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diy solar

Battery fuses & fusing to inverter

KA1J

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Joined
Oct 21, 2022
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I have 4 LiFePo4 100Ah batteries which for the moment will be run parallel. I have a MRBF terminal fuse block for each battery. Fuses are rated at 58V 100A for each terminal. This inverter is a Samlex PST-2000-12 rated at 2000W and with a surge of 3,500W. The battery cables are 5' long & 2/0 AWG. This system will be occasionally moved between the house and the camper for vacation trips so it will require some disassembly.

My question is about fusing: I think the fuses for the individual batteries are proper but I wonder about the fuse to the 2/0 cable going to the inverter. I think that needs to be a higher value. It seems to me it should be able to handle the surge which would require around 300A at 12V. So for me guessing, I need the 100A for the three, and a 300A fuse for the one going to the inverter. Am I guessing right?

A second question; I'll want to series these in a different application with a 5000W 48V inverter. I think the 100A inter-battery fuses need to be the same, but the fuse to that inverter Assuming a surge of 8000W would need to be 8000/48 = 166A, or rounded up the the next higher fuse?

Thanks!
 
MRBF fuses are generally not regarded as adequate for LFP batteries due to their lower AIC (Interrupt capacity). The lower resistance of LFP means their short circuit current can exceed a fuses rating. Some MRBF have a 10,000A AIC @ 14V, and that's probably sufficient, but I would confirm the fuses rating to make certain.

The 100A fuses should be 125A fuses.

Surge is not considered for wiring/fusing. The duration of surges are generally short enough to ensure that wires/fuses for max continuous rating are sufficient.

Fuses protect wires, so you are on the right track. You should have a fuse 1.25X higher than the rating of the 2/0 wire, preferably a Class T

When you go series for 48V, there is no point in having a fuse per battery. You should have a single Class T fuse rated 1.25X the wire rating between the battery and the inverter.
 
Sunshine, Thank you. These fuses indeed are rated for 10,000 AIC. I just placed an order for 125A MRBF Blue Sea fuses.

Now I'm trying to figure out what value I need to fuse with the 2/0 cable. the info I have is: Windy Nation Battery and welding cable: Ultra-flexible, SAE J1127 approved, PURE copper cable manufactured and annealed to ASTM standards. Rated up to 600 volts, operating temperature: -58°F to +221°F, flexible, solvent repellent, abrasion resistant EPDM insulation. I see no chart for fuse size.

I found the Blue Sea Class T fuse block rated to 320A and will get that, I'm just not sure what fusing with this cable to the inverter is proper at this point. I do have a 300A ANL fuse & holder. I believe the T is a better choice of fuse?

Thoughts?
 
SAE J1127 is rated for 330A outside of engine space, 280A inside of engine space.

Fuse is recommended at 1.25X wire rating. You can always use less if you know your equipment can never achieve the rated current.

2000W / 12.8V / .85 = 184A. 184A * 1.25 = 230A (round up to the nearest common fuse size).

A 250A (based on load) to 400A (based on 330A rating) fuse on the 2/0 wire would work.
 
Thank you for the suggestion. 250A should be easy to get and I'll be happy to get on with this. I have enjoyed learning how to safely bring emergency solar backup into the home & to also sufficiently power an inverter so as to not need my Honda 2000i when camping (which should never be used in the AM with sleeping tenters nearby). This brings me safely one step closer.
 

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