Class T fuse + MRBF on individual parallel batteries (x4)?

[SailSolar]

I am upgrading our sailboats housebank from lead-acid to LiFePo4.

New batteries will be 4 x drop in Litime 100ah, group 24,100a BMS, bluetooth versions.

I understand a Class-T fuse is required. I'll be upgrading our old ANL version to a Blue Sea Class T fuse (300amp) in their Blue Sea fuse holder. This is positioned after the batteries are combined in parallel.

My question is regarding before the batteries are in parallel - for the fuse on each individual battery. I have heard a rumor that an MRBF is sufficient for individual LFP batteries in this circumstance. Any insight here?

My thought is to mount 2 x Blue Sea MRBF dual holders (2151) direcly on the Blue Sea Class-T Fuse holder (5502100). This will be mounted immediately next to the batteries. MRBF fuses will be 100 amp. Wires from batteries will be sized greater than 100amp. Wiring from the class-T fuse to a nearby disconnect battery switch then to postive bus bar will be 2/0 (sized greater than 300amp for it's total length to and from main busbars).
The negative side will be similar, but instead of the Class-t fuse, i will have an additional bus bar here, positioned with the same length wires from the batteries.

Essentially I'm utilize the Class-T fuse holder as a positive battery bank bus bar. My thoughts are this will be better than MRBF (3/8" mounting hole), mounted directly on 5/16" battery terminals.

Any thoughts on this plan?

Thanks!

 

[mikefitz]

My thought is to mount 2 x Blue Sea MRBF dual holders (2151)

Suggest a 5191 fuse holder direct on each battery post with a suitable copper washer, and MRBF 100A. This will reduce to zero length unprotected cables. Unless sailboat regs enquire class T fuse after main buss bar you could use ANL.
Your plan is fine electrically but exposes unprotected lengths of unprotected cable.

 

[SailSolar]

Suggest a 5191 fuse holder direct on each battery post with a suitable copper washer, and MRBF 100A. This will reduce to zero length unprotected cables. Unless sailboat regs enquire class T fuse after main buss bar you could use ANL.
Your plan is fine electrically but exposes unprotected lengths of unprotected cable.

Thanks Mike! I was wondering if a copper washer would negate the negatives of a 3/8" 5191 on a 5/16" battery terminal. I'd seen a response from Blue Sea suggesting that the difference in size is not a concern, so adding additional surface contact could only be an improvement I'd imagine. Would you recommend putting the appropriately sized copper washer on the top of the 5191 or on the bottom?

I agree regarding the unprotected lengths of cable, as ABYC requires fusing to be within 7" of the battery terminal, unless you cover the cable in protective conduit/loom/sheathing (I believe), which was my plan if I went that route.

One of the benefits of doing the 2x dual MRBF's was that I could also eliminate the cost of an extra 300a busbar, but it's not the end of the world. The additional benefit of the properly sized stud was the main reason.

Thanks,

 

[robbob2112]

I have used a pure copper wire on top of the MRBF fuse with the lug on top of that. It acts like a bus bar and the dc current passes straight through it. The better choice is to just get wide lugs for the connection to the MRBF.

8 AWG, 3/8 Stud, (Wide Pad) Tinned Copper Battery Cable Ends, Wire Lugs

SELTERM Marine Grade Wire Lugs are made from 99.9% pure annealed tinned copper, providing a high degree of conductivity for maximum current flow. This particular variant comes with a wider pad offering a larger clamping surface and is generally suitable for use with 8 Gauge Battery Cable & 3/8"...

selterm.com

 

[mikefitz]

or on the bottom

Washer on battery terminal below fuse holder metal.

 

[robbob2112]

Like this -

listed in order from top to bottom

• Nut
• Steel lock washer
• Steel flat washer
• Smaller loops for inverter and battery voltage meter
• Large lug
• Pure copper washer - cleaned and polished
• Fuse
• Fuse holder
• battery post

no-ox-id-special on all mating surfaces - note - this is just a temp setup to capacity test the battery - I'll make a real set of cables with heat shrink and the like for when I do it in place.

 

[woundedpig]

Like this -

listed in order from top to bottom

• Nut
• Steel lock washer
• Steel flat washer
• Smaller loops for inverter and battery voltage meter
• Large lug
• Pure copper washer - cleaned and polished
• Fuse
• Fuse holder
• battery post

no-ox-id-special on all mating surfaces - note - this is just a temp setup to capacity test the battery - I'll make a real set of cables with heat shrink and the like for when I do it in place.


View attachment 263870

View attachment 263874

Where did you find washers of this size/thickness/width?

Thanks

dD

 

[wpns]

Where did you find washers of this size/thickness/width?

Thanks

dD

McMaster-Carr has everything you can imagine, I just bought
Copper Washer, for 5/16" Screw Size, 0.344" ID, 0.75" OD

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

 

[robbob2112]

Where did you find washers of this size/thickness/width?

Thanks

dD

Amazon.com