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The Mini battery is about 34 lbs. Maybe try the Deka ETX30L. It is the same as the biggest small Braille (the 21-22 lb one), but less than half the price. You can buy the Deka online or at Autozone. I ran one for about 18 months of year round driving in NH in my E36M3 before it could no longer start the car. I'd expect half that time in the E90M3. I now run the Mini battery in my E36M3.
Another little battery is the older Miata battery. Weighs about 25 lbs but is at least made to start a car.
Not sure what I will try in my E90M3 when the stock one goes. I know it is a big battery. But I also know that small batteries don't last long in new cars with lots of electrical draw.
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