I own a 1995 BMW, 3 series with just over 100,000 miles and have had 3 new batteries installed in the past 9 months. Something continues to drain the battery when parked in my garage for sometimes as short as 2-3 days. One mechanic told me I need to drive the car more often but that just doesn’t sound right to me. Please can you help?
You need a different mechanic, someone who is actually interested in doing a proper diagnosis.
You probably have a short circuit, or a piece of electric equipment that is staying on when it should shut off. Less likely, you have a bad battery or a bad alternator. First step is to have the battery and alternator load-tested.
If they check out OK, then you need to look for a current drain. If you have a multimeter, you can connect it to the battery to measure the current drain when the car is off. It might be as much as a few hundred milliamps when the car is “awake”, but should drop to a couple dozen milliamps or less when the car is “asleep.” (Awake means immediately after you turn it off, or when you do things like open the door or click the remote, which wake up the computer and get the car ready to be driven.)
If you have a large current drain when the car is turned off, you can start pulling fuses to try and identify the source of the leak.
Check the alternator first. When they start to die, they sometimes short and then drain the battery very quickly. Your car is definitely old enough to have a bad alternator.