Inline fuse placement

[KaLvan]

We will be hooking up our auxiliary AGM battery to our starter battery today. Power will run from the starter battery through an isolator to the auxiliary battery. I know there need to be some inline fuses in there, but I am  uncertain about the location of them and also the size. I found some info that said we should use a 60 to 80 amp inline fuse on the power cable around 12 in before the auxiliary battery. But  should there also be one between the starter and the isolator?  

So, what size fuses and where should they be installed?

Also, there will be an on/off switch for the system so we can work on it safely. I am thinking that goes right before the auxiliary battery. We don't live in our van full time. When we are not using the van for camping and just using it around town, I assume I should just shut off the switch to avoid constantly topping off the battery. Is that correct and is the switch be in the right location to do that?

Thanks!

 

[B and C]

Fuses protect the wire. A fuse at each battery. The fuse size will be dictated by the size of wire you run.

Here is a good place to start for wire size needed: https://www.bluesea.com/support/art...oosing_the_Correct_Wire_Size_for_a_DC_Circuit

A continuous duty solenoid connected to the ignition switch to turn power off/on works well for a battery isolator. This is not a starter solenoid. Two different animals although they look the same.

 

[PeterPiper]

Generally speaking, it is best to place the fuse physically as close as possible to the power source. "At the battery" as posted above.
This way the circuit is protected all the way to the source and not just partially through the circuit especially in the case of a short.

I like to resolve these issues by creating a simple schematic of the circuit in question.
That makes it easier sometimes to visualize the situation.

As far as the specific fuse rating to use, it depends on the amp draw of the starter.
You could always start with a 60A fuse and try it.  Use an 80A fuse if it blows.
Overfusing can be as bad as no fuse.

 

[jimindenver]

Since isolators are rated in amps and the ones I have installed called for 4 gauge wire, they were fused for the output of the isolator which was well below the capacity of the wire.

 

[Weight]

As said above by others. Ever positive dc wire needs a fuse as close to the positive battery terminal as possible. In your case a (1) fuse at the solar battery positive and a (2)fuse at the start battery positive. Size the fuse ampere to about 125% or a bit less than the ampacity of the chosen wire awg.
Rule Number One; Fuses protect wires! I use these fuses & holder. https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A