What a bad advise! Buying from AutoZone (I am not familiar with their products) maybe ok, but not registering the battery is stupid. (for many reasons previously covered in other topics). Also, if you change to another type (ACM vs standard) or rating (Ah) you need to code the car accordingly.

OP: A battery should definitely last longer than two years. I am curious how they come up with the 50% number. It is quite hard to "estimate" the remaining service time of a battery...

If your car turns on the check engine light, there has to be a error code stored in the memory. It does not sound right that there are no codes. Maybe you should try another workshop. Are you using a BMW dealership or?

I dont believe that the check engine light is caused by a "bad" battery. If the battery is worn down, you will notice it when you start the car where the engine would crank "slow" or not at all. If it cranks normally and starts with no issues, the battery is fine.

 

Yes, I'm using BMW dealership in Southern California. They kept the car for 2 days and said no code found. But so far the engine light hasn't' come on again. I'll try another dealer, if the light came on again.

 

Typically when they say no code found, it means that they found no codes which they would suspect to be related to the problem.

You can get a cable including CDs with the needed SW from EBay for approx 150 USD. Then you can read out the errors yourself. Much better than relying on somebody else :-)

Here is one
BMW Progman V41 USB OBD Package Dis SSS Ncsexpert Inpa Ediabas | eBay

There are others that are cheaper, but I am not familiar with them. I bought the package above. It allows you to read out errors, reset battery etc etc. You can even do coding.