House battery fuse?

[wamcneil]

Hi all,

The 2003 Dynasty wiring diagrams show a 300A fuse in the house battery cable after the battery switch (see attachment).

But I'm looking up into the cavity behind my battery box and I'm not seeing a fuse there. The positive battery cable runs from the batteries, through a bulkhead post, up to the house battery switch and then up over the frame rail and out of sight.

Do any of y'all know if I should have a 300A fuse somewhere?

Thanks

Walter

Edited January 20, 2020 by wamcneil

[Highwayman]

If you follow the red cable from the shut off switch the fuse is mounted to the bulkhead before it goes to the inverter.

[wamcneil]

Thanks. Mine is different though.

I tracked it down. There's no actual fuse. The 4/0 positive cable runs from the battery switch to the high current run bay in the engine compartment. From the battery boost solenoid, it goes through two parallel 150A circuit breakers. And from the circuit breakers to the inverter (see attachment).

So there's no fuse protection between the batteries and the boost solenoid, but the inverter cables are protected by circuit breakers.

[Jim McGarvie]

My 2002 Knight has a Littelfuse JLLN 250 250A, 300VAC fuse between the inverter/charger and batteries, and it occurs to me it might be good to have a spare. So far the one I've found is on Amazon for $91 (https://www.amazon.com/Littelfuse-300VAC-125VDC-Class-Acting/dp/B00JV3GTIO/ref=sr_1_1?dchild=1&keywords=Littelfuse+JLLN+250A+300vac&qid=1601387634&s=industrial&sr=1-1)! Is that what they really cost? It still might be good to have a spare; it could save a trip. Or might it be better to replace it with a circuit breaker?

Thanks.

Jim

2020-09-28 11.02.44.heic

[wamcneil]

Personally, I wouldn't bother to carry a spare. If you blow that high-current fuse, there's something bad wrong. I would suspect a dead short in the cable or maybe the inverter is destroyed and unsalvageable. If I found the fuse blown, I wouldn't just replace it with a spare and hope for the best like a normal automotive fuse. If your inverter is capable of drawing more than 250A you need a bigger fuse and probably bigger cables to match.

In my setup, the inverter cable goes from the battery, back to the run bay, through half a dozen connections to the parallel circuit breakers and then to the inverter. In my opinion a fuse might be better than a circuit breaker in this case and I'm in the process re-wiring the inverter cables. I've purchased a 300A high current fuse to go between the battery switch and inverter, eliminating a lot of cable, connections and resistance.

I don't have any data to support this, but I suspect that a high current fuse has less resistance than an equivalent circuit breaker.

Cheers,

Walter

Edited September 29, 2020 by wamcneil

[Jim McGarvie]

3 minutes ago, wamcneil said:

Personally, I wouldn't bother to carry a spare. If you blow that high-current fuse, there's something bad wrong. I would suspect a dead short in the cable or maybe the inverter is destroyed and unsalvageable. If I found the fuse blown, I wouldn't just replace it with a spare and hope for the best like a normal automotive fuse. If your inverter is capable of drawing more than 250A you need a bigger fuse and probably bigger cables to match.

Good point, Walter. I agree.

Jim

[rcl285]

Here’s a photo of the rear electrical bay of my ‘04 Imperial which is a contemporary of your coach.  The fuse is clearly visible

 

dick L. ‘04 Imperial

Edited September 30, 2020 by rcl285
Add photo

[Harvey Babb]

Former vehicle electrical system designer here. The fuse is most likely there to protect the inverter from catching fire if one or more transistors fail in a shorted condition. That is a fairly common failure mode, and the circuitry around the transistors will not stand up to the current if it happens.

 

[wamcneil]

deleted

Edited September 30, 2020 by wamcneil

[Guest Ray Davis]

1 hour ago, rcl285 said:

Here’s a photo of the rear electrical bay of my ‘04 Imperial which is a contemporary of your coach.  The fuse is clearly visible

 

dick L. ‘04 Imperial

 Dick ,  that looks exactly like my 03 Windsor elect bay .

[waterskier_1]

On 9/30/2020 at 10:25 AM, Harvey Babb said:

Former vehicle electrical system designer here. The fuse is most likely there to protect the inverter from catching fire if one or more transistors fail in a shorted condition. That is a fairly common failure mode, and the circuitry around the transistors will not stand up to the current if it happens.

 

That fuse is the too disconnect the batteries in case the is a significant current draw.  We are talking 300 Amps, much more than a shorted transistor would draw (it would melt and open circuit long before the 300 Amp fuse or circuit breaker would react.  The most likely fault would be shorted wires to the chassis.  The inverter-charger has a 30 Amp a couple circuit breaker on the input, and additional breakers on the output.  Those are there to protect the inverter.

 

[Harvey Babb]

4 hours ago, waterskier_1 said:

That fuse is the too disconnect the batteries in case the is a significant current draw.  We are talking 300 Amps, much more than a shorted transistor would draw (it would melt and open circuit long before the 300 Amp fuse or circuit breaker would react.  The most likely fault would be shorted wires to the chassis.  The inverter-charger has a 30 Amp a couple circuit breaker on the input, and additional breakers on the output.  Those are there to protect the inverter.

 

Really? Then please explain why on my 2000 Endeavour the fuse is ONLY protecting the short cable going to the inverter while the cable going all the way to the front distribution panel and the generator has NONE?

The 30 amp breaker on the inverter is on the AC side at 120 volts, which is coincidentally the same wattage as 300 amps at 12  volts.

 

[waterskier_1]

1 hour ago, Harvey Babb said:

Really? Then please explain why on my 2000 Endeavour the fuse is ONLY protecting the short cable going to the inverter while the cable going all the way to the front distribution panel and the generator has NONE?

The 30 amp breaker on the inverter is on the AC side at 120 volts, which is coincidentally the same wattage as 300 amps at 12  volts.

 

I just looked at the 2000 Diplomat wiring diagram.  I think this should be representative of how your coach was originally wired.  The 300 Amp Fuse you are talking about is in series with inverter and the battery circuit.  It would protect the batteries and cables between the inverter and the batteries in the case that the inverter tried to draw more than 300 Amps (in the Inverting mode) , would could damage the batteries.  Additionally it will protect the charger and cables between the inverter/charger and the batteries in the case that the batteries shorted out or cable to shorted out when the charger was attempting to charge the batteries.  In any case, I doubt they put it there to protect the inverter from itself.

 

 

Edited October 2, 2020 by waterskier_1
Correction