I'm using a Victron Cyrix (12V 120A) Smart battery combiner to connect the starter and the leisure battery in my RV. I wanted to know when the two batteries are connected to each other, but unfortunately the device does not have any output signal with this information. What kind of circuit can I use to know this? Some kind of continuity test between the two (+) poles of the batteries?
Below there's a diagram with the connections. Thanks in advance for any feedback.
Edit: Based on the suggestion from @joribama I think that measuring the voltage of the two Cyrix poles, can be a good indicator if the two batteries are connected or not. I'm thinking about using the ADC pins of a Attiny85 to measure the voltage using a voltage divider as shown below. Since the Attiny can read up to 5.5V we can calculate that for that limit to be reached the batteries needed to have 22V, so there's a good safe margin. The 100k resistance between the positive batteries' poles and the GND result in a max current of 0.14 mA (~2mW using 14V for the calculations), which I can live with...
With this circuit, using the 10-bit converter, I have a resolution of around 21.5 mV (22V/1024), which seems enough to detect when the cyrix is closed. The Attiny would then turn on a LED inside the vehicle whenever the voltage difference between the two poles fall bellow a certain threshold (to be defined).
What do you think? Will this work?