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I'd get a second battery if I was going to commute 40 miles a day. And a second charger to keep at work.
And go for the downtube mount, not the rear rack battery. That way the rear rack will have the strength to handle the spare battery and the weight balance will be better.
One of the MAJOR advantages of the BionX system is their downtube battery mount. It works very well, and you won't get the handling problems 'Cynergy' described above.

I have covered the labels on my battery with some leather to match my Brooks saddle and grips and am very pleased with the result.

Mike