battery tender?

I have had my 06 rancher since june 12 2007 and the battery has gone dead in that amount of time. Is this typical? The dealer said I need to put a battery tender (trickle charger) on it when I am not riding it to keep the battery from going dead. Nobody mentioned this to me when I bought it so I was unaware. It gets cranked and ridden about once a week so I don't understand why it went dead. Do you guys use a battery tender on your bikes? Thanks.

Also, it was less than a week past the factory warranty expiration date and Honda would not cover the battery. The dealer said that they said if I had been using a battery tender that they probably would have covered it for me. I thought Honda would have taken better care of me than that! Just venting....