Timm's BMW E38 Battery Drain

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 removing fuses to see which one reduces the current demand. This is pretty straight-forward for the rear fuses (situated above the battery) but gets a bit more complicated when those fuses have been checked and you have to move onto the fuses inside the car.

In this situation, have a helper watch the meter and lock yourself in the car (press the remote lock button twice to inhibit the alarm sensors). Again, you will have to wait 16 minutes for everything to switch off, start with the fuses in the drivers oddments tray and move onto the fuses in the glove compartment.