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Per the DIY on e90post this is why you register it. Also the battery is 200 dollars from BMW. Not overpriced considering it lasted over 11 years and a Walmart battery is $120.
Q: I replaced my battery and it works just fine, why all the fuss?
A: Disregarding the need to register and code for a new battery may result in improper charging. Over time, improper charging may have detrimental effects on the lifespan of your cars battery and the operation of the vehicles electronics.
Q: Why do I need to register a new battery?
A: The short answer is that BMW designed the charging system with an algorithm that varies the rate of charge by the age and type of the battery installed. Registering a new battery resets the algorithm back to optimal charging voltage for the specified battery.
The long answer: "The negative battery cable has an electronic control unit on the battery end of it called an intelligent battery sensor (IBS). The IBS measures the state of battery charge and sends this information to the ECU. The ECU uses this information to control how much charging voltage is needed from the alternator to support vehicle electricity needs. A battery deteriorates normally over time and depending upon service demands and driving profile. The IBS reads this and the ECU tells the charging system to increase voltage and amperage output incrementally as the battery matures. Therefore, at battery replacement time, if you don't tell the ECU it has a new battery it will overcharge the new battery causing it to wear out faster.
The registration process takes place in the ECU, which is the main player in this system. Also, when the IBS sees that battery voltage is too low for normal vehicle operation it can start shutting down vehicle functions to conserve power while keeping the vehicle operational. This would be accompanied by a check control warning of excessive battery discharge.”
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