I'm not sure you'll be able to. I don't have the telemetry receiver, so some of what I'm going to put below is semi guess work.
Look at it logically:
The receiver is what transmits the voltage to the M12, so it can only transmit the voltage that it knows about. It knows about the bec, as it's what powers is, so that's what it will send. It would need a direct connection to your drive battery to know what the voltage of that battery was, and I'm assuming it doesn't have this.
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Matthew White
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