Fuse between PWM & battery?

Walnuts

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Mar 26, 2022
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Hey guys...So I am trying to build my first small solar system using the following components:

2x 100W Renogy Monocrystalline
1x Krieger 1100W Inverter
1x Renogy Wanderer 30A solar charge controller

I also have two 12v 20ah LiFePO4 batteries on the way.

My question is....it seems like I need a fuse between the the solar charge controller and the battery. I can't seem to wrap my head around how to get that installed. There are 30A inline fuses that are sold at auto parts stores that have 4" of 12 gauge wire on either side. Do I need to just buy more 12 gauge wire and install the 4" with the fuse close to the battery with a crimp and then combine the other side with more 12 gauge wire so it will reach the charge controller, and use more of that wire for the negative on the battery to the charge controller? I've also read that I don't even need a fuse here.

Besides that, I think I am almost ready to go as soon as the batteries arrive!
 

Browneye

Dr. WattSon
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For a 30A SCC a 40A fuse is correct - it should be 125% of your max current, rounded up. Connect wire 10awg minimum, 8awg recommended.
I used a switching breaker, but an inline fuse is also appropriate, ANL, mega, etc. A switching breaker provides for easy disconnect for maintenance or servicing. Same for the panels - a mini DC two-pole breaker is really handy, unless you can easily access the panel connectors. In m case they're on the roof of the RV.

I have a Tracer 30A...

 

Zil

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We must fuse at the battery positive with a fuse to protect the gauge of the wire. The wire's ampacity.
With (USA) DC we do not fuse the negative.
With any LiFePo battery I would not use an inline fuse from the auto-supply store. They simply do not have enough interrupt capacity for a LFP battery. Even 40 amp hour.
You need to use a voltage drop chart to select the awg needed.
You will need at least 12vdc 100 ampere for the inverter. If using marine grade cable, that would be 6 awg. Larger if longer distance. I fuse for 'next size up' 125 amperes. The CC would be ok with 12 awg, but I would upgrade to 10 awg. Fused at 60 ampere.
All the fuses need be at the battery end of the circuit. We often use BusBars to connect many circuits to a battery terminal. But i have come to these as a combined unit. https://www.bluesea.com/products/5196/MRBF_Surface_Mount_Fuse_Block_-_Common_Source
Keep the battery to fuse block cable short, very short, and you will not need a terminal fuse.
 

Browneye

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You'll likely want some kind of bus bar to connect everything - battery to bus, everything else to bus. Even a four-post helps sort things out a lot.
I also like a master disconnect - like the blue sea ones, so you can just switch the battery off.

1100w inverter should use the 150A anl, and 4awg wire. Welding wire or battery wire is easy to work with, you need a crimper and lugs have cables made up.

20A batteries are not big enough IMO. 1100W at 120VAC is easily 120A at ~12V. Your battery would only run the full inverter load for a half hour or so. 40A X 13V is about 500 watts of battery capacity without even considering losses and not discharging 100%. .

What are your loads, usage?
 

brb58

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The solar controller needs to hook up to the battery on the battery side of the disconnect switch...not to a bus bar. You never want the solar controller connected to the loads of the RV with the battery turned off.
 

brb58

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Uh, no. Why you need a PV disconnect.
SCC needs a battery to operate.
And that is why you connect the solar controller output between the battery disconnect and the battery itself so you can have the battery disconnect off and charge the battery. That also eliminates the possibility of forgetting to do the PV disconnect and putting possibly high voltages to the RV itself.
 

Zil

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All my dc circuits use the Positive BusBar and are fused at that BusBar. My battery disconnect is all the loads. The battery disconnect is on the battery negative and is only for planned maintenance. Loads are shut down for planned maintenance. Including the CC.
True, the CC doesn't like to be connected to the panels without the battery. There is a DPST next to the battery SPST that disconnects the panels and the battery from the CC. I would be a pretty stupid maintenance department to mess-up Disconnect 101.
A Curious Rodent Inspired Short Circuit once "disconnected" the battery positive. The CC actually shut down. The joy?, my relief!, of having a quality CC from Morningstar. I had to disconnect the panels and connect the battery first to get the system back on-line.
 

Levo

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Uh, no. Why you need a PV disconnect.
SCC needs a battery to operate.
Chris, when I threw the main battery disconnect to work on my setup, the SCC remained energised from the PV and I still had 13.6v across the buss bars. Then I remembered from the manual, "never connect the PV unless a battery is connected". By disconnecting the battery first I had broken that "rule" (in reverse). I had to pull the PV positive wire out to kill the system and reset the SCC. Don't you think it makes more sense to have a switch or breaker so as to do things in the right order rather than pulling the wire each time?
 
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Browneye

Dr. WattSon
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Chris, when I threw the main battery disconnect to work on my setup, the SCC remained energised from the PV and I still had 13.6v across the buss bars. Then I remembered from the manual, "never connect the PV unless a battery is connected". By disconnecting the battery first I had broken that "rule" (in reverse). I had to pull the PV positive wire out to kill the system and reset the SCC. Don't you think it makes more sense to have a switch or breaker so as to do things in the right order rather than pulling the wire each time?
Yes, of course. That's why I used a switching breaker to the battery, and a DC breaker as a double-pole switch for the PV input. See pic above.
After my install someone pointed out my switching breaker was too big, and it was a crappy one, so it got a T Tocas 40, and even those aren't all that great. For a 30A controller is likely okay. It doesn't get switched off with any regularity anyway. If you're going to install one of these get a Eaton Bussman one - yeah, they're twice as much, but much better internals.

The double-pole mini breaker works great as a PV disconnect. It needs to be DC rated, and amperage isn't critical as it isn't used for current limiting, but just as a switch.

This is a good vid on this subject:

 
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