i have a 306 hdi 2.0 and all the power has just gone off ?

my question is yesterday i went to get into the car switched on the ignition and everything just went off. now the lights have cum bac on on the second position but the lights have faded and when you try to switch the engine on it goes dead at the same time there seems to b a ticking like the alternator has gone which is what im told it is but when u look underneath at the battery one of the terminals is furred up like hell and just wondered if could it be the battery that either want s cleaning and trying again or replacing ?