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September, 2019


The Testing Blind Spot: Battery Interconnect Strength Testing


By Chris Davies, Advanced Bond and Materials Test Applications Engineer, and Conor McCarthy, Product Manager — Bond Test and Materials Test, Nordson DAGE


B


attery lifetime and stability play important roles in the development of next-generation electric vehicles (EVs). The automotive sec-


tor is booming in relation to EVs, and some manu- facturers already have well-established electric cars on the market today. The Organization of the Petroleum Exporting


Countries (OPEC) predicts that there will be 1.7 million electric cars on the road by 2020, while Bloomberg New Energy Finance estimates it will be closer to 7.4 million, with 2 million sold in 2020 alone. Bloomberg also expects electric cars to make up 35 percent of light vehicle sales by 2040. With so many of these vehicles being intro-


duced onto our roads in the coming years, their safety and reliability is critical. The reliability of these vehicles will depend on overcoming the tech- nical challenges around battery lifetime. Battery suppliers and manufacturers need to make sure that they can match the endurance of well-estab- lished components found in a combustion engine which typically last around 10 years or 150,000 miles.


All car functions including safety features


and driver aids require uninterrupted battery power during different stages of a vehicle journey. If electric car batteries cannot maintain a consis- tent power output and performance, the accept- ance of electric vehicles as a suitable alternative to the combustion engine will not be realized.


Intercell Battery Connections Currently, there are three types of battery


configurations that are widely used: cylindrical, prismatic and pouch. Each one has its potential application whether it be in automotive or renew- able storage. Here, we are exclusively focused on discussing the battery packs made of cylindrical units for use in EVs.


Intercell connection components are crucial to managing the flow


of current between each individual cell and delivering sufficient energy to the powertrain.


To match the power outputs required by high-


ly desirable fully electric or hybrid cars, batteries must be energy-dense with high current-carrying connections that are as reliable as the fuel system in a conventional combustion engine. Currently, most battery testing is focused on the cell electro- chemistry or overall pack robustness, including temperature cycling, environmental, charge/dis- charge, and vibration tests.


However, one critical area is being overlooked


— the intercell connection components. These con- nections are crucial to managing the flow of cur-


from faulty batteries before the vehicles leave the production line. To repeatedly charge and discharge high cur-


rents over time without failure, the formation of the interconnects between individual cells must be controlled carefully. Due to the density of materi- als and dimensions of fully assembled battery packs, many visual inspection techniques cannot be used to identify quality issues. Used in isola- tion, top-down visual inspection is effective, but has limitations. The connections may appear attached to the


surface, but there may be a layer of contamination between the connection and surface —meaning the interconnect has not formed a strong bond with the surface below. It can also be possible that the connection is


Battery packs within the rapidly increasing number of electric vehicles must be robust and have proven long-term reliability.


rent between each individual cell and delivering sufficient energy to the powertrain. Connections that are formed with high resistance can lead to heating effects when current is drawn from them. Likewise, should several of these connec- tions break within a pack, the same current will be drawn from a smaller number of cells lead- ing to overheating and, in the worst cases, even fires.


Several technologies are being investigat-


ed for use as the connection method, including ultrasonic wirebonds, laser welded ribbons or spot welded tabs, each with their own advan- tages and disadvantages. Wirebonding is a well-established ultrasonic technology from the semiconductor industry. The process is high- speed, can use multiple metals, such as gold, copper or aluminum and is flexible to allow “s” shaped connections when components are not aligned. However, the strength of the connec- tion is dependent on the cleanliness of the sur- face to which the bond is applied. Ribbon con- nections can be bonded by laser or ultrasonic, handle higher currents, but have reduced con- nection flexibility. The third method, tab weld- ing, can be used with spot or capacitive welding and offers structural rigidity, but on the other hand, is an inflexible connection.


Connection Testing For all three technologies, accurate testing


of these cell interconnects is critical to under- standing if solid bonds have been made, and therefore reduce the chance of vehicle failures


Schematics of pull testing setup (left) and shear testing setup (right) for the three connection types.


Continued on page 66


made but is very weak, due to a small bonded area caused by a fundamentally weak bonding process. In these cases, using a physical test (such as bondtest- ing) to mechanically stress the bond, reveals the true strength of the electrical interconnect. To perform a mechanical test a shear or hook


tool is connected to a highly sensitive transducer capable of 0.01 cN force measurement.


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