Battery drain on 2 battery system

Hi guys I have s350 2007 car had been left with someone to do work but batteries went flat.I have charged consumer battery via the terminal Post under bonnet but voltage is dropping after a day even. Is there a easy way to check for parasitic drain with this system cheers.

Hi guys I have s350 2007 car had been left with someone to do work but batteries went flat.I have charged consumer battery via the terminal Post under bonnet but voltage is dropping after a day even.
Is there a easy way to check for parasitic drain with this system cheers.

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