This is a great tip, however you should keep a couple of important things in mind. Soldering the battery back on is a bit tricky because you don't want to overheat the battery during the process because it could explode. At the very least wear eye protection. Also, you are correct in that you need to use the keys every so often, BMW recommends each master key be used at least twice per year. I believe BMW specs that a fully charged key will last 15 open/close cycles (I have to look that up and will correct it as needed-at which time I'll remove this note).