i have had this discussion with quite a few people,and,just yesterday,i had it again with a potential customer.dual batteries,or multi batteries,it's like this,if you have a single engin,and you have a selector switch,a standard selector switch the 1,2 all and off sitch,when the switch is turned off,nothing,including your electronics work,you do not have a seperated system,your electronics are not wired to only one battery.this is a matter of opinion and preference,and i hate this system,it's terrible,most people use the "all" position,meaning the batteries are "paralleled",again,meaning you're drawing from both batteries,not just one,they're connected electrically.by running a system like this,you're running the risk of not having enough power to crank the engine when the time comes.you're also running the risk of having the electronics "crash" upon cranking the engine,due to a low voltage situation.also,if you don't run the switch in the all position,you really can't charge both batteries at the same time,menaing,you're gonna have to move the switch position to charge both batteries.
there's a better way of doing this-a much better way...
it's called a seperated and dedicated system.works like this,2 batteries,one for the engine,one for the 12v system,2 simple on/off switches-one switch controls the 12v system,the other controls the engine."bep" and blue seas both make these set ups,i prefer to use a different approach,there's nothing wrong with using either of these 2,i prefer not to.i use a "sense relay",this senses voltage at the batteries,it senses the voltage at the one battery is 13.2v or higher,the relay then engages and allows both batteries to charge at the same time,same thing happens when the engine is shut down,the relay disengages,due to a low voltage situation.trick huh ? there's a little wiring that's involved in this,but it's not that bad.this is an excellent addition to your boat,and i highly reccomend it...