Electrical Test in the Battery module manufacturing process

The electrical connection between the cells of an EV battery is fundamental to its correct performance and safety, so testing the bondings is a mandatory step of the battery manufacturing process.

Different types of test or process controls can be implemented:
AOI with or without X-ray, mechanical traction, measurement of the power absorption during welding, electrical resistance measurement.

But only a precise resistance measurement can certify the good electrical connection of the bonding, guaranteeing the future health of the module which otherwise may potentially be vulnerable to dangerous overheating due to the Joule effect as well as premature degradation of performance.

There are generally 3 types of battery packages: cylindrical, prismatic, and pouch.
Please see the images below for the various battery types.

Image 1,  a traditional cylindrical battery pack, is most commonly used in the EV market, which started over 10 years ago. This battery is  made up of lithium-ion and is  encased in metal. The most common shapes for cylindrical cells are 18650 (18 x 65 mm) and 21700 (21 x 70mm). The cylindrical shape allows for the almost even distribution of pressure inside the cell, resulting in better physical stability of the battery under usage and charging conditions. Once the cells are packed into a battery pack assembly, they can be tied together by their + and – terminal plates and encased in a shroud for easy storage and car assembly. Several EV original equipment manufacturers utilize this package type. One negative is that  after assembling 100’s of cells into a pack, there is unutilized space between the cells, which then reduces the battery output power per area.

The prismatic battery, shown in Image 2, is formed in sheets of anodes and cathodes with appropriate and strategically located insulators, and is configured in packs of various sizes to fit the specific product space allocated for intended use. These packs have found more recent applications in phones, computers, and the EV market as well.  The negatives of this type of battery cell are that more advanced thermal management is needed to ensure product safety, and they generally have higher manufacturing costs over the cylindrical type.

The “pouch”, shown in Image 3, is a relatively new type of battery pack; it is lithium-ion based, like the other types, but it is  packed in pouches that  can expand and contract about 10%.  Pouch cells are typically used in high load current systems like energy storage systems, cell phones, and wearables.  They have also made inroads into the EV market. The battery/product designer must account for this  increase in space in the product design adding to the product size; furthermore,  there is a concern for product safety in regards to punctures of the battery in normal product use.

Battery Testing

There are typically 3 parameters to be tested; shorts, opens, and continuity (resistive).  In the battery manufacturing process,  the main assembly process being tested for is the connection of each cell to its corresponding positive (+) or negative (-) terminal plate or buss bar.

It is critically important to ensure positive and negative cells are not shorted together, and to ensure they are properly connected to their corresponding terminal plate.  Whatever the mechanical assembly process used to connect the cells to the terminal plate (ultrasonically welded, soldered, brazed, or other techniques employed),  the bonds must be tested  electrically or the battery manufacturer runs several risks:  potential field returns, battery efficiency degradation, performance, fire risks, and product life span in the field.

To measure this important connection, typically a 4 wire kelvin test is performed, as shown below in image 4.  This requires techniques of probing or contacting the battery cell and corresponding terminal/buss bar, and measuring a very low value resistance. This measurement will vary from one battery manufacturer to another, but is usually in the milliOhm to microOhm (for prismatic cells) range.  These measurements are essentially tests to verify the manufacturing process, rather than the quality or energy composition of the individual battery itself.  Battery impedance measurements (a 4^th parameter) can also be performed, which gives a general condition of the batteries. These measurements do not stress the entire battery pack: they test the batteries between the two terminal plates and help to detect any weaknesses in the cells.

When all is said and done, it comes down to how fast battery testing can be performed, how reliable are the measurements and the equipment, and is there one common solution  for  all  the various manufacturers:  OEM EV,  wearables, consumables, and even battery manufacturers themselves?  Enter  Seica’s next generation Pilot BT  to help solve the challenges of battery testing. See image 5 below.

Seica’s Battery Testing System

The Pilot BT  (see images 5 and 6) is Seica’s next generation flying prober specifically designed to address the needs and challenges of  today’s battery market.

After seeing OEMs and battery manufacturers struggle with their own designed test solutions utilizing stepper motors, pistons, and other rudimentary automation devices, Seica designed the first Pilot L4T over 10 years ago. Now, with many years of experience in this market segment, the new Pilot BT is able to test battery packs up to 1050 x 865 mm in size. Being fully automated and utilizing Bosch conveyance solutions or others, even longer batteries can be tested.  But the real power of the Pilot BT is its speed and reliability.  With built-in parallel testing capabilities, the Pilot BT can perform 4-wire kelvin testing of up to 16 battery cells at once with a beat rate of up to 2400 cells per minute.  Seica’s test engine utilizes a 200 MHz digital signal processor for extremely fast test acquisition times with data processing through a 1 Gigabit ethernet connection.  Flying probe fixtures mounted on each robotic head can be reconfigured easily by operators to test different battery configurations during shift changes within a few minutes. Built-in system diagnostics and calibrations make the system highly reliable ensuring a tremendous uptime and a very high Overall Equipment Effectiveness (OEE).

The Pilot BT can be easily integrated by 3^rd party automation solution providers in their automated lines.  While Bosch conveyance is used in the standard solution, other conveyances can be employed to allow for product testing of not only cylindrical batteries, but prismatic and pouch batteries, as well.  The Seica battery test line also allows for high volume testing of battery anodes and cathodes whether in sheet form and palletized or in roll form in our Reel to Reel flex system. All of  Seica’s battery testers can be connected to customers’ companywide Manufacturing Execution System (MES) through a proprietary software adapter. This delivers  a seamless production floor integration process enabling  all data to be sent to all departments where the digital factory is deployed.  And finally, hardware and software options for this line also include a noninvasive control unit (see image 7) that monitors the system’s energy consumption, vibration, and temperature.
The benefit of these features is to provide predictive monitoring of events on the production floor, affording customer personnel the opportunity to be proactive in floor maintenance procedures.