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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!

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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...

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    The CTEK 7002 is an excellent charger with "float charge" abilities. Always be aware of hydrogen gas exiting a charging battery. Keep sparks away from the batt. – John Canon Dec 27 '18 at 1:15
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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.

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    As well as the standby current (which is taken by things like remote door locking, alarm systems, etc) lead acid batteries self-discharge by about 5% per month. Even ignoring the self discharge, if the standby current is 10mA (0.01 amps) that would completely discharge a 40 AH battery in about 4,000 hours, which is a bit less than 6 months. – alephzero Dec 27 '18 at 1:53

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