Bmw e39 525i wiki
Suppose a deeply discharged battery is used with less than 9V then the RR might not even charge the battery at all.
So get a loaner battery, check the voltage, if charging system is working correctly then replace the old battery with a new one. If charging system is faulty then repair it, top-up the electrolyte by adding distilled water if required, re-charge the battery using a battery bench charger on 0.
As the OP has mentioned that his bike did not start even after kick-starting, I am also suspecting a faulty charging system.
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