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Parallel batteries and terminal fuse (MRBF fuse) size

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benoit.letourneau@panomed

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Nov 12, 2022
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Good Morning,

Happy new year!!!

I'm attaching 2 batteries in parallel (SOK 206ah).

I would like to put a fuse on each battery terminal. I want the bank to be fused at 250Amp. I understand I could put a 250amp fuse at the positive terminal where the wire is exiting towards my kill switch/busbar, etc (let's call that battery B). But If I want to put one on each battery, do I put a 125amp on battery A and a 250 on battery B or 125 on each? If the bank is outputting 250, each battery will be outputting 125amp. I'm thinking 125 on A and 250 on B (since it is the output of the whole bank at that point)

Not sure if my question is clear enough?

Thank you
 
I hve nearly that same build. I put a 125 amp fuse at each battery and then a 250 amp fuse after combined.
1672590657826.png
 
benoit.letourneau@panomed said:
Thank you! 125 at the first battery terminal and 250 at the second battery terminal (equivalent to you combined).
Click to expand...
Not quite. 125 amps at each battery terminal, so two so far, and 225 amps after combined. That is a conservative setup. Most would not put three fuses in for two batteries.
 
benoit.letourneau@panomed said:
Good Morning,

Happy new year!!!

I'm attaching 2 batteries in parallel (SOK 206ah).

I would like to put a fuse on each battery terminal. I want the bank to be fused at 250Amp. I understand I could put a 250amp fuse at the positive terminal where the wire is exiting towards my kill switch/busbar, etc (let's call that battery B). But If I want to put one on each battery, do I put a 125amp on battery A and a 250 on battery B or 125 on each? If the bank is outputting 250, each battery will be outputting 125amp. I'm thinking 125 on A and 250 on B (since it is the output of the whole bank at that point)

Not sure if my question is clear enough?

Thank you
Click to expand...
Depends on how much current you will draw and on the wire sizes.
The battery fuse protects the wire and the battery.
For LFP cells I wouldn't plan on pulling more than .5c continous from a battery.
The I provision the wire for 1c.
Then I fuse for the wire.
example:
100 amp hour battery.
.5c = 50 amps
1c = 100 amps
pure copper 6 awg wire with insulation rated for 90C or better is rated for 100 amps in free air.
So I would use a 100 amp mrbf fuse as close as possible to the battery terminal.
If you have 3 of these batteries in parralel you can expect to pull about 150 amps continuous from the aggregate without issue.
 
Maximum continuous current for the SOK battery is 100 amps so suggest fuse for each at 125 amps. MRBF is OK for 12 volts with a 10.000 amp withstand capacity.
No further main battery fuse is requires after the parallel connection.
Within the system further suitable fuses or breakers will be needed for feeds to system loads and chargers.
 
mikefitz said:
Maximum continuous current for the SOK battery is 100 amps so suggest fuse for each at 125 amps. MRBF is OK for 12 volts with a 10.000 amp withstand capacity.
No further main battery fuse is requires after the parallel connection.
Within the system further suitable fuses or breakers will be needed for feeds to system loads and chargers.
Click to expand...

Perfect, I put a 125 on battery A and a 125 on battery B. If I pull (for the sake of discussion) 200 from the bank. I know the 200 will be split between the two batteries (so 100 each). The total of the two will be 200 but each will provide 100 amps each. So my question is this: what current will flow through the last fuse (100 or 200?)?

"Within the system further suitable fuses or breakers will be needed for feeds to system loads and chargers."

Yes... Of course and thank you... My question is really about the two terminal fuse.
 
Assuming both batteries in parallel, If you put a clamp amp meter on the positive cable out of battery 1 it reads 100 amps. When you go to battery 2 it reads 100 amps. If you go to the point after combined it reads 200 amps.
 
Ok based on the answers above, I can do 125 for each battery, or 125 for each, and 250 for the combined.
here's a drawing with two options for wiring the Fuse Terminals. In drawing A, the second fuse is combined as current from both batteries is flowing through the fuse. In drawing B I have each 125 amp as current from each respective battery if flowing through the fuse. Which one is best or which one do you prefer?

1672685178938.png
 
Last edited:
John Frum said:
Is the leftside terminal on a battery?
What is the awg of the wire between left and right?
Is the rightside terminal on a busbar?
Click to expand...
Both fuse blocks are on their respective batteries (SOK 206ah)... I should have mentioned it... Sorry. Then the cable exits toward a Kill Switch and Busbar. All wires are 2/0. I changed the image
 
benoit.letourneau@panomed said:
Yes... Both fuse block are on their respective batteries... I should have mentioned it... Sorry. Then the cable exits toward a Kill Switch and Busbar. The wires are 2/0.
Click to expand...
Each battery should be peered to a busbar.
The battery circuit is fused at the postive battery terminal.
The branch circuit from the busbar to the inverter is fused at the busbar.
This is the classic feeder/branch design pattern.

This goes on the battery terminal.

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 www.bluesea.com

It can also go on a busbar like this in order to fuse branch circuits.

PowerBar 1000 - 8 3/8" Terminal Studs - Blue Sea Systems

Complex wiring systems require a single point to consolidate largeand small conductors. The PowerBar 1000 offers a 1,000 Ampbusbar with various size studs and screws to connect conductors andfuse blocks.
www.bluesea.com www.bluesea.com

Only the positive leg of a circuit is fused.
 
I prefer what I put in supposed two. Three fuses for two batteries. If not then, one fuse 125 amps on both batteries.
 
benoit.letourneau@panomed said:
Can you explain why a busbar is better? I'm using a busbar for hooking up all my other circuits but why would using a busbar for attaching my batteries be better than attaching them together? Thanks
Click to expand...
The big learning point for me was the shear size of the 2/0 lugs and how much space and wire they take up. Not impossible to stack them, but fitting all those wires together on 1 stud in a space constrained area is more difficult than I thought. Some bus bars are rated for mulitple attachments, others not. Most busbars say nothing about multiple attachments to 1 lug. Also better fitting and less chance of corrosion on a busbar.
 
benoit.letourneau@panomed said:
Can you explain why a busbar is better? I'm using a busbar for hooking up all my other circuits but why would using a busbar for attaching my batteries be better than attaching them together? Thanks
Click to expand...
Allows for equal distribution of charge and discharge.
Easier serviceability, if you need to disconnect a battery.
Less physical strain on the battery terminals.
No stacked lugs, for better conductivity.
Smaller battery conductor size.
 
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