Timm's BMW E38 Battery Drain - updated
2018 with new method
How to test if you have battery drain - or
just dead batteries
This a common problem with E38's. The car has been working faultlessly,
but leave it for a couple of days and the starter only clicks, there is no
chance of it starting. There is endless advice on the internet as to the
causes of this drain, but in my experience only one in a hundred cases
are due to an electronic fault, most are due to the way the car is used.
The BMW Battery
Most battery 'drain problems' are due to the battery state of charge.
Where a car is used for short journeys, especially in cold weather where
many electrical consumers are in use (heated rear-window, heated seats,
blower on full-speed, headlights etc), it is possible that the alternator
cannot sufficiently charge the battery. In these situations a fully-charged
battery's state of charge will slowly reduce to a point where there is
insufficient energy to operate the starter motor.
Where a battery is operated with a low state of charge over any period of
time it will be permanently damaged due to sulphation. This effectively
reduces the battery's capacity (the amount of energy that it can store),
and its ESR (effective series resistance) which reduces the amount of
current the battery can supply during cranking.
Sulphation can occur in a matter of months if the state of charge is
maintained at 20%. I have often heard It can't be the battery, it's only 6
months old. Unfortunately, yes it can, if you do not maintain a good state
of charge the battery is not going to last long, even if it is the OEM
Bosche unit. More information on the BMW can be read here.
Maintaining a good state of charge
Each time you start the car a large amount of energy is consumed. This
energy has to be replenished by the alternator, the amount of time it
takes to replace this energy is entirely dependant on what consumers are
operating and the RPM of the engine. With many consumers operating
(especially the blower, lights and heating elements) the alternator will not
replenish this energy unless the RPM is greater than 1500RPM. Any lower
than this and the battery is actually being discharged. If very few
consumers are operating the battery will charge even down to 600RPM.
These situations are quite normal, especially in the winter. If the journey
is quite short then the alternator will not replenish the energy consumed
when starting the engine. Over a period of time the state of charge will
reduce, sulphation will increase and the battery will slowly lose its
efficiency. If your use of the car is similar to this, then you will have to
maintain the battery using a battery charger. Choose a battery charger
that is automatic, these types fast charge the battery and then change
mode to maintain the battery without over-charging. My 740i has a BMW
charger permanently situated behind the battery, in winter or where the
car is little used, I run a mains lead under the boot lid and leave it on
charge.
Longer winter journeys may replenish only a proportion of the energy
expended, but once the blower speed is reduced and the heating
elements switched off the battery will start to charge correctly. However,
it may take a journey of many hours to reach 100% state of charge.
Before blaming battery drain always ensure that the battery is in good
condition and is fully charged using an automatic charger. Our BMW
batteries have a capacity between 90 and 110 Amp-Hour. This means
that a 5A battery charger will take 22 hours to fully charge a discharged
battery. A 10A charger will take 11 hours.
Measuring the state of charge
The state of charge can be measured using a voltmeter. This
measurement must be taken after the surface charge is removed. This is
simple to do, switch the ignition to position II and switch the lights on
full-beam for 2 minutes. Switch the ignition and lights off and measure
the battery voltage at the battery terminals. The battery being measured
here is essentially fully discharged:
The state of charge is determined using this
Table:
I measured my 840ci voltage today, the temperature is around freezing
and I measured 11.9V which means I have a 10% state of charge. It is
at these voltages that sulphation damage will occur and the capacity of
the batteries (it has two) will be reduced. If I had left the car another
few days it would not have started. I have put the batteries on charge.
To maintain good battery condition, the state of charge must kept above
60%. Any lower than this and the battery will deteriorate. A battery kept
over 60% state of charge may last a good 10 years, a battery kept a
20% will be destroyed in a little as 6 months. Sulphation can be reversed
to some extent by using a very slow charge and by agitating the
electrolyte (driving over bumpy roads!). Generally, once sulphation has
occurred the battery needs to be replaced.
Measuring Battery Drain
The BMW E38 electrical system demands high currents for a period of
time after the ignition has been switched off. Increased current will be
seen for up to 16 minutes after the car is switched off and fully locked.
During this period a number of electrical consumers are still in operation
including the heating and ventilation system, the memory system and
certain illumination.
It is important when checking for battery drain that the electrical system
is not disconnected. This is because disconnecting the battery may reset
the system that is demanding current and the evidence may be lost. It
also means that you will have to wait for 16 minutes for all the
consumers to switch off.
The correct way to measure current is to fully lock the car with the
ignition key removed except for the boot. To mimic the boot being closed
and locked push a screwdriver shaft sideways into the lock situated on
the boot lid:
This will operate the lock mechanism and switch off the boot light. The
Ammeter is now connected between the negative battery post and a good
earth point on the chassis. Ensure that the Ammeter probes are
connected to the correct terminals on the meter (usually the 20A or 10A
terminals). Now unbolt the earth strap from the chassis of the car.
Using this method the Ammeter forms the connection between the
negative battery post and the chassis, maintaining the electrical system
during the test. The current demand may be as high as 6A during the
initial period so ensure the Ammeter is capable of conducting this much
current.
After 16 minutes the current should reduce to 50mA (0.05A). If the
current reduces to this reading then you have no current drain as 50mA
or less is the correct current consumption. If the current is above this
then there is some problem that needs investigation. At 50mA, a battery
in good condition and fully charged will provide enough capacity for
starting for a period of 70 days. At a current drain of 500mA the battery
will last 7 days.
Real Battery Drain
If your car is the one in a hundred that has true battery drain then
there are a few likely contenders. I have personal experience of at
least one and have been involved in the repair of many others.
These are in no particular order:
The Telephone
If you have replaced the
telephone shown to the right
at any time then you
probably have the wrong
software for correct
operation. The Motorola
V3688/V3690 is the correct
phone for the BMW E38.
However the replacement
phone (usually from Ebay) is
the Motorola V50. Although
these appear to operate
correctly with the MID or
OBC they fail to switch off
when the ignition is switched
off. This means that the
complete telephone system
remains operating and this will consume high currents, high enough
to discharge the battery in a few days.
Ensure that the replacement phone has the correct BMW operating
system or disconnect the phone when the car is parked.
The Sat-Nav System
Whenever the car is unlocked, even just the boot, the Sat-Nav
prepares itself for operation.
During this preparation the
disk will spin-up, some data is
read and written and if the car
is locked again the unit will
hibernate.
Where disk-errors occur the
unit may fail to hibernate and
this will increase the current
demand. A good way of
checking for this is to listen to
the rear left-hand wing after
the car is switched off. Where
a problem occurs the drive can
be heard spinning and
searching...a sure sign of
failure.
The only way to cure this problem is to replace the head unit, it
could be the time to upgrade to a MKIV unit!
The Ignition Switch
The ignition switch is a multi-
pole, multi-throw switch and
although the engine stops
when the ignition switch is
returned to position 0, it is
possible that other circuits are
still live.
Sometimes this problem is
immediately noticeable as the
radio or OBC will stay on,
other times the current drain
is harder to trace. A simple
way to check for this problem
is switch the ignition off and
then unplug the inline
connector that serves the
ignition switch.
Remember that you must wait 16 minutes after doing this to ensure
that all systems have timed-out and have now gone into
hibernation.
Corrosion under the electronics in the boot
There are many connectors in
the electronics bay to the rear
of the LHS boot space.
Unfortunately, the rear
clusters are know to leak,
especially on pre-facelift
models and those fitted with
after-market lights.
The water will track along the
boot floor and will eventually
reach the multi-way
connectors that connect the
Sat-Nav, CD changer,
Telephone and Video Module to
the main looms. The water will
eventually be damaged by
electrolytic action and will
cause short-circuits and
excessive current-drain. The tell-tale sign of this problem is the
discolouration of the in-line connectors.
The repair involves the replacement of the rear-cluster gasket or
replacement of the rear-clusters themselves on the pre-facelift
models as these are known to leak. After the source of the leak is
discovered and rectified the connectors can be scrubbed using a stiff
brush and WD40 to remove the water and discolouration. As long as
the connector body is not burnt this should solve the problem.
I-Bus problems
If you have the MKI or MKII
Sat-Nav systems you may
sometimes find that the
system freezes and cannot be
used until the battery has
been disconnected and then
reconnected. If you have this
problem it means that there
are I-Bus errors where the
communication has failed
between components on the
bus.
In this situation it is possible
that the systems connected via
the I-Bus do not hibernate correctly and may consume current when
the systems should be switched off. These problems are always
solved by disconnection of the battery and it is this reason that you
must not disconnect the power when performing current-drain tests.
One way around this problem is to switch off the RDS system on the
radio as this reduces I-Bus traffic.
Trafficmaster System
Not only does the
Trafficmaster system often
cause a loss of monitor
picture, it also can cause
excessive current demand
when switched off. The
subscription has usually
expired and you only see the
image as shown here so it
might as well be permanently
disconnected.
The Trafficmaster control unit
is normally fitted on the outside of the 6-CD changer and has
(depending on model year) a pair of connectors. When used with the
MKI and MKII Sat-Nav units a 'Y' cable is usually employed to connect
both the Nav and Trafficmaster unit to the Video Module. The 'Y' cable
can be discarded and the Nav unit directly connected to the Video
Module.
Harder to trace problems
Where all of the above
possibilities have been
discounted then you will have
to start the hard slog of
testing each fuse for current -
this must be done without
removing the fuses as this
gets rid of the evidence.
There is a new method of
measuring the current being
passed by each fuse without
removing them and this is all
explained here:
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