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X3 F25 (2011 - current)
The latest X3 brings some added style and some new features to the BMW SUV family. Talk about the new F25 now!

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  #1  
Old 11-19-2013, 02:15 AM
jgroarke jgroarke is online now
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Battery cell depletion

Here's a story.

I ran the battery flat somehow a week or so back, at the time I didn't know why / how. Some seriously sketchy starting times (werrrrr werrrrr werrrrr start) were followed a day or so later by nothing at all, computer reset, red screen etc...

Anyhow, to keep this story short, I got the BMW rescue out. They re-charged it and told me to go to the service center. They didn't have the right testing equipment for the battery test. I took it to dealer, they kept it 2 days (overnight for drain test). They "checked" battery, alternator, leak test overnight, nada.

Conclusion? The computer said I had done too many short journeys. It's bloody HAL! I argued nothing different to the way I'd been driving for the past two years but to no avail.

Anyhow, as fate would have it, two weeks later on the day of me moving house, same thing happened! Car stranded in hold house. Damm.

Call BMW service out (different guy this time, very nice chap), and he did the same. Charged BUT this time he had the "old school" testing equipment. He told me that BMW service centers now only have the option (via edict from Der Vaterland HQ) that they can use computer diagnosis (ISIS?). This is how they tested mine. However, this chap said that system was pretty much useless on 90% of duff batteries. Lo and behold he tested with ISIS on his laptop, "BATTERY OK". Tested with his old faithful tester "CELL DEPLETED - NEW BATTERY NEEDED". I saw with my own eyes.

Needless to say I complained to dealer for wasting two days of my car, however I believe their hands are tied by up above.

Technology is not always progressive, I hope this helps someone.

Last edited by jgroarke; 11-19-2013 at 02:17 AM.
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  #2  
Old 11-19-2013, 08:52 AM
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jcatral14 jcatral14 is offline
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Sorry about your battery problem.

I always thought that the way that BMW designed this system, your problem will be quite common. As I understand it, the alternator only kicks in when braking so if you do a lot of short trips and do not use the brakes enough, sooner or later you will wind up with a depleted battery. The alternator doesn't work as much and the battery does not have a chance to recharge. I think this will be a problem for drivers who anticipate the flow of traffic and don't use their brakes as much.

I wonder if the system is smart enough to detect the low battery charge and automatically activate the alternator to charge the battery regardless of whether the brakes are used or not? Perhaps a solution is to use a battery tender at night?

Please keep us updated what the resolution is.

Best,

Jay
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Last edited by jcatral14; 11-19-2013 at 08:53 AM.
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  #3  
Old 11-19-2013, 09:29 AM
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Quote:
Originally Posted by jcatral14 View Post
I wonder if the system is smart enough to detect the low battery charge and automatically activate the alternator to charge the battery regardless of whether the brakes are used or not?
Yes it is, the system will detect low charge and bypass the regenerative braking system.
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  #4  
Old 11-19-2013, 09:50 AM
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Originally Posted by The X Men View Post
Yes it is, the system will detect low charge and bypass the regenerative braking system.
Thanks! Good to know. But why did OP have the issue? Perhaps the system didn't detect the low charge?
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  #5  
Old 11-19-2013, 12:25 PM
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Quote:
Originally Posted by jcatral14 View Post
Thanks! Good to know. But why did OP have the issue? Perhaps the system didn't detect the low charge?
Sounds like the OP has a bad battery and BMW is refusing to replace it. Most of the owners who have had their battery replaced under warranty had to do a lot of complaining to the dealer and BMW. Most auto manufacturers do not like to replace battery under warranty, sometimes it really comes down to how hard you want to push it and how much is your time worth.
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  #6  
Old 11-19-2013, 09:20 PM
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German cars historically have not taken well to short journeys and incompletely drive cycles and with how intensive the electronic uses are thesedays, the problem gets exacerbated. You can push for a warranty or goodwill battery replacement but ultimately if you go weeks without taking a highway driving, the vehicle isn't going to be very happy. That's just the way these cars are, and there's no amount of complaining you can do that will change what the engineers do.
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  #7  
Old 11-20-2013, 02:08 AM
RhoXS RhoXS is offline
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Originally Posted by jcatral14 View Post
I wonder if the system is smart enough to detect the low battery charge and automatically activate the alternator to charge the battery regardless of whether the brakes are used or not?

Jay
My opinion that follows is only from what I observed by continuously monitoring battery voltage and not from some credible source such as a BMW document.

I monitored battery voltage and noticed under normal driving it sits between 13.2 Volts and 13.8 volts depending on load and outside temperature. It jumps up to the normally expected charging voltage of about 14.6 volts or so when the display indicates the battery is being charged. I also noticed voltage does not seem to respond to consequential changes in electrical load (except for a very small but momentary transient). This means to me the alternator output is responding and maintaing whatever the electrical load happens to be.

13.something volts is not full charging voltage but it is much too high for the battery open circuit voltage so the alternator is constantly putting some energy into both the battery (charging) and electrical system (carrying normal load) at this voltage. Therefore, I think the system is actually simpler that what most people surmise. Basically, I think it just continuously maintains a voltage that is consistent with about an 80% charge. Since the battery capacity is supposed to be 25% oversized for the needs of the car, this effectively continuously maintains a battery containing the equivalent amount of energy as a properly sized battery. The 25% remaining allows someplace for the braking/coasting down hill energy to go. If the battery was maintained at full charge it would be damaged by trying to jam more energy into it, especially at a high rate. Also, a normal lead acid battery would be permanently damaged by not maintaining it at 100% charge continuously. That is why BMW uses a so called AGM battery.

I am reasonably certain the system controls alternator output voltage electronically by likely modulating field voltage. The alternator is not mechanically clutched in/out and is obviously not just either charging or not charging.

In summary, even though the battery is not maintained 100% charged, I am reasonably certain it is maintained at no less than an 80% charged. Therefore, since 80% corresponds to correct battery sizing, there is no reason to see any issues, different from a "normal" car, with maintaining the battery properly charged.

The 2014 brochure states the alternator is rated at 2940 Watts which translates to probably 5 Hp engine load assuming it is running at 100% electrical capacity. Since the alternator probably rarely operates at 100% capacity, and is apparently always running at some load anyway, I just do not believe the amount of energy saving from capturing braking and coasting energy is worth all the effort and really perceive this feature as much more a marketing tool than anything all that useful.

Last edited by RhoXS; 11-20-2013 at 06:45 AM.
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  #8  
Old 11-20-2013, 09:02 AM
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From the BMW website:

Brake Energy Regeneration.

Make use of every watt: by charging the battery only when your BMW is braking, coasting or decelerating, Brake Energy Regeneration improves fuel efficiency by up to three percent and ensures that the full power of your engine is available for acceleration.

Today's vehicles require much more electrical energy than older models, due to the much wider array of electric and electronic on-board comfort and safety systems. This energy is created by the generator (also known as the alternator) which converts the engine's power output into electricity. In conventional systems, the generator is permanently driven by a belt connected to the engine.
BMW's Brake Energy Regeneration operates differently: the generator is activated only when you take your foot from the accelerator or apply the brake. The kinetic energy that would otherwise go to waste is now used efficiently, converted into electricity by the generator and stored in the battery.
Producing electricity in this highly efficient way delivers an additional advantage: when you apply the accelerator, the generator is deactivated - so the full power of the engine can be directed to the drive wheels. Brake Energy Regeneration thus increases fuel efficiency while simultaneously enhancing driving dynamics. As a safety precaution, the Brake Energy Regeneration system monitors the level of battery charge and will, if necessary, continue to charge the battery even during acceleration to prevent a complete discharging of the battery.
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  #9  
Old 11-20-2013, 10:16 AM
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jcatral14 jcatral14 is offline
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Quote:
Originally Posted by The X Men View Post
...when you apply the accelerator, the generator is deactivated...
Hmm, maybe some sort of electro-mechanical clutch? Then when the system detects low charge condition, the clutch is electrically coupled to the generator? Just guessing.

If that is how it works it's a pretty ingenious system albeit complex.
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2007 Mystic Blue 530i - ED 12-15-06
2003 Grey Green 330i
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  #10  
Old 11-20-2013, 10:55 AM
RhoXS RhoXS is offline
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Quote:
Originally Posted by The X Men View Post
by charging the battery only when your BMW is braking, coasting or decelerating,

Producing electricity in this highly efficient way delivers an additional advantage: when you apply the accelerator, the generator is deactivated - so the full power of the engine can be directed to the drive wheels.

As a safety precaution, the Brake Energy Regeneration system monitors the level of battery charge and will, if necessary, continue to charge the battery even during acceleration to prevent a complete discharging of the battery.

This cannot be accurate. I do not believe it.

First, I am making an assumption that system voltage is controlled via accurately controlling the alternator field and the alternator output but not by a DC to DC voltage regulator. I will verify this assumption by actually measuring battery terminal voltage with the engine idling (and supposedly not charging) and comparing it to the system voltage via the cigarette lighter outlet between the two console cup holders.

Assuming there is no DC-DC voltage regulator there is no way the system voltage will be as high as 13.2 (or even higher) unless the alternator is driving it there. In other words the alternator must always be putting out energy because the system voltage never drops below 13.2 volts when the engine is running.

The open circuit voltage per cell for a lead acid battery is 2.1 volts. Therefore a six cell battery will have an open circuit voltage of 12.6 volts. Without a charger connected, this voltage will decrease under load, the amount of decrease is dependent on the discharge current and state of charge.

Nevertheless, I am 100% sure the alternator, by always maintain 13.2 volts or higher, is always driving energy into the battery as must happen if the battery is connected to a system that runs at higher than 12.6 volts. This also means the alternator is putting out sufficient energy to maintain the current electrical system load. Again, if this was not the case, there would be a net load on the battery and the battery terminal voltage would drop below 12.6 volts. The battery is a sealed AGM battery so it is not possible to directly measure specific gravity. If it was possible, I would bet the specific gravity would almost always be representitive of an 80% state of charge.

My voltage measurements when driving are from the cigarette lighter output between the two console cup holders; not at the battery terminals. However, the voltage at this cigarette lighter output immediately drops to about 12.5 volts or so when shutting down the engine so I think it is highly likely it is indeed representative of battery terminal voltage and not the result of a voltage regulator independent of the alternator.

Last edited by RhoXS; 11-20-2013 at 04:07 PM.
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  #11  
Old 11-20-2013, 02:25 PM
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RhoXS, it is possible that what you are saying is true, that the charging system maintain a minimal charge when at idle and acceleration, increase the acceleration rate when braking.
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  #12  
Old 11-20-2013, 11:47 PM
newyankee newyankee is offline
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I agree with RhoXS.

I can't say with any certainty how the BMW electrical system actually works, because it is a computer controlled car. But as a Physicist and "chief mechanic" for 4 older cars and one Cessna, I am familiar with conventional electrical systems.

I installed a LED voltmeter in the front cigarette lighter immediately after I picked up my car 20 days ago. When the engine is off, the voltage is 12.6V or lower. With the engine running, the voltage stayed in the mid 13V range for the first few days, and now jumps up to 14V or 15Vmax quickly after starting.

In my experience, a system will produce a maximum charging voltage (15V) when the battery is fully charged, but will decrease as the battery sucks up charging energy.

I believe my battery was somewhat depleted during the delivery process to the west coast, and spent a few days charging at around 13V. Now that it is fully charged, it jumps to 14V or more within 20 seconds after starting. Also, FYI, I find the battery at 12.4V after sitting for 1 night, and as low as 12.2V after sitting for 2 days.

It is also my opinion that the braking re generation is an eco-gimic. Sure, de acceleration energy can be passed through the engine to the alternator, but unlikely to amount to much, and I am convinced that the battery is maintained at a fully charged state during normal driving.
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  #13  
Old 11-21-2013, 02:23 AM
RhoXS RhoXS is offline
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Originally Posted by newyankee View Post

It is also my opinion that the braking re generation is an eco-gimic. Sure, de acceleration energy can be passed through the engine to the alternator, but unlikely to amount to much, and I am convinced that the battery is maintained at a fully charged state during normal driving.
I think "fully charged" means 80% of the design capacity of the installed battery; a battery that is intentionally oversized with respect to the actual needs of the car.
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