batterys are tricky. because bmw changed the ah a few times.
your best option is replacing it with the same ah that you currently have.
Dont forget if your vehicle does not have a voltage regulator. you will also need to register the battery with the car otherwise you will be going through a lot more than batteries.

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2009 BMW E60 LCI 528Xi


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2002 BMW X5 3.0i E53