I took part of the summer off, going on vacation and spending time with my family so not much work being done on the 320i for the past three months. I was able finish the dashboard and all the 12V wiring. I installed the AVC2, which is a module for controlling the J1772 connection to the charger. Back at the beginning of June when I was getting ready to go on vacation I had charged the batteries up to 30AH and 375V which is half capacity and a cell voltage of 3.2V. I assumed with nothing connected and even the manual disconnect switches open that the battery pack would just sit there and stay charged and be ready for me to drive it when I got back. Just a couple of weeks ago I wanted to test out the AVC2 connection and turned on the car. I always check the JLD404 to verify the voltage and state of charge. To my horror instead of reading 370V the pack was only reading 77V!!. That meant the batteries had been discharged to less than 1V! Not a good practice for lithium batteries of this type. Over discharge can cause large losses in capacity because the lithium gets tied up in the cathode material. I do not know what caused the discharge. The only thing connected to the batteries was the BMS. The current leakage in the switches used in the BMS is only a few microamps at 800V. At the voltages the BMS switches see the current leakage should be sub-microamp so it would take a long, long time to drain down 30AH ( more than 30,000,000 hours!). Something else must be going on. My biggest concern was the state of the batteries. I measured the voltage of every battery. They were all around 0.6V and the lowest was 0.25V and the highest was 0.85V. No battery was at 0V so the bottom balancing I did earlier might have saved all the batteries. I removed one of the batteries from the front battery box and connected to my lab bench supply and charged it at 1A until the voltage was 2.7V. That did not take very long - about 60 min. I then could put the battery on my Powerlab charger - that charger will not work on batteries discharged to a low level. I charged the battery at 10A to 3.65V and the battery looked just fine. The charge curve was normal and the battery charge capacity was 64AH. As received from CALIB the average capacity of all the batteries was 66AH so a very small loss in capacity. The problem I had was with the Bursa charger for the main pack because it would not work on a low pack voltage so I had to charge groups of batteries up to 2.7V using the Keithley 2420 source meter I have. Earlier this year I had used that power supply for bottom balancing some of the batteries. I have a program I wrote for doing the bottom balancing, which is essentially what I would be doing again. I charged batteries in groups of 10 to 18 up to an average voltage of 2.7V at a 1A charge rate. I carefully checked the voltage of all the batteries during this initial charge. After all the batteries were bottom balanced I could charge at the Level2 rate with the Brusa charger. Once again I charged the pack to 375V and 30AH and carefully monitored all the battery voltages during the charge. Nothing looked out of the ordinary. I then finished fully charging the pack to 60AH and 413V. Everything still looked ok, except one battery went to 3.65V when the average charge voltage as just 3.45V. I stopped the charge because that battery went to full voltage and I did not want to overcharge that battery, even though I will probably replace it. The question now is how did the pack become discharged in just a couple of months. I know earlier from making measurements there is no leakage path to the car ground. I only measure capacitive voltage when I test from the battery peak voltage to car ground. However, recently when I was changing out that battery in the front battery box that went to 3.65V during the charge I got a shock when I brushed against the battery box. The battery box is grounded, but the batteries should have no reference to that ground. When I measured from the last battery in the front box to ground I measured 53V, which should not be there. I started disconnecting things and found the ground reference was in the Delphi DC/DC converter connection. When I removed the high voltage cable the voltage reference to car ground went away. So either the DC/DC converter is the issue or the cable is. It might explain how the pack could be fully discharged in such a short time. Using a 33k ohm resistor I measured the leakage to car ground from the high voltage point of the battery pack. I got a 2.5V drop on the resistor indicating a 0.076mA current flow. That is well inside the spec for the Delphi DC/DC converter. The voltage that appears to car ground is disconcerting and is probably what gave me a shock. But that small amount of current leakage should not have discharged the pack in just a couple of months. Something else must be going on. The BMS was the only other component that was connected. I still need to connect that and measure any current leakage. When I first started testing the BMS I found that the 3.3V DC/DC converter on the board was not working. That explains why the BMS was not sending any CAN messages when the USB was disconnected. I need to repair that board so I can do some more testing.
Other fun I have been having before the first drive is installing the carpeting and front seats. The carpeting I got from a company called Stock Interiors. When I received it I checked it against the old carpeting. It seemed to have the right shape and contours in the right place. Aftermarket new carpets never have any of the holes cut so I had to cut the holes for the gear shift, e-brake, throttle pedal and the seat mounting points. I did not realize at the time that the contours for the seats areas were not quite right. I struggled to get the carpet in the correct position before I started cutting. It seemed like the carpet was too small in some areas and too big in others. The carpet over the transmission tunnel is loose, but under the seats it is tight. The info on the company's website says it should take 3 hours to install the carpet. I don't know what universe that got that number from but it took me two days to install most of it. I still don't have the kick panels or the rear seat section installed. That number is probably for a late model car that does not have as many contours as the 320i. I got the driver's seat installed, the steering wheel and center console. Everything I need to make the first test drive.
A video of all this fun is avialable here.