Tuesday 4-30-09: Battery Testing:
The long tedious testing of the individual batteries has begun. After talking to our contacts at A123 and other EV teams, we decided that the two most important indicators of a battery's health are voltage and impedance. We plan to first measure the voltage of all the cells that A123 donated to the team and throw out any cell that are not between 3.296 and 3.340 volts. The cells have been sitting for 3 years so if the voltage is within this range, it is a good indicator that the cell is sealed well and that there is very little current leakage through the cell. This range was given to us in the A123 battery pack design manual that is available to the public. This design manual has a lot of very useful information about building a pack and particularly about building a pack with lithium iron phosphate batteries. Today I checked over a 1000 cells. My goal is to get all the cells checked before the car arrives next week. Today, Mike and I also went to the electrical engineering lab and used their impedance tester to check a box full of good cells (cells that passed the voltage test). The average impedance of the cells was 7 milliohms with a very small standard deviation. It seems to me, and the other team members agree, that we only need to measure the voltage of the cells in order to tell if they are good or not. In other words, if a cell passes the voltage test, it should pass the impedance test as well. If we decide that this is a reasonable assumption to make, we will be able to save a lot of time by just testing the voltages of all the cells.
Cell Level Battery Pack Fuses:
Today I ran a couple of tests to determine if the individual cell fuses are sized correctly. Our nickel sheet is .010" thick and the tab fuses are 5.5mm wide. A123 sizes their fuses to be 3.6 mm wide and .007" thick nickel sheet. They rate there fuses to blow at 2100amps in .1 seconds. Our fuses are currently about twice the cross sectional area of A123's. We wanted to investigate this further so we ran a few tests using an A123 26650 cell. We estimated that an A123 cell can output 300 amps in a short. We cut two test strips of nickel. One that was the size of the fuses that we are currently using on the motorcycle packs and one that was the size of the A123 fuses. Both blew when we shorted the battery across them. This was a good sign because the purpose of the fuses is to protect against a short. After talking it over, we decided to stick with the 5.5mm width rather than reducing it to 3.6mm because we do not want to risk the nickel heating up when we rapid recharge these packs. I went ahead and ordered enough nickel to finish the motorcycle packs and also build one of the car modules. We bought our nickel from National Electronic Alloys Inc. They have a very good selection and they also have good prices.