THere are various ways to test the 12vdc battery. IF it's original, it might be worth it to just replace as it will probably need it fairly soon, anyway. There's a thread or two around that describes how to do that. When a new battery is installed in a modern BMW, you should 'register' it with the computer so that the charging system can optimize its life. There are places that can do that outside of the dealership. The dealer tends to charge way more than the task requires (about 5-minutes of work to register it). A weak 12vdc battery can cause all sorts of weird things to happen. The ambient temp can make the voltage go up or down, and it may have just been a little warmer when it worked.