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FCP Euro Service Kits

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Battery keeps going flat?  It could just be the battery is damaged, and unable to maintain the proper charge, or it could be the alternator is not working properly and the battery is not getting recharged as it should.  There is another potential cause, one that is much harder to diagnose and fix.  The term parasitic battery drain refers to elements in that car that can use power even when the car is turned off!  Whilst older cars were 100% off with the key out of the ignition, modern cars always have at least some power usage when in 'standby' - the obvious example being the remote control receiver waiting to receive instruction to unlock the doors.

Math time

Batteries are usually rated in 'amp  hours' or AH - this defines how much current can be supplied by the battery over time.  A 100AH battery as found in say a BMW E38 can deliver 100 amps over 1 hour, or 50 amps over 2 hours, 25 amps over 4 hours, etc.  The parasitic draw of an E38 should be around 50mA (milliamps).  This means that an E38 should be able to stay in standby for over 80 days!  (50mA translates to 50mAH over an hour, and then 1.2AH in a day, then 110 divided by 1.2 gives 83).  Take that with a pinch of salt though - because having your car still in standby after 80 days does not mean there is sufficient power available to turn the starter motor, most likely it won't even be able to unlock the doors!

Nevertheless, this example should show that a car should be able to sit for a number of weeks quite comfortably even with the standby electronics active.  Let's say that you have a DVD based satnav system that failed to power down and kept the disc spinning - that could be 4A of draw.  4A translates to 96AH over 24 hours (4 times 24), so in this case your car will likely be unstartable after a day - that's a serious problem!  The most common problems fall in between these extreme and you may find the battery dead after a weekend.

If current draw is more than 50mA you likely have a problem

IMG_3658 Set your meter to 'A'

To diagnose this problem you need to measure the current drawn by the car - measuring current means you have to connect the meter inline with the power - not just in parallel as you would do with a voltage measurement.  Usually the best way is to disconnect the negative line from the battery - connect that to one meter lead, and connect the other meter lead to the negative battery post.  So power runs 'through' the meter.  At this point you'll probably find a massive reading!  This is because the car is likely very much awake still - if you open the trunk on a BMW to access the battery you'll find the trunk light goes on - so that's drawing power - but also the module that looks after the lighting and door locks is also still active.  For many older BMW's the magic time to wait is 16 minutes - at this point the electronics are meant to fully shut down.  (Unfortunately some newer cars require 30 - 60 minutes).  So if your current draw is more than 50mA at this point you likely have a problem (and again - 50mA is an estimate - actual figure will vary depending on what features are in the car - and I believe the newest F01 7 Series only has a 20mA draw due to more efficient electronics).

There are usually some tells you can check on whether the car is in standby - such as leaving a door open - the internal lights should go out after that 16 minute mark.  On the E38 the light by the gearstick will be illuminated even after the car is locked - and only go out after 16 minutes.

So now you know you have a problem - what next?  Well, you can just pull fuses and relays one by one and check when the current draw drops on the meter.  The devil is in the details here again though - often electronic modules can share fuses - or the opposite - multiple fuses for the one unit, like the E38 integrated TV/radio controls.  So even after isolating a fuse you may have to proceed to disconnecting individual modules attached to that fuse.  Another detail when swapping fuses in & out is that when you reapply power to some modules they may spring into full life - not standby - so you may have to wait another 16 minutes - or power cycle the car as sometimes a master module instructs sub-modules to go to sleep rather then them deciding for themselves.  Even older car electronics can be a complex business I'm afraid!

Capacitors dry out with age

IMG_3661 BMW LCM module

When you find a bad module - what can you do?  Well realistically replace it like for like, with either a new module or one from a scrapped car.  Most modules are damaged by heat or water or even just time as capacitors dry out with age - repairing is sometimes possible but this is an even more specialist area.

Personally I've found the BMW MK-II satnav to be a culprit for many drains, as it seems to fail over time.  I also once found a drain caused by the LCM (light & check control module) which had incurred water damage, and I hear the GM (general module), TV module and earlier cell phone units are common problem points.

 

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About the Author: Bryan McPhail

IMG_2691Bryan is a longtime BMW enthusiast in Florida.

 


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Written by :
Bryan McPhail

Bryan is a longtime BMW enthusiast in Florida.


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