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.
Best Answer
In a dual battery setup, all accessories required to be powered with the engine stationary would be connected to the accessory battery.
With the engine running the isolator relay would parallel the primary and accessory batteries. With the engine stationary the isolator relay would isolate the accessory battery from the primary battery system.
With the engine running, both the batteries would be charged by the alternator and would together feed all the connected loads when required.
With the engine stationary, the isolator relay would ensure that the primary battery does not get depleted by the accessories being powered.