Originally Posted by swamp2
Cleveland: Useful and valuable anecdotes. You may be right that those of us trying to get the Braille battery to work may never get there. However, this may not be entirely apples to apples with your experience and efforts. It sounds like you were trying some really small batteries (motorcycle batteries).
No one here (well at least lucid and myself) believes they can outsmart BMW/BMW engineers. Well you clearly can with regards to performance... but that is another topic. What part of the design for worst case scenario principal don't you understand? Let me repeat for the 3rd time: The OEM battery needs to handle various combinations of very cold weather, lengthy use of accessories with the engine not running and weeks of idle time. Cold weather in particular is a huge tax on a battery both for starting (CCA) and reserve. It sounds as if you do not believe the OEM system has a tremendous amount of extra capacity for warm weather, no extended use of accessories with engine off, and a mere couple of days between uses. Can you tell us your thoughts on this particular point? You just can not argue that the OEM battery both has no significant extra reserve but at the same time it is the smallest that will work reliably when not heavily taxed.
Do you have any idea why for you a 33 lb battery seemed to magically work when a 20 lb one wouldn't? 33 lb was surely significantly lighter than the OEM battery. Where do you draw the line?
Woah, too many questions at one time...
Ok, I need to get something out of the way right off the bat...
In regards to your motorcycle battery comment:
EVERY SINGLE ONE
of the small "Lightweight Racing Batteries" on the market are simply re-branded, re-engineered, MOTORCYCLE
batteries. (more or less)
All of them. It's important that you know this...
The internal lead plates have been redesigned to squeeze out a little more power...but that's about it. Go to any Suzuki or Harley-Davidson dealer if you need any confirmation. These batteries are essentially motorcycle/4-wheel atv/jet-ski/lawn tractor batteries. (not kidding)
These are not, and never will be, SAE-approved STARTING batteries for automotive duty.
Putting an aftermarket brass screw-on SAE battery terminal (with a tiny 3/8" threaded stud on the bottom) does not magically turn a motorcycle battery into a automotive battery.
Don't even get me started on the whole "Pulse Cranking Amps" (PCA) or "Hot cranking amps" (HCA) thing...
These terms were invented by small lightweight racing battery manufacturers to help market their products better. Those numbers are going to be higher because of the skewed testing conditions. They cannot be used as a direct comparison tool with a conventional full-sized SAE approved starting battery.
Conventional SAE approved battery manufacturers use this basic criteria on virtually every battery you will ever find in the marketplace:
a) Cranking Amps @ 32°F
b) Cold Cranking Amps @ 0°F
c) Amp Hour (Ah) Capacity: Measured using the C/20 standard
d) Reserve minutes BCI: Using a 25 amp discharge rate @ 80°F (26.7°C) down to a 10.5V cut-off point
These are solid battery performance parameters, and they are only ones that really matter at the end of the day. Any comparison should be made using these yardsticks. I admit it, I got sucked into the high artificially ginned up numbers
these little batteries can produce under certain lab conditions. They don't fair quite as well, when you compare them 'straight up' to a full sized SAE starting battery using the specs listed above.
And sometimes, it sends up a red flag
on how some of their numbers don't add up. (performance wise) After running several of these small batteries, I suspect there is a lot of 'rounding up' involved in their claims. (and that's being a bit generous)
So let's not go down that road...
Once again, battery power is primarily a result of SIZE
Technology can help you tweak this a little, but the overall footprint
is the key.
Larger battery case (dimensionally) = More room for LEAD plates with a larger surface area + Larger AGM sandwich material that absorbs the acid = More powerful battery in every category. Period.
This is a fact, not my opinion. Power output (CA, CCA, Ah, Reserve min.) are directly proportional to size
It's a pretty simple formula. It's not rocket science...
You can't have a 1500 cranking amp AGM battery that is the size of a shoebox. That's ridiculous.
And this is coming from the biggest lightweight battery fanboy you could have ever known just a few years ago.
I have done a complete 180 degree flip-flop on this issue. That is not only due to my own personal experiences, but also many other owners who I've corresponded with who went down the same rabbit hole with me.
It "sounds" like a great idea...in the beginning.
Here are just a few of the issues that tripped me up:
1) SMG pump/accumulator cycling
2) OEM iPod adapter/charger not shutting down
3) Bluetooth phone module (ASSIST tcu) including random calls to assist after the car was shut off...
4) My Radar Detector
5) OEM Home Link garage door opener module
6) OEM DVD Nav drive constantly rebooting itself
7) OEM CD changer cycling through discs
8) OEM SIRIUS satellite radio tuner not shutting off
9) OEM factory alarm and clown nose beacon
10) Light control module
And on, and on, and on...
The above items are just a small sampling of the modules that are largely hidden inside the car.
All these things are controlled by BMW proprietary I-BUS (CAN-BUS) communication interface software through the cars ECU.
I wasn't aware of how many systems and subsystems are connected to this system.
I never factored this into my original plan
, and I ended up paying the price.
You seem to fashion yourself a very clever person (as do I), well that can sometimes be a hindrance to your ability to see things in an objective manner. I'm sure you believe your theories about why this WILL work. They are probably as sound as my initial theories were so long ago.
You are making the exact same arguments now, that I did in the beginning. I was convinced that I had figured out exactly why the BMW electrical engineers had designed a 800 CA 70Ah battery for my car, and how I could successfully trim that down by a substantial margin with no ill-affects.
You are using the same defense arguments I did, and didn't matter. I was blindsided by several things, including the various electronic modules all over the car. (that were working against me)
Unless you drive the car everyday (keeping this battery fully charged), you will have some reliability issues with a battery that size going forward. I can promise you this will happen. It may not die on you completely like some of the smaller sized batteries I tried to use, but you will find the breaking point where the car will barely have enough power to start the V8 engine. I had a number of accessories in my car for the cell phone and iPod, so maybe your a little better off if you don't add any accessories to the car.
The E92 M3 has even more of these electronic control modules than the E46 did, and I'm sure the next M3 platform will have even more. You can't stop the advancement of more electronic based control systems. That is here to stay. These devices are tucked away in every corner of your car, and they are all controlled by a central 32-bit processor in the ECU by proprietary CAN BUS software.
Do I have to paint you a picture?
Sometimes computers don't shut down everything (properly) like they are supposed to...
And when they don't...the battery takes the brunt of it. The BMW Electrical Engineers must have factored this in the final battery size and power specifications of this car. Are you saying you know for a fact they did not?
I spoke at lenght about this very issue with a BMW Field Engineer who works for BMW North America. He travels all over the country diagnosing the toughest service related cases that BMW gets. When my car was in the service bay, we spoke about the electrical systems in late model BMW cars, and he would certainly disagree with your hypothesis on why the OEM battery is so large. Some portions of your theory are correct (climate), but the overriding theme that you don't seem to get, is that it's a forgone conclusion that the CAN BUS will not always shut everything down as it should. It's apparently so important, that BMW can actually refuse to honor any legitimate electrical warranty repair if you remove the OEM battery from the car. (and you don't replace it with a unit with the same specs)
Check this out with your service advisor if you think I'm kidding...
The 90Ah battery in the trunk was designed as much as a safeguard against total electrical failure in case the software program goes off the reservation, as it was for any concerns about cold climate conditions. (or someone playing the radio after the ignition is killed)
You need to throttle back just a bit here. It's clear to me that you are assuming you know the exact reasoning behind their decision making process.
That is is a bit presumptuous don't you think?
Forget the "theory" of what's supposed to happen. That's how I got burned...
I found out much later, that BMW has issued many bulletins on defective control modules or ECU software glitches that don't shut down things when they're supposed to.
There is no engineering workaround to this random fault. If you figure one out, please let me know...so we can patent the idea and get filthy rich.
You only have one battery. And you have to think of it more as an insurance policy (security) instead of a target (to dump weight). I use to think of my OEM battery in that way, and it turned out to be counter-productive for my non daily driven street car.
The safety margins with these racing batteries are smaller than you think...
As I said to Lucid, I wish you best, and I really hope things work out for you in the end.
I'm just relaying some of potential downsides of this mod (that I experienced in spades), because you rarely get the other side of this story.