According to BMW's instructions, when additionally installing a radiotelephone, in order to provide a reserve of electricity, a second battery of the appropriate capacity should be installed with a relay that disconnects it from the vehicle's electrical circuit. BMW service specialists will be happy to advise you on this matter.
Important recommendation:
Application of mobile communication systems (car radio, radio devices, etc.), not intended for use with your vehicle may cause interference. BMW cannot control each product and therefore cannot accept any liability.
In this regard, before buying such a system, we recommend that you seek advice from a BMW service.
To ensure the reliable operation of your BMW, do not install any mobile phones or other communication equipment whose antenna must be located inside the passenger compartment.