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      04-10-2011, 10:12 AM   #8
Lotus7
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More than you every wanted to know about BERs

Hopefully, you'll not think this is too tedious, but even though I'm usually a staunch advocate of new technology, I'm beginning to wonder if the net gain of the "simple" BMW BER system isn't more marketing hype than a cheap source of "something for nothing" as BMW states in their literature.

BMW has publicly stated that the BER can improve fuel efficiency by "up to" 3% on the European Urban Driving Cycle (ECE-15). I've also read reports that state that no statistically valid gains were observed on the US EPA cycle (which is only slightly more “aggressive” than ECE-15. There are obviously no gains whatsoever in longer, highway driving situations.

Because of the low charging rate during steady state driving, a BMW with BERs will likely have a battery that normally averages a charge level of 60 to 70% most of the time. A conventional, standard “wet” battery will have significantly reduced life under that condition, and will also not like the periodic high charge rate of the BER. BMW has anticipated this condition and has specified and supplied new X3s with AGM (Absorbed Glass Mat) batteries. The AGM battery is also somewhat larger than needed to allow enough charge to be able to start the vehicle in cold climates when the battery is only partially charged. However, AGM batteries currently cost about 2 times more than standard “wet” batteries. The “larger” battery size required also adds to the cost, and adds a little extra weight to the vehicle (which hurts fuel consumption a tiny amount).

The bottom line: You might save a few pennies a week on fuel if you do mostly “urban” driving, but if you keep the car more than 4 or 5 years and need to replace the AGM battery, it will cost several hundred dollars more.

Last edited by Lotus7; 04-10-2011 at 10:18 AM..
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