Battery bus bar design and laser welding

A battery assembly includes a plurality of battery cells each including a cell tab and a bus bar connected to the cell tabs of adjacent battery cells. The bus bar including a pair of 180 degree bend regions that each define a channel for receiving a respective cell tab and a cut-out region defining an opening having opposing edge portions that allows direct access to the cell tab within the cut-out region. A weld line connects the cell tab to at least one of the opposing edge portions within the cut-out region.

FIELD

The present disclosure relates to a battery bus bar design and laser welding methods.

BACKGROUND

A battery assembly for hybrid, plug-in electric (EV), or other type vehicles includes a plurality of battery cells. Cooling fins, repeating frames and foams, and the battery cells are stacked to form a battery module or battery pack. The battery module for a long driving range EV may contain hundreds of battery cells.

Currently, after stacking and assembling the battery cells and other components into the battery module, every two or three adjacent battery cells are welded together with U-channel bus bar sheet metals to form serial/parallel electric connections. Specifically, each of the battery cells has at least two tabs or electric terminals (one positive, one negative) that are welded to the U-channel bus bar sheet metals. In certain applications, the tabs of each of the battery cells are bent such that the tips of the tabs are aligned after welding. Unfortunately, due to the difficulty of precise tab bending and position control during welding, uneven tab height still exists. Uneven and misaligned tabs affect the overall weld quality between the tabs and the U-channel bus bar sheets. Furthermore, because of the irreversible nature of current welding techniques, such as laser welding and ultrasonic welding, removing a defective or “bad” battery cell from a fully assembled battery module requires cutting all of the tab connections in the battery module, thereby rendering the “good” battery cells unusable for re-weld or restoration to another battery module.

It would be desirable to develop a battery module and a method of assembling the battery module that is more reliable and less susceptible to defects.

SUMMARY

A battery assembly includes a plurality of battery cells each including a cell tab and a bus bar connected to the cell tabs of adjacent battery cells. The bus bar including a pair of 180 degree bend regions that each defines a channel for receiving a respective cell tab and a cut-out region defining an opening having opposing edge portions that allows direct access to the cell tab within the cut-out region. A weld line connects the cell tab to at least one of the opposing edge portions within the cut-out region.

DETAILED DESCRIPTION

With reference toFIGS. 1-3, a bus bar10is shown connected to a cell tab12of a battery cell14. As shown inFIG. 2, a battery module uses a group of bus bars10, each including a copper portion16that is laser welded to a group of negative cell tabs/terminals12A which are made of a copper material, and an aluminum portion18that is laser welded to a group of positive cell tabs/terminals12B made of an aluminum material. Each of the copper and aluminum portions16,18of the bus bars include a 180-degree bend region20that defines a generally U-shaped cross section in which the cell tabs/terminals12A,12B are nested during assembly.

As shown inFIGS. 1 and 3, a cut-out region22is provided along a portion of the bus bar10that is between a pair of bend regions20. As shown inFIG. 3, the cut-out region22defines an opening having opposing edge portions24between which the cell tab12is received. The cut-out region22allows direct access to the surfaces of the cell tab12and the edge regions24for welding.

As shown inFIG. 2, the bus bar10is free floating so that it is capable of reducing gaps in the laser welding and thereby improve weld quality on the welds. By free floating it is meant that the bus bar10is not embedded in a rigid molding that inhibits the movement of the bus bar10. Therefore, there will be reduced stresses on the welds due to vibrations in service. As shown inFIG. 2, the position of the adjacent battery cells14are shown misaligned with the free floating bus bars10nonetheless shown cocked for fully receiving the misaligned cell tabs12therein.

In addition, with reference toFIG. 3, the cut-out region allows either a butt joint or fillet joint welding operation for connecting the cell tabs12to the bus bars10without associated problems that occur due to varying gaps in bus bars without a cut-out region or that are not free floating.

As shown inFIG. 4, the depth of the cut-out regions22between the bends20can be deep enough so that the cell tabs12extend beyond the opposing edges24so that the extending portion26of the cell tabs12provide “filler material” for butt welding in a direction of the arrows “A” shown.

Alternatively, as shown inFIG. 5, the cut-out regions22between the bends20can be deep enough so that the cell tabs12extend beyond the opposing edges24so that the extending portion26of the cell tabs12enable fillet welding in a direction of the arrows “B” shown.

As a further alternative, as shown inFIG. 6, at least one of the edges24along the cut out region22can be coined with a step region30for providing a reduced thickness region requiring lower laser energy for a fillet weld “B”. Alternatively, the edge24can be formed in a wedge shape32as shown inFIG. 7for providing a reduced thickness region requiring lower laser energy for a fillet weld “B”. The reduced thickness obtained by coining or wedging at the weld joint provides for even heat distribution.

With reference toFIG. 8, the bus bar10is shown including opposing edges24along the cut-out region22having two different levels for increasing the laser welding target size and absorbing heat from the welding operation. The uneven edges24of the bus bar10and extending portion26of the cell tabs12provides a fail-safe condition if the weld were off target, and in over-welding conditions when the tab is completely melted, and help absorb additional heat for the fillet weld in the direction of the arrows “B”.

With reference toFIG. 9, the bus bar10and cell tabs12A,12B can be clamped by a clamp mechanism40during the welding operation. The clamp mechanism40can close the metal gaps and enhance heat dissipation during the welding operation.

As shown inFIG. 10, the weld process can be performed using an oscillating laser welder50.

Precision laser welding is used to partially melt and metallurgically connect the edges24of the bus bars10to the terminals12in order to avoid ultrasonic welding that could inflict too much energy into a cell terminal12and in turn damage a cell's internal electrical connections. During welding, laser energy will be aimed at this surface. The result is that the bus bar10and battery terminal12are attached to each other along the length of the cut-out region22in which the battery terminal12resides.