I just acquired a used deep cycle marine battery and when I went to charge it the charger "flickers" between "almost full charge" and full charge". That is, it will switch between charge and charge complete each second.

What does this mean???

all chargers are different read your manual all so may want to check the battery with a hydrometer to see if all cells are good JMHO

If your battery is full then it is okay.  Function as design!

the trigger back and forward is the maintainer feature.