Join Date: Jan 2012
Location: Jersey Shore, NJ
It's pretty hard to dry out a cell in a good battery with a wall wart battery tender. They generate very low current once the battery hits about 12.6 volts (resting voltage). However ... if one of your other cells is dead (shorted), and the overall voltage never gets to 12.6, sure the current can "cook" the good cells dry. Takes a good while and you still need enough current from the trickle charger, but I've seen it.
You battery is likely toast. If you want, fill it with distilled water, charge it, take it to Autozone or Sears and have them test it (free). You'll know immediately. I'm betting the dry cell is not the bad cell, another is bad ... else the float current would not have been able to cook the water out of it.
Car batteries live in harsh conditions. 5 years is a solid average life. If you are nice to them, they will last 6-7 years. If you are tough on them (heat, cold, vibration, lots of starts, lots of accessories), you'll get maybe 4-5 years or less. Any battery that fails within 2 years was defective from day one (or treated very very badly).
Don't waste your money on extended year warranty batteries. It's just marketing. The prorated money you get back in those last two years is trivial and really just sends you back to the place you bought the first one to get another one.