Flydat

I don't know much about the Flydat but it sounds like it detected that the 12V supply was low. The 12V is not needed to run the engine and so all was well in the air, however, after stopping the engine, the 12V was then required to start the engine but there was insufficient charge in the battery.

My first port of call would be the battery charging circuit. If it is not delivering 13-14V across the battery when the engine is running then you more than likely have a problem with the regulator, the wiring or the connectors. It is not uncommon for the connections into the bottom of the regulator module to burn out. I would start by looking there.

If the battery cannot be recovered with a  regular battery charger then you may have a battery fault. It always pays to check the wiring for chaffing and the integrity of the connections, particularly the grounds.

Let us know how you get on.