Home made battery isolator... will this work?

[POS carb]

I'm thinking of adding a small battery in the rear of the Vic to help the audio system out a bit, but I am cheap and I was wondering if this will work and which configuration is best.

The battery up front is a group 65.
I want to add a group 51 in the trunk with an isolator for the system and I was thinking about using a Ford-style starter solenoid as an "isolator" so that it connects the trunk battery to the main system whenever the key is On and the radio is on. Once the radio is turned off or the key is turned off the trunk battery will disconnect from the main supply. I'd have to add a relay to the remote wire that gets a switched 12v source but here's the basic idea:





In summary:
I could do 2 different configurations.
a: the trunk battery is connected whenever the ignition key is on
b: the trunk battery is connected whenever the ignition key is on and the stereo is on

I want to do configuration "A" so that the battery stays topped off but I would like an expert opinion first.

To do B I just add a relay in line to the solenoid with a 12v switched ignition source to the 30-pin and a remote wire to trigger the relay (85/86)

[89T]

I'm thinking of adding a small battery in the rear of the Vic to help the audio system out a bit, but I am cheap and I was wondering if this will work and which configuration is best.

The battery up front is a group 65.
I want to add a group 51 in the trunk with an isolator for the system and I was thinking about using a Ford-style starter solenoid as an "isolator" so that it connects the trunk battery to the main system whenever the key is On and the radio is on. Once the radio is turned off or the key is turned off the trunk battery will disconnect from the main supply. I'd have to add a relay to the remote wire that gets a switched 12v source but here's the basic idea:





In summary:
I could do 2 different configurations.
a: the trunk battery is connected whenever the ignition key is on
b: the trunk battery is connected whenever the ignition key is on and the stereo is on

I want to do configuration "A" so that the battery stays topped off but I would like an expert opinion first.

To do B I just add a relay in line to the solenoid with a 12v switched ignition source to the 30-pin and a remote wire to trigger the relay (85/86)

that's the way to do it. Be sure to go directly to the Battery up front. Just find a power source that is only on when the key is in the run position. That way if you are washing the car and have the radio on, it wont kill the main battery.be sure to run a bosh relay and circuit breaker's or fuses.

[dieselgus]

Solenoids & Relays | Steel & Phenolic Body Solenoids24059 | Cole Hersee - Littelfuse

A Ford starter solenoid is not rated for continuous duty. We use the CH ones all the time in law enforcement vehicles and have seen zero failures due to duty cycle on the past 5 years. Not too badly priced.

1315 Battery Separator - Smart Charge Priority Systems

These also work quite well.

[Hauntd ca3]

use a voltage sensitve relay or vsr
it will isolate the rear battery so you can run your stereo when the car is not running and once the car is running and the main battery voltage is high enough it will turn on and charge the rear battery.
they are commonly used in motor homes to charge the house batteries off the vehicles charging system while you are driving along.

[POS carb]

A Ford starter solenoid is not rated for continuous duty.

Thanks, this was one of my concerns