I connected two 4 cell battery in parallel and did a flight test. There was a sudden roll to the left, the reason of which I cannot find. I am attaching the log file along with this. Kindly look it up and let me know if you can understand the reason. Also look for the RCin VS RCout graph for roll channel. Initially in the earlier two flight test that I did, battery fail safe got engaged even though the voltage was way above the failsafe voltage.