Dual Batteries (again!)

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Jul 5, 2007
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Alright...I've read all the FAQ's, searched all the threads, and surfed the internet until my eyes crossed. There's about as many opinions as there are posts. I've given it my best shot and now I'd like some help before I start cuttin' and wirin'.

My intent (like many others') is a simple system that isolates the primary (start/run) system from the accessories...yet may be combined if required.

Where I REALLY need help is the application of circuit breakers and/or in-line fuses (and their rating).

So...please review my schematic and feel free to comment. Thanks!!
 

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Joined
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I can't help too much, but here's what I know having just added a second battery. I used the Hellroaring BIC-95150B isolator, hooking it up as an auxiliary battery.

In the instructions (available at the hellroaring.com website), they discuss another approach which sounds like what you want, starting battery isolation.

It suggests, but doesn't say it's required, this:
"If your main vehicle loads are connected to the battery or starter rather than the alternator side, then you may want to move these connections to the alternator side."

It also suggests:
"For complete isolation on the starter side battery, ensure that the ignition circuits are connected through an additional relay."

That should help. Sounds like you could take several approaches, although thinking through all the details of implementation could give you a headache:doh:

The way to go would be to just move whatever feeds the fuse panels as they describe above. I wouldn't change the OEM fuses individually, just change the feeds. You could also add an auxiliary fuse panel while you're doing that if you have more than a couple of add-ons.

Regardless of the particular install you're doing, I wished I'd done added an aux panel to start, instead of using individual in-line fuseholder. Either way works, but the penal will be neater.

The Hellroaring isolator gives you the ability to combine batteries.
 
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If you plan to ever combine the batteries during winching then you'll need a very heavy main set of cables to link the two batteries. Something that will take 1/2 or more of your winches full power needs. Check your winch manual but probably 1/2 of 400-650 amp draw? You probably will not want a fuse on that circuit. For all others determine what kind of draws will be applied and go from there.
 
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Short clarification. The combine feature on my Hellroaring isolator is only used for starting, as necessary.

To feed power to my winch, I use a Blue Sea switch to it from either battery or both combined. For this circuit, I used 2/0 welding cable.
 
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In your diagram the only fuse you may need is one between your AUX positive link and your inverter(depending on where it is placed). Any other additional wiring you take from the LINK needs to be fused/circuit breaker to suit the size of the cable you are running. Remember, the C/B is to protect the cable, not what is plugged into it.
You also need to wire in your automatic battery isolator, as the diagram does not show it, or are you going to open the bonnet every time you stop and isolate the AUX via the 4 position switch?
You can of course add high current fusible links to your batteries, but if you wire/insulate/protect your cables you may not wish to.
Personally I would wire the following
Main battery to isolator 1
Aux battery to isolator 2
Winch to isolator C
Automatic battery isolator across terminal 1 & 2
Positive link to Aux battery positive terminal
Fused circuits from link to auxiliary circuits (fridge, radio, power outlets etc)
Isolator switch stays in position O at all times normally
Auto battery isolator charges and disconnects your Aux battery every time you start/stop
When winching turn isolator to 1, 2 or C (your choice dependent on your batteries)
If battery is flat, turn isolator to C to combine batteries for jump start.
 
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I'm just completing my dual battery/ alternator upgrade, and I have not added fuses for any of the main battery/ winch cables. As mentioned previously, there are no automotive type fuses/ circuit breakers that I am aware of that are designed for these types of amp draws. Best practice is to use proper wire routing/ protection to prevent sharp edges from cutting into the wire insulation. I soldered all my heavy gage lugs (this what Mil-spec connectors use). I also applied two layers of heat shrink to the wire/ lug conection. I did add a 175A megatype fuse from my "new" 130A alternator to my battery isolator. I am running my vehicle chassis electrical loads from the fuse box under the hood to my second battery so that my starter battery will only be used for engine start.
 

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