Battery

I purchased my Taurus in May of 2011.  Since then, I have had to replace the battery three times, so four batteries in a five-year period.  I brought it to the dealer here in Ft. Lauderdale, Fl.  They told me that nothing is wrong and that the reason is because of the heat.  My car is in a garage in my home, and at work, it is also in a garage, rarely in direct sun.  I don't think that four batteries in five years is normal, but perhaps someone here could enlighten me.