Expert: John Kingery - 5/12/2008

Question
Hi, I have a 1995 BMW E38.  My 5 year old battery recently died.  I replaced it 3 months ago and I frequently have to jump start the car.  The first 2 months I could tell that the battery was low and the car would turn slowly.  Now, at random time, the car will not turn over at all unless I jump start it.  The battery has been tested and is fine.  The alternator seems ok.  Could it be a relay or something else??

Answer
You need to check the electrical drain on the system.

Using a meter, determine the drain on the system when it is at rest.  A good way to do this is to disconnect the battery and put the meter between the battey and the cable.  (Keep the meter outside the trunk so you can see it.)

The current draw of the car should be about 0.7 amps when everything is shut down.  It should stay at that level for 16 minutes, then kick into a "sleep mode", with a draw of 0.02 amps.  It should stay at this level indefinitely.

You may find that something is causing the car to "wake up" out of sleep mode, and causes the battery to drain fast when it isn't running.  To find this, it's a tedious job of checking every circuit.

Another way to check this is to disconnect the battery cable every time you park the car, then reconnect it when you start it.  If it starts fine, it's draining when it should be in sleep mode.