You need to connect a volt meter and see if you get around 13.75 V or so when the engine is running. The idiot light is quite dumb as it really only indicates full failure but not low charge rates.
FYI There is an article in a recent Roundel that tells how to up the charge of Bosch altenators by fitting a diode in the sense circuit as they tend to charge at a lower voltage than others. My Explorer shows 14.5V constant and the first battery failure was 8 years into ownership. The X seems to hold nearer to 13.85V and I had to fit the second replacement battery just after I bought it at 2 years old! (First replacement was for a VW fitted by dealer to sell it!)

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2015 X5 xDrive35i MSport
2013 X5 xDrive35i