BMW M5 Forum and M6 Forums banner

Can the e39 m5 run with no battery connected?

9K views 30 replies 22 participants last post by  IlandMan 
#1 ·
Anyone tried it?
 
#4 ·
Will not run.
 
#6 ·
PLEASE, do not even THINK of trying this!

If you disconnect the battery with the car running, the alternator has nowhere to send the electricity it's creating. The voltage regulator ceases to work, and the alternator goes "full field", meaning it starts putting out 18 + volts. This amount of voltage will FRY your computers.

I've just sent a 2011 Mini home, (by flatbed truck), that had seven fried computers....at least! (I couldn't tell if there were more, because there was no communication further down the bus lines) The car is now, essentially, a paperweight, because the owner doesn't want to spend ~ $10,000 to find out how many other computers were dead.

And yes, all this happened when the owner decided to switch batteries, with the car running. I see this every year or so.

BMW technician training makes strong emphasis on this, early on.
 
#8 · (Edited)
#11 ·
Not that I would suggest anyone try to run without a battery, it is just the scary tail you tell is not plausible. The Alt is totally controlled by the DME via the grey wire that goes to the DME. The DME will shut down the alt when the voltage you see on the OBC reaches ~14.7v. Really it is higher than that but because the car running consumes so much power that is all you see. If fact when you first start the car the DME keeps the alt off to prevent drag on the motor, then it slowly allows the alt to put out more amps.

It is a possibility bad things could happen but BMW engineers do there best to make sure bad things don't happen.
From the WDS.
Alternator

Alternator
Compact alternator with multifunction controller (MFR)

  • The alternator with multifunction controller features only one threaded pin for the terminal connection B+ (terminal 30). The connection D+ (terminal 61 E) is located in the black connector (2-pin or 3-pin depending on version) at the alternator. The terminal 15 connection is also located in this connector for voltage supply of the controller. In some models the load status of the alternator is signalled to the engine management.
Compact alternator with multifunction controller (MFR) and start load response

  • The alternator with multifunction controller (MFR) and start-load response does not differ externally from the alternator without the start-load response function. The difference only concerns the time limitation of the rated current during the start procedure.
Start load response function:

  • This function is intended to shorten the engine starting procedure particularly at low ambient temperatures. The task of the function is to eliminate a braking torque acting on the engine as the result of full excitation and current output. As a result, during the first few seconds the load current is limited to 0 A in order to achieve the full power output in the following few seconds increasing at a rate of 10 A per second.
Multifunction controller:

  • In the case of the multifunction controller, the battery charge indicator lamp is activated by means of an electronic switch integrated in the controller. The controller measures the voltage difference of terminal 30 and terminal 15 internally and switches terminated 61 E to ground in the case of fault. The indicator lamp lights up.
Fault detection with multifunction controller with/without start load response:

  • Failure of the belt drive (U<sub>alternator</sub> = U<sub>terminal 15</sub> / no phase signal)
  • no charge due to a fault (U<sub>alternator</sub> = U<sub>terminal 15</sub>)
  • Interruption in excitation circuit
  • Overvoltage by a defective, conductive controller output stage (U<sub>alternator</sub> > U<sub>setpoint</sub>)
  • Break in charge line (U<sub>alternator</sub> - U<sub>terminal 15</sub> >= 3 V +/- 0.5 V)
 
#12 ·
The way I read WDS, the DME doesn't control the alternator. The controller in the alternator just grounds terminal 2 when it detects a fault which lights the charge light on the dash. This is what the WDS write-up above is saying. The signal from the 12V supply at F18 to the light in the cluster to the alternator controller just "passes through" the DME from terminal 1 to terminal 13 on its way to the alternator. I don't know why that signal would pass through the DME, unless it was because the designers were leaving their options open? Or it may be related to the statement above that "In some models the load status of the alternator is signalled to the engine management". Idle speed control?

The controller simply grounding the cluster warning light so it lights is supported by some bench testing I did on an alternator. I wired pin 2 on the alternator connector to the probe on a 12V test light and connected the test light crocodile clip to battery +ve so it simulated the warning light. I also had pin 1 and the main stud connected to battery +ve. When I spun the alternator at low speeds with a drill, the test light stayed lit. When I got it up to about 2000 rpm, I could feel the alternator load up and the light went out.

But having said that, there have been quite a few threads on here about battery connections going open circuit while driving. This is equivalent to disconnecting a battery, I would say. In every case I recall, the car just died. No modules got fried. Looking at WDS, maybe that was because it was the ground wire on the battery that went open circuit, which isolates the alternator as well as the battery. But if the +ve terminal becomes disconnected, now the alternator is directly supplying all the main fuses, without the dampening effect of the battery on voltage.

But maybe the controller is set to supply 2V above battery voltage, so maybe it just shuts the alternator down when it sees battery voltage drop to 0V due to the disconnection?

Interesting question.

bmwdirtracer's example shows a modern car loses control of voltage and fries stuff. I would have thought the designers would have been smarter than that. I mean battery terminals can come loose for other reasons. But I'm not confident enough to try this on my car.

Malcolm
 
#13 · (Edited)
Please read again. Read between the lines. Sometimes they write things on a "do they really need to know our secrets" basis. Yes if you ground the gr wire the alt will work as normal. It will not shut off on decel and the startup mode will go, likely so will all the other safety things. If the DME controls the ground for the reg then different things are possible. Next time you get a bad reg have a look they are no different than any other reg, that said hard to say what is in silicone chips.
It clearly talks about startup behavior, the reg is not doing that.

Also from your own driving you should have noticed that the volts on the OBC always drop to the expected battery volts when the fuel turns back on after decel, the alt is turned off and the alt is not turned on again until idle is stable. Reg is not doing that.

Lastly how is the reg monitoring the voltages at the different terminals? Sorry it is not that smart. The most important indicator that they are talking about the DME as the controller, look where the ground for the battery light goes, yes to the DME. There is only one reason both of those wires go to the DME and it is not to have it pass through the DME for future options, if they need options they change things. You must read between some lines in the write ups in the WDS but when you add the drawings it makes things clear as long as you understand they write that to tell techs how to fix the car not how they engineer the car.
Diagram Floor plan Plan Passive circuit component Parallel

Edit
Something else interesting I found The light does not usually get controlled by going to ground, they normally are connected between two positives and as one side becomes drained it acts as a ground and the light first lights dimly and gets brighter as the as the imbalance gets greater the light gets brighter.

According to this drawing, which should be up to 2001 3 even though the other drawing should be from 2000 09 up, That is how it is wired or was wired. X213 is the splice joint for term 30 but on US cars it is really X209. Strangely there is no difference between the alts post and pre either of those dates yet the wire from the alt still goes to the DME. That is not a passthru back to term 30 as the other wire is already hooked to term 30.
Text Diagram Floor plan Line Parallel
 
#14 ·
On later model cars (like the N63 engined cars), BMW certainly does program the DME with a charging strategy:
Enginerdy: Why BMW's N63 twin-turbo V8 eats batteries

However, firstly, we're talking about a voltage SPIKE, and no matter how fast the computers are, they will still see the spike, before it can be controlled.

On Thursday, I also had an E60 M5, that was intermittently lighting up the dash with almost every warning light, as well as registering a list of warnings on the i-drive screen. When hooked up to ISIS/ICOM, there were literally dozens of codes, in a dozen computers, many indicating bus line communications failures, steering angle sensor failure, wheel electronics failures, speed sensor failures, and SRS issues.

But the DSC, the SRS, and a couple of other computers showed codes for "Overvoltage > 16volts". The DSC registered this code 34 times.

The alternator voltage showed exactly correct on a DMM, and also through ISTA, so I cleared the codes, and drove the car to see what came back first. In a trip around the block, the overvoltage codes reset.

Therefore, I ordered a new Valeo alternator, replaced it (3 hours!),cleared codes, and drove a long test drive......no codes, no lights.

My point being that while the DME has maps/strategies, at least in later cars, for controlling alternator output, it can't stop that overvoltage until AFTER it occurs, by which time the overvoltage can have already fried a computer or two -- and often does. It's worth noting that there are different codes for "Overvoltage > 16volts" and "Overvoltage > 18 volts". There would be no such codes written into the programming, if the DME could indeed prevent the overvoltage from happening.

The E60 M5 was lucky, no further damage.
 
#15 ·
Not my experience with an E39, I have seen at least three that have had the alt over charge and they all shut off ~ 14.7v OBC. That would likely be 15.7 - 16.5 in a real measure. I suspect this system on this car is the half breed to the systems later used. I think that is one of the reasons it refuses to to fully charge weak batteries. A SNFU in the computer logic.
 
#24 ·
OP certainly isn't a troll.

As for why you'd want to do it, perhaps you have any number of settings in the car you want to keep and not erase by dissing a battery. Maybe this is a lazy way of folk keeping their radio stations tuned to memory, etc etc.

As for aftermarket, have you seen how long it takes to set a system up from scratch, sometimes even with the settings written down? Maybe this was just a quick question to see if an hours worth of twiddling could be saved, perfectly plausible to me from even that one angle as a genuine question, so don't be so quick to chastise or make fun of IMO.
 
#26 ·
#27 ·
That will only work if the car is a sleep. If the nav turns on it will suck that 9V flat in a second or two. I got away with using my battery charger last time but have been warned that it should be a battery and not a charger. I did not really grasp the tech words he was using but trust him. He is an electrical engineer.
 
#29 ·
I googled History of Automobile Batteries to see how long vehicles have been using batteries to no avail. This leads me to assume that car batteries have been used in their current (excuse the pun) form for as long as the modern engine has been around.

Why after 130 years of motoring we're wondering if our cars would run without batteries just baffles me.
 
#30 ·
I would never attempt to disconnect the battery while car is running but I have an issue with battery terminal. The nut holding the positive terminal is stuck in the middle. It doesn't hold the terminal tight at all so I was gonna go to Mechanic tomorrow to fix this. What makes me scary is I've been driving the car for last couple days about 15-20 miles. A couple of times I attempted to start the car, power went out and I had to go back to trunk and re-positioned the terminal. Also there might have been a good chance that the terminal got loosen again and battery could've been disconnected when I was driving on bumpy road. I don't see any warning light came on yet. Is my car fine?
 
#31 ·
I've had a similar thing happen where the car just died altogether during a torrential downpour. Luckily for me I had just moved off from a red light so I was able to safely coast to the side of the road. Once I was able to check I found a loose terminal, tightened it and luckily there was no lasting damage to any of the electronics.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top