Hi there, I tried using a battery isolator from the alternator to my 2 batteries. 1 is a deep cycle marine battery, which I will use for the system. The second is the stock BMW battery, which is used for the starter/lights/etc. Both are located in the trunk.
I have decided, for conveniences sake and since the original idea was more expensive and didnt work, to seperate the two with a relay. This way, when the ignition and alternator are off, the 2 are seperated.

However, my question:
What size relay do I need?
I cannot find a 120 amp one anywhere around here-
how do the contact ratings work? What would happen if I used a 80 amp? Would the current jump the relay when its off? Or would it just simply restrict current flow?

which size would you recommend? I have a 120amp alternator that is putting out 120 at ~14volts confidently (just recently replaced)

thanks everyone!

 

Ya, isolators also drop almost a full volt across themselves, so they're not very efficient.

Use an RV solenoid, not a relay. These are common and can be bought at most auto parts stores for 30 or 35 bucks. They have no voltage drop, and are good to prolly 150 or more amps, and can easily be doubled up for more amps.

I use one myself, and have for years, never had to replace it yet.

You can just barely make out the solenoid pictured here in my van, follow the red cable off the battery toward the "front" of the picture, and you'll see part of it, shiny silver.

 

Make sure you get a "Constant Duty" solenoid, most autoparts stores sell them, get the highest amperage you can find.

 

Does this replace the need of an isolator?

 

yeper...id visit princess auto if you have one in your area...we use them here for wiring hydraulics...you may need to use a pair to get a high enough current rating