Jim, the keep-alive for the radio really shouldn't kill a well charged battery in any length of time. A battery has a certain amount of "self discharge"--that is, it just loses charge sitting around, and of course if it doesn't get completely charged up, the situation will be worse. I suspect that's all that's going on. When my cars sit around a long time, I put them on the charger for a while. A battery tender probably is a good idea, too, although I haven't ever used one.