What are the main differences between the cheap scheme 1 and the more expensive scheme 2?
I have no shore power but i will add PV in the near future.
In terms of end user, not too much.
The difference is that the Argo fet feeds back voltage information to the alternator, and is capable of compensating for volt drop between the alternator and the argofet, resulting in slightly improved performance at the end of the boost / bulk charge.
I used a Cyrix-ct 120A relais in a similar installation.
The relais was located between the starter battery and the service battery. The solar charger was connected to the service battery. BUT - the Cyrix relais is a bi-directional relais, that mean, it measures the voltage of both batteries (in case the relais is open) and closes the contact when the voltage is rising on the one or other battery. So you can charge the service battery via alternator, but also can charge the starter battery via solar (when service battery is full).
But with that feature both batteries are not really isolated from each other because you can't deactivate this automatism.
With the switch you can force the relais to connect both batteries what I sometimes used in winter when the starter battery was a little weak to start the diesel engine.
If you still want to isolate the batteries, you need a standard relais without any "Intelligence". With a switch you can then only manually connect or disconnect them.
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