It is not healthy for any battery used in automobiles to be run down to empty. The danger lies in un-equal cell capacity. As you continue to discharge the battery the cells which are in series continue to put out the required current however the weaker cell continues to have the current pass thru it which in essence puts a reverse charge on it destroying it. Be careful. Also any on board electronics that requires a keep alive voltage will be re-set. I remember something in the manual for my FEH about the engine and transmission control unit need to be re-trained if the 12 volt battery is disconnected.