had tt in storage for 5 weeks, had battery disconnected and stored in tt, went to hook up tt, dead battery. have American marine deep cycle battery,came with tt, 4 months ago. want to charge battery at home with battery charger. can find no caps to inspect water level so assume this is a sealed battery? there is a clear glass or plastic port about the size of a dime on top of the battery, what is this for? thanks for help...