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Fusing Aux battery cable


reb78

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I fitted an auxilliary battery last year in the back of the 110 to power accessories like the National Luna fridge and Eberspacher when camping. I used two old jump leads that the clamps had broken on to connect the batteries via a T-max split charge. Messing about today, I thought... this positive lead really ought to be fused. The biggest draw on it would be if I ever use the aux to jump the car if the main battery is ever flat. 

What size fuse should I be looking at using and what connections to connect it up in (presumably some sort of inline holder)?

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I like these:

https://www.vehiclewiringproducts.co.uk/p-355-strip-midi-fuse-holder

Or, for expandability, these:

https://www.vehiclewiringproducts.co.uk/p-357-midi-mega-fuse-holder

I don't think any fuse worth fitting is going to stand up to a jump-start so I'd always do it from the un-fused side. 50-70A should be plenty for aux stuff, you want it to blow before the wire melts.

If you want one of #1 and a fuse to go in it, PM me your address I've got surplus from my re-wire.

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The problem with fusing cables carrying the kind of current required to crank a cold 2.5 litre diesel is that the total resistance of the fuse and the connectors used to terminate the wires connecting to it is greater than that of the cable itself, and since it's only operating at a nominal 12V that makes the voltage loss across the fuse significant. 

A better solution is to ensure the cables are mechanically protected, including using good quality rubberised cable (e.g. welding cable) with good quality crimps at each end, protected by self-adhesive heat shrink  to support the cables in the crimps, and extra mechanical protection where the cable might rub. I use a short length of radiator hose split along its length and then cable tied back over the cable on any rub points.

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On 12/20/2020 at 11:39 AM, nickwilliams said:

The problem with fusing cables carrying the kind of current required to crank a cold 2.5 litre diesel is that the total resistance of the fuse and the connectors used to terminate the wires connecting to it is greater than that of the cable itself, and since it's only operating at a nominal 12V that makes the voltage loss across the fuse significant. 

A better solution is to ensure the cables are mechanically protected, including using good quality rubberised cable (e.g. welding cable) with good quality crimps at each end, protected by self-adhesive heat shrink  to support the cables in the crimps, and extra mechanical protection where the cable might rub. I use a short length of radiator hose split along its length and then cable tied back over the cable on any rub points.

So its all setup like this at the moment - the cables are protected in conduit as they run to the battery in the rear and pass through proper glands where they go through the bodywork. I bought a nice crimper to do the battery connections.

On 12/19/2020 at 8:00 PM, FridgeFreezer said:

I like these:

https://www.vehiclewiringproducts.co.uk/p-355-strip-midi-fuse-holder

Or, for expandability, these:

https://www.vehiclewiringproducts.co.uk/p-357-midi-mega-fuse-holder

I don't think any fuse worth fitting is going to stand up to a jump-start so I'd always do it from the un-fused side. 50-70A should be plenty for aux stuff, you want it to blow before the wire melts.

If you want one of #1 and a fuse to go in it, PM me your address I've got surplus from my re-wire.

Thanks for that Fridge I will drop you a PM - (just tried and it says you cant receive PMs?)

On 12/19/2020 at 10:27 PM, TSD said:

A little more thought would lead to the conclusion it should be fused at both ends?

The other end is probably ok as its a very short run from the relay to the aux battery. I have run out of space in the battery box to install other bits neatly so the relay went in the rear next to the aux battery and the Tmax controller is mounted back there too.

Mikes mention of the amps when jump starting did raise another question though - I guess the relay itself might not be up to it... its a 250A relay I beleive?

Edited by reb78
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14 minutes ago, Bowie69 said:

250A is plenty for cranking, that is a continuous rating, so for 5 secs it could well be 1000A rated.

Thanks. So I wonder if a bypass to the fuse could be fitted like an isolator that is usually left disconnected other than in the need of a jump start?

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2 hours ago, reb78 said:

Thanks for that Fridge I will drop you a PM - (just tried and it says you cant receive PMs?)

The other end is probably ok as its a very short run from the relay to the aux battery. I have run out of space in the battery box to install other bits neatly so the relay went in the rear next to the aux battery and the Tmax controller is mounted back there too.

Mikes mention of the amps when jump starting did raise another question though - I guess the relay itself might not be up to it... its a 250A relay I beleive?

Looks like my inbox is full :( I'll try and empty it a bit and/or drop you a PM with my email.

TSD's point about fusing both ends is that the entire cable is connected to one or both batteries most of the time, so it can feed power to a fault from either end of the circuit. if the fuse on the AUX end blows you've still got the main battery feeding amps in t'other end.

Cheap (and moderately expensive) battery switches/relays don't enjoy starting current, even the 250A rated ones. In all honesty I'd happily compromise occasional jump-start convenience for electrical safety - after all, you shouldn't need to jump-start your truck unless there's a problem.

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