes perforated and the anode and cathode become welded together causing an internal short, thereby reducing the open circuit battery cell voltage and/or electrical capacity. Therefore, the battery cell loses functionality and, in some cases, bursts.
What is needed is a terminal strip system that will reduce heat transfer to the battery cells during welding.
In one embodiment, a battery terminal strip for connecting to a terminal of at least one battery cell is disclosed. The battery terminal strip includes a strip of metal that has at least one weld point formed in the strip of metal. Each of the weld points having a protrusion on a first surface of the strip of metal such that the protrusion contacts a terminal of a battery cell of the at least one battery cell thereby forming an air gap between the first surface of the strip of metal and the terminal of the battery cell, thereby reducing heat transfer from the strip of metal to the battery during welding.
In another embodiment, a method of connecting a battery cell to a terminal strip is disclosed including providing a terminal strip comprising a strip of metal having at least one weld point formed in the strip of metal. Each of the weld points having a protrusion on a first surface of the strip of metal such that the protrusion contacts a terminal of a battery cell of the at least one battery cell thereby forming an air gap between the first surface of the strip of metal and the terminal of the battery cell, thereby reducing heat transfer from the strip of metal to the battery during welding. The method continues with positioning the battery terminal strip against the battery terminal of the battery cell such that the protrusions of the weld points contact the battery terminal and fusing the protrusions of the weld points to the battery terminal by discharging a spot welder at the weld points. The above steps are repeated for subsequent weld points.
In another embodiment, a battery terminal strip for connecting to a terminal of at least one battery cell is disclosed includes a strip of metal having sets of four weld points and a terminal strip vent formed in the strip of metal. The terminal strip vent is located substantially equidistant from each of the weld points and each of the weld points have a protrusion on a first surface of the strip of metal such that the protrusion contacts a terminal of a battery cell of the at least one battery cell thereby forming an air gap between the first surface of the strip of metal and the terminal of the battery cell.
The invention can be best understood by those having ordinary skill in the art by reference to the following detailed description when considered in conjunction with the accompanying drawings in which:
Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Throughout the following detailed description, the same reference numerals refer to the same elements in all figures.
Referring to
Another disadvantage of this interconnection method is the terminal strip 10 blocks the battery cell 8 vent hole (not visible because it is blocked by the terminal strip 10). Many battery technologies, such as lithium ion, include a vent hole 6 (see
Referring to
As the spot welder discharges energy at the weld points 22, intense heat bonds (welds) the protrusions of the weld points 22 to the battery terminal 9/11. Because the terminal strip 22 is made of metal which is a good conductor of heat, during the welding process, residual heat conducts across the terminal strip 20. The weld points 22 create an air gap (e.g., a 0.006 inch air gap) between the terminal strip 20 and the battery terminals 9/11. Since air is a poor conductor of heat, more of the heat generated at the weld points 22 conducts down the terminal strip 20 instead of conducting through the battery cell terminal 9/11 and into the battery cell 8, thereby reducing heating of the battery cells 8. Eventually, the heat conducts into the air and into neighboring battery cells 8, but the maximum temperature allowed by the manufacture of the battery cells 8 is not exceeded by the welds made at the battery cell 8.
The vent holes 24 in the battery cell terminals 20 provide an escape mechanism for gasses that build up within the battery cells 8 that escape from the battery vent holes 6 when, for example, the battery cell 8 is over charged, over discharged or damaged. In some embodiments, a terminal strip vent hole 24 is formed or molded in the terminal strip 20. The terminal strip vent holes 24 in the battery terminal strip 20 align with the vent holes 6 in the battery cells 8 to provide maximum exhaust surface area should venting occur.
To connect a battery cell 9 to a terminal strip 20, the battery terminal strip 20 is positioned against at battery cell terminal 9/11 of the battery cell 8 such that the protrusions of the weld points 22 contact or touch the battery cell terminal 9/11. If the battery terminal 9/11 has a vent hole 6, the optional terminal strip vent hole 24 is aligned with the vent hole 6 of the battery cell terminal 9/11. An electrode of a spot welder contacts the weld points 22 and discharges to fuse the protrusions of the weld points 22 to the battery cell terminal 9/11. The steps are repeated for subsequent weld points 22, optionally waiting for the terminal strip 20 and the battery cell terminal 9/11 to cool between weld steps. In embodiments in which the terminal strip 20 is a copper strip 20, the spot welder electrodes are preferred to be resistive electrode. It is preferred that these resistive electrodes be made of molybdenum. Other electrodes made from copper, copper-tungsten (AMPCOLOY) and tungsten do not create as good of a weld as does molybdenum,
Equivalent elements can be substituted for the ones set forth above such that they perform in substantially the same manner in substantially the same way for achieving substantially the same result.
It is believed that the system and method as described and many of its attendant advantages will be understood by the foregoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely exemplary and explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.
