Any Smart Tec users out there have to replace the battery in their "sending unit" yet. My Smart Tec arrived with a bad battery in the sending unit (9 dives then dead). I was just wondering how long they are supposed to last? Hate to get half -way through a dive trip and have to change it out again. (rather do it before the trip). Anyone have to do this yet. Or did I just get "lucky" with my initial factory battery?