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How to diagnose what's wrong with a battery that goes dead once a month?

38K views 37 replies 10 participants last post by  uncmozo 
#1 ·
For the third time this month, the car is completely dead after leaving it for 8 hours. Twice before I needed a jump, and, then it worked just fine.

I'm, I guess, panicking, so, if you have good references, please shoot 'em my way as I'm not sure what the problem is. I think it could be the FSU but I have no evidence that it is ... but I'm gonna call AutoHaus tomorrow to get a new FSU anyway.

The battery itself is only about 2 years old. Is there an indicator that goes on if the alternator isn't working? This only happens after a long park (overnight or during the work day).

The lights were NOT on, nor was anything plugged into the cigarette lighter. I'll debug if it's the battery by picking up a density gauge (I have a voltmeter but I don't know if that will tell me much).

Is there a good troubleshooting DIY for a periodic dead battery? Bear in mind, it's perfectly good 9/10ths of the time; but, lately, about once or twice a month, it's totally dead (the horn won't even blow ... sometimes it clicks when I try to start it; sometimes it's totally dead).

But, as soon as I connect the donor jumper cables, the car springs back to life and acts normally. Until the next time.

Please advise, (Sorry I don't have time to research but I gotta' get it jumped now!).
 
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#2 ·
Ill try and help out here...for one how old is the battery? An old batt will do lots of crazy things including die after a few hours. Also, lock the car at night and wait 20 minutes. Then check if the small orange light next to the shifter is off, if not...your car isnt going into "sleep" mode. There are many possiblities, just throwing a quick couple out.
 
#4 · (Edited)
It's time to start compiling a list of possible causes of battery drain ...

I guess, panicking
OK. Now I'm less panicked as the E39 is now working fine again.

- I didn't see your help until now so I'll check that little orange light by the automatic shifter (I always wondered what that was ... what is that light there for anyway?)
- I have no idea how to check the voltage regulator (but now that I'm running, I can look it up)
- The 2-year old battery looks pristine (no crud or anything on the terminals)
- I can never read the color of the sight glass (what kind of perverse test is that?)
- I don't see anywhere to open the battery to get to the sulfuric acid density so I didn't buy the density gauge yet)
- I don't see any light that indicates charging (or no charging for that matter)
The electrical problems I know of is the FSU has never been replaced (to my knowledge) and the interior blower was exhibiting some minor wierdness so I ordered a new $72 Final Stage Unit (FSU) from AutoHaus.

In addition, my recent DICE Silverline 2nd-generation iPod touch radio mount won't charge any iPod so perhaps the iPod is drawing the battery down. Tom, over at EAS, has been wonderful and is replacing the Dice Silverline, so, until it arrives, I unplug the iPod whenever I leave the car.

Now that I'm running again, I can dig for some good diagnostic procedures to find out WHAT is draining the battery?
 
#5 ·
Here is how to check your system
This is a battery maintenance DIY (I am still on the OEM battery, and it cranks great even at -30°C - no problems)
This is a battery voltage check
And here a DIY to replace the OEM battery - attention at the vent.
Now you have to find the leak. If you did not changed the FSU, it might be that one. But, if the A/C does not behave strange, maybe something else?
 
#7 ·
Check the trunk

I had the same problem, then one day I noticed my trunk light had melted and burnt. The actuator in my trunk locking mechanisim had died in the open position triggering the trunk light to stay on, unknown to me. I had that replaced and all was good.

I found out the actuator was dead when I tried to use my remote to open the trunk and nothing happened.

Good Luck!
 
#9 ·
Until my FSU arrives, what I'm doing now is to listen attentively to the sounds inside the cabin after I remove the key in my E39.

If I hear the faint sound of the fan blowing from deeply inside the dash, I start the car back up and fiddle with the A/C and heater settings, punching random buttons, until when I shut down the E39, I no longer hear the faint fan.

So far, this takes two or three attempts but the good news is my battery hasn't died overnight after doing this procedure. Lesson learned.

I'm not sure if it's working, but, at least, I feel like I'm doing something about it.
 

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#10 ·
Glad you sorted it out.
That pic of the FSU, it's the old one you pulled right?
I think I spied somwhere a thread with pics of the new style FSU, and it looks smaller, but with more cooling "fins", compared to the old one that has fewer but bigger "fins".

Also, there was a thread on this forum, and I cannot find it (I tried) - a member from France I believe, had the curiosity to open up a "damaged" FSU to find out the failure mode.
His point was that the electronics are pulled out by the silicone type material they are imbedded. The silicone, after years of hot/cold cycles seem to shrink or pull out of that "mold" where it sits together with the electronics. Some nice pics were posted as well.
 
#11 ·
Let's take apart the next FSU that goes bad (& post pictures)

That pic of the FSU, it's the old one you pulled right?
Nope. That was a reference pic. My replacement FSU hasn't arrived from UPS yet so mine is still safely ensconced next to my glovebox. :)

Also, there was a thread on this forum, and I cannot find it (I tried) - a member from France I believe, had the curiosity to open up a "damaged" FSU to find out the failure mode.
I would LOVE to find that thread! I searched and didn't find it. I also googled and didn't find it. When mine arrives, maybe you guys can help me disassemble it so we can determine the failure mode in the investigative autopsy.

 
#14 ·
Hmmmm..... unplug the FSU? Most people said it was a bear to unplug the FSU. Sure, it's only one bolt, but, that bolt is apparently way way up there and very hard to get to unless you're really lucky and you get it on the first shot. In essence, my assumption is that it's way too inconvenient to unplug the FSU nightly.

As for the drain on the battery ... I don' t know how to measure it. I generally don't mind buying useful diagnostic tools (if I can reuse them for other cars), so, if you have a suggestion as to what to check and what tool to use, let me know. (It might be on the previously referenced threads.)
 
#16 ·
Since you need to remove the FSU anyway to install the new FSU, go ahead and remove the old FSU and leave it out. Then see if the problem goes away. If it does, you can sleep better knowing you have diagnosed the right fix! If the problem recurs, back to troubleshooting. You should replace the FSU when the new one comes in, even if it is not the right fix, since these FSUs seem to have a limited lifespan (at least the original ones did). Good luck. I am betting it is the FSU as mine caused battery drain issues before it caused noticeable fan issues.
 
#20 ·
:) I know about the basics of electricity. I do have access to a Radio Shack voltmeter ampmeter. But there is no way that meter is going to take the amps of current if I put it in line with the battery.

So we need those clipon things that go around the wires, don't we?

I realize the ampmeter needs to be in the circuit (not across the circuit) so that's why it's not so simple as inserting my $15 radio shack voltmeter/ampmeter into the battery circuit.

How much current are we looking for and what two wires do we break to insert the ampmeter?
 
#28 ·
Five month old battery died yesterday. Had AAA test it, and they found the battery was draining even with everything shut off.

E39s seem to have the same common problems. So far, I've had a non-opening trunk, inconsistent A/C fan speed, and a broken CD changer - all of which are potential battery drainers.

I've fixed the trunk problem (broken wires leading into trunklid), and the CD changer hasn't worked the past 4 years or so, so I'm guessing it's the FSU. Guess I'll bring the car to the indy mechanic tomorrow again....
 
#29 ·
they found the battery was draining even with everything shut off.
You seem to have the same problems I had ... (except the CD changer which I don't have since I only have the single Business CD) ... but I do agree ... E39 problems seem to be relatively common ...

The FSU isn't all that bad to replace ... it's 50:50. Either you will replace it so quickly you'll wonder what all the fuss is about; or, you'll be cursing the evil ears on the FSU connector like some of the rest of us (me included).

Of course, I broke off the evil ears ... and then my FSU came right out ... but I don't recommend such draconian satisfactions ... :)

Bluebee
 
#33 ·
Long term progress report ... It has been two years since the FSU was replaced and the battery hasn't died since!
In the past two years, we've learned to stay away from the GKR FSUs (which my original FSU was) in favor of the Sitronic/Bosch for the E39.

If you're interested in the details, xref is here.
FSU (aks FSR) fan blower speed resistor autopsy and wiring diagrams (1) (2) (3) (4) ... (1) (2) (3) (4)

 

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#34 ·
Just to update this thread, since I often refer others with dead battery overnight symptoms to it, this seems to be a reasonable diagnosis:

- If your battery goes dead overnight (intermittently)
--> Then test your battery drain (1) (2) & test your alternator output (1)

- If the drain and alternator are good but the battery still dies overnight
- Then feel the FSU to see if it's hot a few hours after shutting the car down
--> If the FSU is hot, then unplug the FSU & order a new FSU

- If the FSU wasn't hot yet the battery is still dying, then unclip the FSU
--> If the problem goes away while unclipped, then order a new FSU.
Note: Only order the FSU brand recommended in this thread!

Note: You may also note oddities in the fan blower operation; but then again, you might not as it seems to often be intermittent.
 
#35 ·
#36 ·
#38 ·
I had serious battery drain. First I thought my old battery died so I replaced it. After 4 days, that battery was dying, I had to charge it every other day. Finally after 2 weeks that battery crapped out and they replaced it under warrenty - dead cell. Tried all the FSU tests, FSU seemed ok. Began testing for parasitic drain, when I found the 'hood open' limit switch unplugged. I had done brake work last month, checked the brake fluid, and forgot to plug the limit switch back in. That was the source of the drain.

Sometimes I am my own worst enemy.

Jerry
 
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