Battery Caharging on board
In addition to the dangers of heavier than air volotile gases such as gasoline vapour and propane; a battery, while being charged also generates a small amount of equally explosive hydrogen gas (think Hindenberg) any of these volitile gases could be ignited by a connect / disconnect spark. If you must charge on board, ensure adequate ventilation to remove all of these gases (think bilge blower) prior to connection or disconnection. Also to limit the possibility of sparking near the battery, make charger leads fast to battery terminals then plug it in at farthest reach of it's power cord. This should eliminate sparking at the battery (assuming the red is to + ,black is to - terminals).