Does the ECU use the auxiliary battery during istop?

[sunwalker]

Does the ECU use the auxiliary battery during istop?
Meaning when the engine turns off & the a/c etc is then run from the auxiliary battery during istop.

I've been trying to figure out why my abs, tracking, hand brakes mil (warning lights are on) for my 2011 Nissan lafesta / Mazda 5.
The fault codes from the scanner are many, but include low voltage supply, resetting the abs module worked (so far) after I charged the aux battery on repair mode for a day (need to purchase a battery diagnoser).

My thinking is the car engine stops (istop) then all electrical including the ecu draw only from the aux battery, so if the aux battery is failing then the ecu turns on the mil abs etc fault lights.
Is this the case?

Sorry just realized I posted this in N. Leaf. probably should be in  General Nissan Owners Lounge