Switching to backup power supply in my camper project

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Joined Jul 19, 2018
As a part of my efforts to improve the reliability of the power system in my camper, I'm looking for a solution to power the BMS if and when the 12v LiFePO4 main battery becomes too depleted. I've put in a 12v "D" cell pack to power the BMS that is switched on when the BMS disenables the discharge of the Main battery by opening the coil of SPDT relay an toggling to the "D" cell power pack.. the problem: It is slow. It appears that the time it takes for the voltage to drop in the campers circuits (perhaps to some built in capacitance) keeps the relay coil active too long and the changeover to backup D cell pack is slow. If I could find a circuit that would work instantly it would be a plus. I'm thinking of a MOSFET design?

While we're on the subject I appear to be getting EMF triggering of the circuit that controls my high amperage contactors on power up. Is there a way to mitigate this?
let me add this as a possible starting point :
my control source goes high when source 1 reaches a point selected as where I want to switch to the back up source 2. The total load at Vout would be no more that 500ma
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