Dual battery function

[Bob 3500HD]

My 3500HD came with dual batteries.  Trying to figure out how they function.  The passenger side battery is connected to the starter and alternator (starter battery).  The drivers side battery has an isolator switch on top (backup battery).  I assume the backup battery is isolated with the truck off.  How do the batteries drain at that point?  If I run my truck camper off the truck batteries does it drain the starter battery or the backup battery?  There is no information I can find anywhere on how these are wired up and function. The information I find on the web seems to be that the isolator should isolate the starter battery.  It doesn't look like that on my truck.

[mkeddie161]

Pictures of isolator and under hood area

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[mkeddie161]

Currently adding second battery as we speak

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[mkeddie161]

Without any pictures I can't say if u have a isolator
but from the stock Diesel( not saying u have a diesel) that I've seen there is nothing to separate the batteries they just work together all the time
Kill one kill both


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[Bob 3500HD]

There is an isolator.  But it looks like it’s on the secondary battery.  I’d just like to understand how the whole thing functions.  For example, which battery drains

[davester]

The Duramax's get their dual batteries hooked up in parallel with no isolator, as they need the power of both to get the engine going in adverse conditions.  Don't know your setup, maybe you can figure it out by getting the build sheet for the truck, to get what option it is, and go from there?

[Bob 3500HD]

Nothing in the owners manual and I've searched my ass off on the web.  Seems like GM would have some sort of written explanation published somewhere

[mkeddie161]

Like I said post up some pictures or can't help out much....if it's from the factory I dont think it's a isolator....

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[Bob 3500HD]

[Bob 3500HD]

[mkeddie161]

So that definitely looks like a isolator it looks like the 100amp fuse would be ur power coming in(charge wire) and the only thing powered by the battery is the 30 amp fuse...


Warning always think that a disconnected wire is gonna blow up in ur face... I'm not responsible

The way to see what is happening is to disconnect the 100 amp fuse and take a tester and see what power is coming to the cable that was hooked to the fuse if it's powered then that should be ur charge wire.

If the isolator side of the 100amp fuse is dead(0volts) then ur second battery will only charge when truck runs and discharging should not kill start battery

With out some testing it's hard to say how it is working just need more info wish I knew they made that before I started my project

Wish I knew about that isolator sooner



My bought fuse block is from a diesel pickup...it has a 400 and a 175 amp fuse in it


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[Bob 3500HD]

Here is the fuse layout on the starter battery.  The 400 amp goes to the starter.  The 250 is the alternator.  The others disappear into the wire harness 

[Bob 3500HD]

If the starter battery is drained by my camper with the truck off I don’t see how the secondary battery can start the truck through that 100 amp fuse

[georgecdisc]

I thought it operated as such:

 

Truck off = batteries isolated

Truck on = batteries isolated

Truck running = batteries connected

 

100A fuse at isolator is so alternator can charge second battery (when truck running)

 

30A fuse at isolator goes to 12V+ of 7way trailer plug.

 

I think this means you can drain your second battery with a camper and still start your truck with primary battery.

 

I may be wrong, hoping someone else can back this claim.

 

...i did a little experimenting when wiring in my battery maintainer quick connects (one for each battery). My memory is foggy on those results.

 

[diyer2]

^^^^^^^

This makes sense.