Will battery run out if I store my BMW without running for few months ? That is what my mechanic said. But it does not make sense. Please tell me if that is true or not!
Yes, it will.
If you have power in the place where it will be stored then it is best to put a battery charger on it - the latest ones have smart sensing and will not overcharge the battery if left connected for long periods.
One make among many you could check out is CTEK...
All modern vehicles with electronics use some power, even when everything seems to be turned off. That standby current is called "parasitic draw," usually between 10 and 100 milliamps. A good battery can handle that draw for a few weeks/months.
However, it is common practise to disconnect the battery negative post before storing the car. Upon re-connect, you will need to reprogram your radio code and stations. Also, the engine computer (ECM) will automatically re-learn your driving habits.