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First: The "battery light" isn't there to tell you that you need a new battery. It indicates a problem with the alternator. The problem is that so few failures actually seem to bring on the light it's probably only useful enough to tell you that the belt broke.
Second: You need to start measuring voltages if you want to diagnose this yourself. "Leaving it" or describing how many lights are on isn't going to help. All batteries new and old need to be charged while the car is running. A one-time failure could be the battery developing an internal short between uses causing it to be "suddenly" useless when you try to restart it. A battery in this condition will read ~10.8V. Repeated failures are probably because the battery isn't being charged, meaning the real problem is the alternator.
With the engine not running, the battery should be 11.8V-12.5V (with a complex array of possible voltages depending on state-of-charge, current load from computers powered up while you're measuring and ambient temperature - it is a chemical process, after all). With the engine running the system voltage should be anywhere from 13.0V-15.0V with wide variance because of the way newer BMWs handle the batteries depending on chemistry (flooded/AGM) and state-of-charge (learned by the IBS system). If you still see 12.x or 11.x with the engine running you need an alternator, not a battery.
Third: If your battery has gone dead but is still viable, charge it with a plug in charger and not the alternator. Recharging a completely dead battery is very hard on the alternator. That makes a lot of internal heat, isn't really designed for that and it will shorten its life. Alternators are meant to operate current demand and "top up" starting batteries. Of course if you're in a pinch there's nothing you can do...
Fourth: If you replace your battery, register it. There's no need to use a dealer battery or a dealer to register it, but get it done. Though you don't need a dealer battery, some batteries are better than others. I really like the Deka/East Penn "Exact OEM" line. I have a Deka 9AGM49 group 49 AGM battery in my E90 right now.
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