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Looking forward to the results!

 

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Sounds like a fun project.

Is the new aux/optima battery run in parallel, with the switch between the engine battery and aux battery?

If so you may wanna google 'mismatched batteries parallel', as this setup supposedly can shorten the life of your either battery. In this case the mismatch is presumably age, deep cycle vs starting, amp hours, etc. I suppose this is a YMMV situation.

EDIT: having gone back reread the posts, its evident you're familiar with this already.

Alternative setups are
1. use a battery selector switch (A, B, A/B, OFF) to manually isolate the batteries altogether. perhaps this is the intent of the setup you're going with i'm not sure the disconnect switch you linked would allow you to truly isolate one battery from the other.
2. use a solid state isolator / smart battery isolator (non-diode, no voltage drop)

you can search boat and rv forums blogs, forums for more details.

on that note, those same forums consistently give good reviews of http://www.genuinedealz.com/ for sourcing custom cables. I didn't use them myself but figured I'd throw it out for anyone considering something similar in the future.

 

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Great point and agreed, that setup was ok for my boat but is not wise in this case.

 

sweet set up


nice job