grounds for parallel battery wiring

I am a little confused on the grounds for the batteries. I understand that I connect positive form alternator, to positive prime or first battery, the positive form that to my second battery in back. then the negatives will be connected to each other from first one to second (back one). Bu here is where i hope you can help me understand. I do not know whether to actually "ground" the first and second battery, like to the chassis, or if only one should be grounded to the chassis, or if wiring both the grounds counts as a ground(nothing connected to the chassis)? thank you, and please get back to me because i really need a second battery as i just got a HO 300 amp irragi alternator. Thank you!Lanzar car audio

You do not need to ground the 2 batteries to each other. I know some companies like irragi will reccomennd it for a "closed loop system"   You do need to ground both batteries to a solid chassis ground.   If you have the extra wire, it is a great option to ground the 2 batteries together via cable. In my experience is has produced less noise, and more stable voltage conditions between the front and rear batteries2009 0-1000 Trunk WR 154.0DB 2009 1001+ Trunk WR

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While I have little experience with this, the idea is not to create potential differences between the two areas. If you ground battery 1 to the fire wall and the radio gets 12.5v, then battery 2 is grounded to the trunk as is the amp, but the amp reads 12.6v, then you might pick up some additional noise. 0.1v probably isn't worth worrying about, but hopefully I am explaining it right. It never hurts to add grounds. Maybe run an equal sized wire from the firewall to the trunk area. But battery to battery most likely isn't necessary.