The relationship between the two batteries is strong, if not critical. The IMA control and management systems (which encompasses the Hybrid Battery) can only power up if the 12V battery is healthy and with a good voltage level. Once the hybrid systems are powered up then the power flow typically occurs from the hybrid battery to the 12V unit in the form of a recurring charge.

In summary: A poor or faulty 12V battery spells trouble to the hybrid system's ability to function properly (if at all).

Cheers;

MSantos