The battery level went from 37% to 0% and therefor a forced landing was done without any prior warning. How is this possible?
We will need to see your flight log to answer your question. You can upload your log to this link and share a link back here to the uploaded file if you wish others to assist you.
It appears there are two issues here. One is your Battery#1 cell. It was way off from the other 2 and It looks like it collapsed at the time of autoland. Second is the issuance of full down throttle prior to autoland and the full reverse elevator. You were already about to crash in to the sea before the battery collapsed.
Why would a battery do this? And without any low battery warning?
On the video you can see the drone wasn’t really close to water and dropped down quite a bit before going under.
The battery has 3 cells. Each of these should remain in balance within a certain tolerance factor. If you look at the log you uploaded, you will note several over differentials. Cell #1 especially so. It was far lower and collapsed. Below shows the cell I am referring to.
This is what I mentioned in reference to the full throttle down. You were already descending before autoland initiated.
One possibility, if your battery was not topped off just before flight, is that the battery had entered into the discharge state to reach 50% for storage, and you flew with a partially discharged unbalanced battery. During that discharge, the cells are not balanced as they are discharged, which could easily lead to the cell imbalance observed. Topping off the battery just before flight rebalances all the cells, which will prevent this type of cell imbalance, which can lead to catastrophic failure.