Confused on fuse and cable requirement

[Razzman]

New guy trying to get some clarity, I'm seeing so much conflicting information and advice I'm confused at this point as to what is reality and what isn't. To be clear I'm no electrician but know basic 12v electrical systems.

I cleaned up the trailer factory battery wiring and added two SOK 100ah lithium batteries in parallel. Trailer came with a Zamp 10A solar controller and 200w solar panel on the roof. I'm being told I should fuse between the battery positive and busbar (because of the lithium batteries) with 1/0 cable and a 250A fuse, that the 4AWG isn't big enough.

I'm confused as it’s light loads on this and there's nothing different from the factory other than organizing and adding the batteries. Just seems excessive to me.



What I did

• Added two SOK 100ah lithium batteries in parallel (non-Bluetooth) in place of the two 6V GC2 batteries.
• Added a Victron SmartShunt.
• Added two busbars.
• Added a 30A ANL fuse for the existing external solar wall port, replaced the existing inline fuse.
• Added a 30A breaker for the roof solar to disconnect from batteries.
• Used 4AWG cable on batteries and to extend to the busbars.

1. This runs the trailer 12v systems only: heater, LED lights, 12V TV sometimes, water pump, propane fridge and propane water heater.
2. NO inverter installed (and don't plan on having one)
3. If we need DC we either plug in to 30A shore power at campground or use our 4500w generator and plug into that.
4. Batteries have never dropped below 85% overnight and charged back full by noon typically.

 

[pollenface]

Typically one would size fuse and cable to 1.25x the biggest load.

You can use an online voltage drop calculator (or phone app) to calculate voltage drop for any length of cable and size accordingly, if you're within 3% it's fine.

 

[mikefitz]

I'm being told I should fuse between the battery positive and busbar

Use a MRBF fuse/ holder direct on each battery positive, value 125 amps, 4 awg is OK.

MRBF Terminal Fuse Block - 30 to 300A - Blue Sea Systems

Easily and economically satisfies ABYC 7 circuit protection rule by mounting on a 3/8 battery post, battery switch or bus bar.

www.bluesea.com

The issue with lithium batteries is that under system fault conditions very high currents can flow, perhaps 7000 amps. The fuse type must be able to interrupt this. MRBF is rated to 10000 amps at 14v. Link fuses are only rated to 2000 amps.

Additional fuses are need in cable runs from the positive buss bar suitably rated.

There is a small cable connected to the battery positive, is this fused?

 

[Razzman]

Use a MRBF fuse/ holder direct on each battery positive, value 125 amps, 4 awg is OK.

MRBF Terminal Fuse Block - 30 to 300A - Blue Sea Systems

Easily and economically satisfies ABYC 7 circuit protection rule by mounting on a 3/8 battery post, battery switch or bus bar.

www.bluesea.com

The issue with lithium batteries is that under system fault conditions very high currents can flow, perhaps 7000 amps. The fuse type must be able to interrupt this. MRBF is rated to 10000 amps at 14v. Link fuses are only rated to 2000 amps.

Additional fuses are need in cable runs from the positive buss bar suitably rated.

There is a small cable connected to the battery positive, is this fused?

Mikefitz - Thanks for the quick reply.

Use a MRBF fuse/ holder direct on each battery positive

So I need one on both batteries? This works correctly even though they are parallel connected?

Additional fuses are need in cable runs from the positive buss bar suitably rated.

These leads all terminate at the fused 12v power center

There is a small cable connected to the battery positive, is this fused?

This is the power lead from the Victron SmartShunt and is inline fused, just can't see it behind the battery.

 

[Razzman]

Just ordered two each of the following to correct this;

• 2 - Blue Sea Systems 5191 Fuse Block Terminal 30-300 AMP
• 2 - Blue Sea Systems Terminal Fuses 125a
• Pack - M8x1.25 to M10x1.5 Double End Threaded Stud Screws (SOK terminal thread is M8 - fuse block is M10)

 

[mikefitz]

M8x1.25 to M10x1.5 Double End Threaded Stud Screws

Neat solution, I have used copper washers in the past.

So I need one on both batteries?

Although very unlikely, with a single fuse one battery could fail and current from the 'good' battery will flow into the 'bad' battery.

 

[Razzman]

Neat solution, I have used copper washers in the past.

Although very unlikely, with a single fuse one battery could fail and current from the 'good' battery will flow into the 'bad' battery.

Thanks for all your help mikefitz!