840 Parasitic Drain

[jolo]

I replaced my General Module (GM) and still drain my batteries overnight. I subsequently turned to this forum and realized I had incorrectly done a parasitic drain test before purchasing the new GM. I just performed the correct parasitic drain test and when I pulled either fuse 33 or 43 from within the luggage compartment fuse block (Passenger compartment and luggage compartment lighting fuses), the amp draw dropped from 60 to 16. I had the headliner and trunk liner out of the car with the dome/interior and trunk lights modules unplugged (no lights attached and therefore no lights on). So I plugged the dome/interior module back in (the trunk lighting module is tucked away somewhere in a box for now) with the lights off, and both circuits are still drawing current. So, the passenger compartment and luggage compartment lighting circuits are drawing current (60 amps) when not activated. I did have the car doors closed, and the trunk button depressed even though the lights are not installed.

I assume I have a short somewhere in those circuits. Any suggestions for next steps to resolve this are appreciated.

[olinjohnston]

The consumer cutoff relay should kick in at precisely 16 minutes. To test this, just open the trunk or switch on a reading light and set a timer. If the lights do not turn off, I would swap that relay.

[Craig540i]

I would double check the current draw - 60 amps is an awesome lot for the interior light circuit. That's the equivalent of a 720 watt globe!

[jolo]

Right Craig-mA

[Timm]

60mA is close enough to where it should be - if your batteries are failing then possibly the batteries themselves are damaged - it doesn't take more than a couple of weeks at 0% state-of-charge to kill them.

[jolo]

You may be right. With the batteries out of the car after recharging, one comes up to 12.34 and the other 12.20. I believe they should sustain a charge of 12.6; is that correct? The reason I felt those two fuses are the issue is the drain is .16 to .19 when either of them is pulled out of the fuse box in the trunk. Seems odd it is either one, but I assume they are connected circuits. With both 33 and 43 installed, the draw is .59; pull 33 out the draw drops to .19. Put 33 back in and it jumps back to .59. Pull 43 out and it drops to .19. None of these courtesy light modules is plugged in at this time as the interior has been torn down. None of the relays are warm after the car goes to sleep.

[Timm]

The battery voltage is very low for after a charge - after sitting for 24 hours you would expect 12.6V = 100% state-of-charge.

Pulling fuses loses evidence - instead, measure the current with them in place like this:

http://www.meeknet.co.uk/gen_bmw/Cal... BMW Fuses.pdf

[jolo]

Thanks Timm, very helpful. I tested the mV at the fuses and all in the trunk checked out except the 7.5 amp fuse #33 for the Passenger Compartment and Luggage Compartment. It reads a constant 2.7 mV. Now I need to determine why it's drawing so much current. I am guessing the lower 7.5 amperage circuit runs the Luggage/Trunk Light and the 10 amp runs the Interior Lighting. I guess for now, if it is just the trunk light, I can pull the fuse and use the car while I try to isolate the problem.

Thanks a lot Timm.

[Timm]

One thing worth saying is - I think the source of your problem is your dead batteries rather than an excessive current drain - 60mA is fine, and good batteries will support this drain for many weeks....

[jolo]

Thanks Timm. I will start with new batteries. I am considering Interstate Part # MTP-91/T6. Do you have any better recommendations?

[Craig540i]

Agree with Tim, 60mA is fine, especially as our cars have 2 batteries.

[jolo]

Thanks Craig, Timm And no concerns re the 2.7 mV at the 33 fuse that controls driver side windows and interior lighting? Guess we will know when I get new batteries in the car. I had car appropriate Exide AGMs last one year. Now I have Optimas. I have been told Interstate or General Motors/AC Delco equivalent AGMs are the way to go.

[Timm]

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.

[jolo]

Just ran another test three more times and each time I left my multimeter probes connected until it finally settled down from .59/.60 to .41/.42. I think that definitively points to the batteries. I dis-connected the two battery grounds and connected them together ( I now have a quick connect/disconnect to the ground post with a plastic screw knob. I connected the positive multimeter lead to the ground bolt and the negative lead to the negative battery post. I am now reading less than .50 so I am good re parasitic draw. I think the Optimas may have been a mistake for this car. I am convinced the old alternator killed the new Exides. So, I have a rebuilt alternator pushing ample voltage (13.6 to 14.4), and a new General Module. New batteries should wrap this up. Thanks for all the valuable advice. I will advise if the new batteries solve the problem.

- - - Updated - - -

Timm, with a two battery system as with the e 31, can I connect a trickle charger to the jumper posts under the hood. I had tried that and the trickle chargers (I tried several sizes) just got hot and did not maintain the batteries. I guess I need to connect my battery charger to the jumper posts rather than a trickle charger. Do you recommend I set the charger to 2 amps or 15 amps and deep cycle auto, or regular auto?

[Kimolaoha]

. . . can I connect a trickle charger to the jumper posts under the hood. I had tried that and the trickle chargers (I tried several sizes) just got hot and did not maintain the batteries. I guess I need to connect my battery charger to the jumper posts rather than a trickle charger. Do you recommend I set the charger to 2 amps or 15 amps and deep cycle auto, or regular auto?

Everything Timm said is (of course) spot on, and the way to prevent sulphation for a car driven short distances (or a garage queen) is to invest in a SMART Charger - not a trickle charger.

One tried and true smart charger is the Deltran Battery Tender Plus (Amazon - about $45). My preferred method is to connect the included quick-connect pigtail connector directly to the posts of the driver side battery (it will also charge the passenger side battery since they are connected in parallel) and run the end of the pigtail under the trunk lining to the rear trunk compartment area, where it can be easily connected/disconnected to the Deltran unit. Others have also had success by connecting the pigtail or clamps to the passenger side battery or directly to the charging posts under the hood, but since the cable is thin enough to exit the trunk with the trunklid closed, I've found that connection to the driver side posts expedient and neat (the Deltran is then plugged into an outlet near the rear driver's side 'corner' of the car).

http://www.amazon.com/Deltran-Batter...attery+charger

http://www.amazon.com/Deltran-Batter...battery+tender

I've done this for 20 years, 24/7, in three different cars and my batteries last for ages. There is newer CTEK technology that also works, but these Deltrans are reliable and economical workhorses. If your batteries are already shot, this will not Lazarus them (sulphation is irreversible) but I certainly wouldn't invest in new batteries without this tender at the ready. Everything you ever wanted to know is at www.batterytender.com.

[jolo]

Thanks Kimo; I have that Deltran tender on my 33 Coupe (I incorrectly refer to it as a trickle charger). I tried it and other higher amperage Deltrans on the posts under the hood of the e31 to no avail, but my batteries were probably already gone. Once I have the new batteries, I will connect the Deltran as you suggest and I should be fine. Thanks much for the tip, very helpful.

[Timm]

Yes, a smart charger is the way forward. You can connect it to either battery or the B+ post and the chassis connections in the engine bay - they are all joined together!

[Jolo3361]

Thank you both very much. You have been extremely patient and very helpful.

Kind regards
joe

[tommydogs]

Thanks Timm, this is an extraordinarily informative post!

[jolo]

Timm/Kimo: Have the batteries connected to the Deltran Battery Tender. Still need to purchase new batteries but the tender is keeping me going. I threaded the inside of the B+ terminal, cut off a metric bolt to fit and then drilled and threaded a hole in the bolt for a smaller metric bolt to attach the positive Deltran lead. The negative lead fit under the existing ground bolt on the strut tower. I connect the Deltran overnight. It is the 5 amp variety and works extremely well.

Olin: I discovered after finally driving the car that the Consumer Relays in the trunk get hot and stay warm when the car is in sleep mode. Obviously you were onto something there. The glove box light does not turn off when the car should be sleeping. I will attach the other interior and trunk lights and see if they go out. If they don't then the Consumer Relay is bad. I think it is since it is staying active when the car is sleeping and because it is getting quite hot when the car is running. Both the upper and lower one get hot, particularly the upper one (both are green in my car).


IMG_0593 1.jpgIMG_0592 1.jpg

[Dash01]

Assuming your two green consumer relays are the exact same model number, have you swapped them to see if that makes any difference in battery drain?

And, such relays are pretty simple devices. So, removing their caps and inspecting/cleaning/resoldering may cure their internal faults, so the relay can be refurbished and used again.

Also, see Shogun's post about a fuse-testing device which plugs into the fuses without removing such fuses from the fuse box. These diagnostic devices sell for less than ~$15 on eBay, plug into your multimeter, and help with rapid tracing of electric faults.

[vet73]

You could also look into a thru hull connector often used on boats. Has a waterproof three prong USA type 120 volt male end with an easily removed cover that is on the outside and solder connects on inside for leads from charger to connect to. Gives a neat connect hookup by simply plugging end of extension cord into a "plug" on the car. Nothing to get shorted by repeated trunk closings and pinching of wires. Only requires a 1.5 inch or so hole thru body in some obscure location.

[jolo]

Olin mentioned a potential fault with the Consumer Relays and Tom Carter suggested checking to see if they stay warm after the car goes to sleep. Well they are quite hot long after the car has been parked and stay hot. I pulled both relays and parked the car overnight and both batteries maintain a state of charge at 12.5 volts without the Deltran connected. I connected two new inexpensive 30 amp TYCO relays and they also get hot. I ordered the BMW (TYCO) replacements pictured on line, but I have not connected them yet because I feel I have a bigger problem with those two circuits that is causing the Relays to heat and not cut out the circuit. Before installing the new BMW relays, I want to know what other issue may be causing the problem with these circuits. Since I am not the original owner, these could be the incorrect relays for the consumer cut-off even though they are BMW Siemens (part no 61.36-8 353 447)? Tomorrow, I will check with my local BMW dealer to see if they can validate the part no.

IMG_0608 1.jpg

[Dash01]

You could start by checking on REALOEM with your VIN number, and see what relays that shows.

Could be the relays themselves are fine, but you have a short(s) in the circuit which sends juice to open the relay(s).

[olinjohnston]

So this general module... Did you replace it with a new one, a used one, or a repaired one?