2700 Battery Testing

The Goal

In order to build a battery pack for a solar car, nearly 1400 small cells are used to create one large pack. Our team ordered excess cells expecting some to perform excessively better or worse. Sorting through this many cells can be difficult, however. To do this, We decided to create an automated test sequence and then employ members from nearly every subteam to test these cells.

In order to shorten the time testing took, we took a sample of full discharge tests, then performed partial discharges to compare the rest of the cells. In total, the testing took about 2-3 weeks, compared to the years it would take if we had done full discharge tests.

Materials

• Raspberry Pi
• 16 Relay Board (10A)
• BK Precision 8502 DC Load
• Keithley 2450 Source Measuring Unit
• Signal LEDs
• Laser engraved testing setup
• Four Point Battery Holder

The testing setup is laser cut and engraved. After all the time that had been put into the testing, I figured it was worth a few hours to make the setup look sleek.

The Keithley SMU allows for four point measuring, meaning that we were able to create a load on two pins and take a seperate measurement on the other two. This allowed for precision measurements but also made creating discharge curves easier. The BK Precision was able to pull up to 10 amps, while the Keithley could only pull 1 amp. When performing partial discharges, the BK was able to create a more representative curve in a shorter amount of time.

The Raspberry Pi was connected to a monitor with a mouse and keyboard. The GUI I had created allowed the users to start, the test, as well as enter data during the test. This included the cell ID that was automatically entered by a barcode scanner and the Analog impedance as measured by a handheld device. I later adapted the program to allow users to connect to the Keithley via IP address through the interface rather than editing the code. As we troubleshot the process, I added catch statements that prompted manually entered data if it was out of the expected range. I also added an override for the cell ID in case the wrong cell had been scanned.

How To Run the Tests

In order to test a cell, the user first had to set up the Keithley, BK, and the test unit. This was made as user-friendly as possible through minimal connections, detailed instructions, and color-coded electrical tape. Once set up, and the test program launched, the user interface made testing really easy. to start, you click the start button, and it prompts you to scan the cell. Taking the scanner and scanning progresses the program, where another prompt pops up asking for the analog impedance. This measurement is taken, the battery is placed in the holder, and the user presses continue. The program then takes the open-circuit voltage, DC internal resistance, and capacity ratio (discharge test). These tests were determined to be achievable while giving a good characteristic of the cell for comparison. As the test runs, the red LED is lit up signifying that it is not safe to remove the cell (it is discharging at 10 amps). When the white LED comes back on, it is safe to remove the cell and start the next test.

The user interface made it easy to test lots of cells, as it automatically stored the data as we went, and by the time we ran tests, it was mostly bug free and few issues were encountered.