how to wire multiple batteries

i'm currently working on my 1996 w202 c class mercedes in which i have 4 yellow top optima batteries to put in the boot 1 to replace the existing car's battery and the other 3 to run my car audio and other extra gizmo's (air suspension, undercar lighting etc etc). i'm trying to create my own master piece but have little experience of wiring more than 2 batteries together i've usually used a split charge relay in the past but am sure that it's probably not the best way of doing this, so advice would be greatly appreciated on this problem. also cos the mercedes has the battery in the boot as standard would it be advisable to change the original power cable that runs from the front passenger footwell via the vehicle standard connection point to the boot and the cable within the engine bay that connects the alternator to the opposite side of the same connection point. i haven't uprated the alternator yet as the car's in bit's but am planning to but unsure of a suitable replacement what should i use

What you need is a Multiple Battery Isolator. You can get them at RV Centers.

They Are Great Big Diodes with Heatsinks.

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !