Accurately testing 18650 li-ion cells is a very important step in the process of re-using cells from laptop battery packs, or designing a new battery pack in order to create matched modules and maintain a minimum amount of balancing current to keep the modules balanced.
During my first term at the University of Waterloo, I joined Midnight Sun, the solar car student design team and am working on designing a new battery pack for the next car, MSXIV (Midnight Sun 14). During this process, we are looking to accurately test every single cells that we putting in to the car in order to determine their capacity and internal resistance. This will allow us to create perfectly matched packs if the testing is completed accurately.
New cells are being used in this car, and the tests must be able to distinguish cells that fall within the manufacturer’s specified tolerance ranges for the cells. That means that the testing method that is chosen must be accurate to 10mAh for the capacity and ideally less than 1mOhm for the internal resistance.
And so started my journey of figuring out how to test all the cells in a timely manner while not spending tens of thousands of dollars on proper commercial testing equipment.
Starting with, one of the cheapest and most popular cell testers on the market, the LiitoKalaa Engineer Lii500.
I have 9 such testers, part of my cell testing station, and used 9 different cells to test each one, one slot at a time.
Each module was tested with one cell in the same slot multiple times to determine the repeatability of a measurement in the same slot, then the cell was moved to a different slot to see how the measurements compared.
The results can be seen on this spreadsheet, and were somewhat surprising. For tests of the same cell in the same slot, the values did not vary too much, within a range of 20mAh. However, when the cell was moved to test different slots, results were changed to a spread of almost 100mAh for some testers, with the average spread between the 4 slots around 50mAh.
Given these results, these testers are unsuitable for determining small differences in capacity between a batch of new cells, but for testing cells from laptop pulls they are perfectly acceptable. Keep in mind though, that the results will be within a range of around 50mAh from the value that they show.
Test Conducted By: Micah Black
Written By: Micah Black