I did installed an isolation solenoid on my friends jk last summer, along with two optima yellow top. don't remember what brand it is though. If my memory is good, when the switch is off, batterys are isolated. if the second (used for all accessories) drops below a certain voltage, the solenoid will engage and link both batterys together to charge the lowest. once it is charged it will disengage to isolate them from each other. in fact, the switch serve only for starting. if your main battery is low, press the switch to link 'em both for more starting amps for 1 minute, then disengage by itself.
Of course, there is a solid state chip attached to the solenoid to manage all that.
Gonna ask my friend to chime in to confirm.
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