What kind of battery is it?  From experience and that of others Optima batteries seem to not do well if not kept charged.  If taken care of Optima batteries will last for years but if they aren't maintained seem to have a short life...more so than standard design batteries not maintained.

Is the top of the battery kept clean?  You can have current running across the top of the battery between the terminals if there is any kind of dirt there.  

You might get your battery warranty adjustment in cash and try buying a battery elsewhere.

 

A battery tender will help maintain a battery, however if you do store your car during the winter, if you fully charge it up before storage, unhook the cables, and maybe recharge it again 3 months into storage, you should be ok...  the tender on the otherhand, you will still need to make sure the battery is charged up, and that if you have a battery in which you can check the water level, do so. Popping the caps on the battery and taking a flashlight to look down the wells you will see the fluid and just under the surface you'll see the plates in the battery.  The wells have a little projection from the top of the battery case and project downward about 1/4-3/8 of an inch, if you should need to add water do not fill it above that projection, as you could risk the chance of the fluid 'boiling' out of the cap while it charges, either with the alternator or a battery charger.  Reason i mention this is that with the battery tender if it is connected up constantly, you might loose fluid... and if there is fluid lose, there will be a potential for a battery failure...  Yep, a battery tender under 1 amp , usually they run about .5 to .75 amp( or 500 milliamp to 750 millamp)