Maybe the battery has become sulfated, and is now unservicable. Often when storage batteries are deeply discharged or sit unused for long periods ( no charging or load ) they sulfate, resulting in a condition where they never reach a full voltage per cell. They act partially shorted, and can draw huge currents under charging, yet do not retain a charge.
Some chargers check the battery voltage to determine its state of charge, shutting off when the battery reaches the correct voltage. Some chargers are more sophisticated in sensing when a battery is full, pulsing the current, letting the battery rest, then check voltage. Others simply apply a fixed voltage, and when the battery reaches that voltage current drops off, by simple Ohms law.
Yours sounds like a fixed voltage charger, and if the battery never comes up to that voltage, it will continue to draw a large current from the charger.
I hope this battery is a "deep-cycle" type battery that is able to withstand being completely discharged. It sounds like yours is, but I am not sure. If it is not, it may be damaged.
Check the condition of the battery. Is there enough electrolyte in each cell, measure the cells with a hydrometer, check their voltage, etc. Depending on the battery type, this may or may not be possible.