Fuses in dual battery system

[BurbanAZ]

I just put my dual battery in the other week and am using a solenoid isolator like I always have done before. I always put a fuse in before and after the isolator because I read somewhere that I needed to.

I put a fuse before the isolator because I only had one fuse laying around and am going to throw another fuse in after I run to the parts store. I was thinking though, what's the point of having 2 fuses. If one fuse blew it would still open the system, I don't see why people say you need one on both sides of the isolator.

Just curious because I was thinking about it.

 

[DiploStrat]

Two Batteries = Live at Both Ends

A normal circuit is only live at one end, with two batteries, your system is live at both ends. You want a fuse as close to each battery(s) as possible, the normal spec is 18 inches. If you fuse only one side, the wire may still be hot from the other side, depending on whether your relay is open or closed. And if the relay is open, and you have fused only one leg, then the other leg is unfused.

My cables run nearly twenty feet under my truck; I do not want a twenty foot arc welder underneath.

 

[dwh]

Yea, technically, you need a fuse to protect the wire wherever there is something (battery, alternator, whatever) that can supply power TO the wire. And the fuse or breaker should be as close to the power source as possible to protect the full length of the wire.


But in reality...in my truck both batteries are under the hood (in other words, inside a big metal box), and none of the factory battery wiring had any BigAss(tm) fuses, so I'm not running any fuses or breakers in my battery wiring. I've got a dumb solenoid split-charge relay setup.

Of course, the tap off the aux battery to feed the house circuits is fused. Well...actually Bussman Shortstop breakers. From the aux battery through a 50a breaker, then from the 50a through a few 20a to feed a few aux circuits.

 

[BurbanAZ]

Yea, technically, you need a fuse to protect the wire wherever there is something (battery, alternator, whatever) that can supply power TO the wire. And the fuse or breaker should be as close to the power source as possible to protect the full length of the wire.


But in reality...in my truck both batteries are under the hood (in other words, inside a big metal box), and none of the factory battery wiring had any BigAss(tm) fuses, so I'm not running any fuses or breakers in my battery wiring. I've got a dumb solenoid split-charge relay setup.

Of course, the tap off the aux battery to feed the house circuits is fused. Well...actually Bussman Shortstop breakers. From the aux battery through a 50a breaker, then from the 50a through a few 20a to feed a few aux circuits.

Yea for the price of a fuse I guess it's just worth having 2 fuses in the system and being safe and not having my truck burn to the ground. The fact that I've never seen factory battery cables with fuses is one of the things that got my wondering if I really needed multiple fuses in the same system.

I have a blue sea fuse panel for all my accessory stuff that I'm going to wire in too and that will be fused as will all the stuff connected to it.