Don't drive your car with a bad battery, it can damage components.
The OE battery is the new style AGM battery, which is more expensive than regular batteries, however you can get an Interstate battery for a lot less and works just as good.
There is an important thing, the battery replacement has to be programmed to the car, otherwise the Intelligent Charging System won't see the new battery and won't charge it correctly. I'd suggest to take your car to an independent BMW specialty shop who has the OE or equivalent diag. computer and let them replace and register the battery.
Zee