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Discussion Starter #1
Before I start, I know this is no M5 but I'm looking to get any help wherever I can.

I've been fighting a power drain for a few weeks now. It's enough to kill the battery in under 12 hours. I'm having some trouble with my AC system at the moment, so I unplugged my FSU in case that was the power drain. I'm having trouble figuring out exactly where the power drain is coming from. Ran some tests last night with a voltmeter and discovered that while the car was "sleeping" after sixteen minutes of inactivity with the doors unlocked, it was still drawing 3 amps of power. I locked it, waited another sixteen minutes, and presto! Power usage dropped to around .1 amps. Satisfied that I could simply leave my car locked for the time being, I disconnected the voltmeter which disconnected the battery from the car. I reconnected the battery, tested it, started right up. Two hours later I was upstairs in my house when I hear the all too familiar E39 alarm blaring in my garage at two AM. Battery was dead by the time I got down to the car. Could this be some computer module in the car going bad and resetting when the battery is disconnected? Could it possibly be the blower motor failing and drawing power even though the FSU is disconnected? I'm trying to save myself a trip to my local dealer to get it professionally diagnosed. Thanks for any help anyone can provide!
 

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3 angles:
1. Battery: How old is it, and did you load test it?
2. Power drain source: Visit the sticky DIY Guidepost.
3. Alarm: I have not heard of this going off due to low power. Check the sensor.
 

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Discussion Starter #4
3 angles:
1. Battery: How old is it, and did you load test it?
2. Power drain source: Visit the sticky DIY Guidepost.
3. Alarm: I have not heard of this going off due to low power. Check the sensor.
1. The battery is a Bosch 850 CCA which is about a month old so I would presume it isn't bad.
2. Thanks for the source! I've been doing a lot of research and it seems that it could be any handful of things.
3. I haven't had any issues with the alarm except this one time when it just went off, I presumed it was because the battery was almost dead and caused one of the sensors to trip. I had a friend tell me that it was normal when the car is about to die for the alarm to go off. Maybe I coincidentally hit the key fob in my pocket?

This afternoon I jumpstarted her and once she had a charge I removed fuse 57 for my telephone (which I have already had removed from the car for some time). I also unplugged my aftermarket LED Angel Eyes. I then locked her and went to work. Came home to find that she's still alive and doesn't feel to have any less power (starts and no dim lighting). I don't know if this was a coincidence of disconnecting the battery and having a module reset, or if either fuse 57 or the angel eyes were the issue in the first place. Definitely confusing.
 

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Not a dig in any fashion, but there are quite a few of you 540 owners here now. They are not M5s they really are not the same in any form or function. There is a e39 forum here E39 5 series Forum - BMW M5 Forum and M6 Forums
if you all posted here your problems and of coarse watched the forum you might find you would build a forum as good as this one but for your cars.
 

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Discussion Starter #6
I have to politely disagree. They are very similar in almost every regard as far as the electrical system is concerned. The E39 M5 is based off of the 540 with many vast improvements, but the base vehicle is essentially the same.
 

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A 0.1A drain in sleep mode is still far too much though. The car should draw no more than ~0.05A (50mA) once it has gone to sleep.

I had exactly the same problem and my car was drawing about 110mA, which flattened it very quickly. I followed the guide on here written by Street Dragster (I think it was) regarding fitting an ammeter in line with the battery (shounds like you've done the same) and pulled the fuses out one at a time.

It turned out that my car had the westfalia detatchable tow hitch (I didnt even know it was there!) and there was a relay for the electrical trailer socket that wasn't powering down even in sleep mode, that carried on drawing power. Once I removed the fuse to that that, the drain dropped by over half to just below 50mA. The next big hitters I found were the GPS nav and the telephone, removing them dropped it to below 10mA, although I didn't permanently remove those fuses of course, as I use those features regularly.

All I can suggest is you do the same. You'll find it!

Have you got a multimeter that will read to these sort of levels? You do need one that will read better than 0.1A.
 

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ETA, with regard to the alarm, my car did exactly the same the first time it went flat, and I do recall reading on here somewhere at the time, that a low battery voltage can cause the alarm to go off.
 

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Discussion Starter #9
Thank you for the information. I'll run more tests tomorrow morning before work. The multimeter I was using was the first one I found. I have better ones that I'll probably use this time. As stated above, I yanked my telephone fuse and as of right now it seems to have reduced, if not solved the problem. We'll see tomorrow morning. I'll keep you updated.
 

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I guess the only other thing I'd add as well is that in my experience, once a battery goes flat like that, it's never the same again and you can never truly rely on it, so I'd suggest once you fix the drain, you replace it anyway.
 

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There are some similar things but the quirks are definitely different. You know these cars are the same from? Vast experience with an M5? 540 is closer to a 525 than an M5. You need to read this, about off topic. http://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/89128-pointers-moderators-about-posting-m5board.html . Yes 540 is off topic it is not an M5. And politely or not you guys need your own forum built up you are the only ones that can do that. At min you should post in the correct forum and cross link to here.
 

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It's also worth checking the alternator output, as it could be that your battery is not getting recharged when the engine is running and the alternator is on its way out, which will exacerbate the problem you have. Put a voltmeter across the battery terminals with the engine running and you should have at least 14V there.
 
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