Dual Batteries

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If i do an isolated dual battery setup, can i use a common ground between them, or do i need to isolate the ground for the 2nd battery?

Common ground is ok. I have a 12 volt system with 2 batteries . The auxillary battery is isolated from the cranking battery once the engine is turned off and is used to run the fridge for short periods while I recharge the deep cycle battery.
It means I always have 1 good battery to get me going
Not sure what you mean by "common ground"?

The chassis is grounded to the whole body. The only thing not grounded together is the engine block because it rides on rubber motor mounts. But the engine is grounded to the chassis anyway.
Definitely connect the battery negatives together! They'll be connected together through everything on the truck through the frame anyway. And if you lose a ground on one battery when you're starting (due to a corroded terminal, etc) with the batteries combined, you're starter may find a return path through a more expensive route. Like through your harness, electronics, ... crackle, smoke, burn.
@NLXTACY, EOE_Overland - well, technically, i _could_ isolate them. it would be work, but i could do it (i would make sure that nothing on the circuits from that battery grounded to the chassis, but rather a ground i rand directly from the battry), that's why i was asking if they should share a common ground.

thanks for the info guys.
Yeah, you could. But that's a whole lotta work. As long as your 12V positive circuits are isolated with an isolator, battery switch, combiner, or other isolator-du-jour, you're in good shape. Isolating the grounds too will eliminate the opportunity to combine your batteries to start if your starting battery fails. Not sure how you're wiring the system; you may not want this feature? But that's an option I'd always want to keep. Or if you're really worried about complete isolation but keeping a combiner choice in the mix- put a on/off switch in the connecting cable between the battery grounds, and wire the truck the way you mentioned (keeping the DC negs seperate). Then you can throw a switch to make the connection between the grounds. Not the way I'd go personally, but it's a fine option if you have a reason to keep the batts totally isolated.

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