I want to make a dual battery setup on my car. Doing some research I came across the two main techniques to do this, both based on isolate the two batteries being fed from same alternator to avoid charge issues.
Wanting to avoid the voltage drop and heat issues of diode isolators, I started to check mechanical isolation (relay).
There are a lot of ready to deploy products in this line, most of them looking like this. But I'm kind of stuck on how they work. They look like simple big fat spdt relays.
While the instructions are clear and mount the batteries to it doesn't look like a problem, I'm puzzled. Most of the instructions wire 12v directly with the relay coil.
If I'm not wrong, this will make the relay change it's state when I turn the car on, charging just one battery. Then when I turn off the power, relay should change again and charge (but no, because the alternator would be off) the other battery.
I am missing something? Is this not a relay as i know it? Or it is a solenoid? Should I add a switch to control the relay?
As an example, here is a mounting diagram. I am missing something simple af and I don't know what.
Edit for clarification:
The purpose of this setup is the usual one among the car and audio hobbyist. I want a secondary battery to make use of some gadgets without compromise the main battery.