redarc vs12 voltage sensor

The problem is that you need to be able to control the INPUT VOLTAGE to create the required trigger voltage. Obviously you could just sit there with all the appliances - headlights, vent fan, ... - turned on (engine off) and wait for the voltage to droop to your desired set-point, but that's going to take a lot of your day and get old really quickly.

Plan B is to source an adjustable DC power supply and connect that to the relay input. It would need to be a low ripple supply. Then simply set the power supply output to your required set-point (using an accurate digital multimeter) and adjust the VS12 pot as per the instruction sheet (clockwise increases) to get the required result.

I know that not everybody has such a PSU at hand, but that's the only real way to set it properly that I can see.

Yes Nick, there is a very easy and economical way to do this.

Obtain a 1k potentiometer (Jaycar RP-7504 $2.50 or similar) and connect it as shown in the photo below. The output voltage to the sensor can be varied between 0 and 12 volts by simply rotating the knob-shaft of the potentiometer. Do not have any load connected to the sensor during the test.
Connect your multimeter to the leads to the sensor to monitor the applied voltage.

Does anyone have suggestion on what voltage i might set this to ?
I want to save battery the from damage from over dis charge .
deep cycle wet cell 100ah battery running few light & 40 lit fridge
leaving lights/fridge on etc just in case not situation .
cheers