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Old 10-07-2018, 14:56   #1
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Battery Cable Fusing

To all the ABYC expert's here,


How do you fuse battery interconnection cables between batteries that are in two locations?


Here is more details.
I have 6ea 12v batteries that comprise the house bank.
All are connected in parallel but three are located in the Port battery box. And the other three in a starboard box.


The pos and neg cables that connect the two boxes are 48" long each.
This pos cable runs in it's own PVC conduit, the entire length of the cable.
Also the pos feed is from the port box. While the neg feed is from the starboard box.


My question is how do you fuse the 48" long interconnect cable?


ABYC states that only start cables can be fuse free. All others are fused with special length exceptions.
So how do you fuse while keeping the battery resistance balanced?
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Old 10-07-2018, 16:22   #2
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Re: Battery Cable Fusing

From your description, note sure of your layout, but in general I would fuse each three battery bank at the battery positive / 48" lead with a fuse rated a 1/2 of the amperage of the "weakest link" wire. I use these for such purpose:

https://www.defender.com/product3.js...0030&id=983740

https://www.defender.com/product3.js...0030&id=983740
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Old 10-07-2018, 16:27   #3
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Re: Battery Cable Fusing

I prefer Class T or ANL style if you can rig a solid safe mount point very close to the post

With those MRBF, make sure there are no stresses bearing down diagonally or sideways on the posts themselves, case cracking can result.
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Old 10-07-2018, 16:37   #4
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Re: Battery Cable Fusing

Quote:
Originally Posted by Jeanneau 45.2 View Post
From your description, note sure of your layout, but in general I would fuse each three battery bank at the battery positive / 48" lead with a fuse rated a 1/2 of the amperage of the "weakest link" wire. I use these for such purpose:

https://www.defender.com/product3.js...0030&id=983740

https://www.defender.com/product3.js...0030&id=983740



Hi Jeanneau 45.2,


The problem I have with your method (two fuses, one at each end of the pos 48" interconnect) is the resistance created by the fuses.


This added resistance will cause the three batteries on the Pos feed side to accept more charge and discharge deeper than the other three.
Causing the two sides to become out of balance, sort of speak.



The only way I can see to eliminate this condition is to also install two fuses on the 48" neg interconnect.

Which of course is nonsense, as far as protection.


Currently right or wrong, I'm leaning towards "NO" protection on the interconnects, beings this cable runs in its own PVC conduit.


BTW, we are talking about 4/0 cables and proper protection on the pos feed cable.
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Old 10-07-2018, 16:42   #5
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Re: Battery Cable Fusing

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Originally Posted by john61ct View Post
I prefer Class T or ANL style if you can rig a solid safe mount point very close to the post

With those MRBF, make sure there are no stresses bearing down diagonally or sideways on the posts themselves, case cracking can result.

Hi John,
The main battery fuse is a 350A ANL. But I do use MRBF's off of the main busses.
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Old 10-07-2018, 16:47   #6
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Re: Battery Cable Fusing

Yes they're fine, as long as you (and others later) inspect to confirm the above provisos.

And of course batts well fixed in place even if everything goes sideways.
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Old 10-07-2018, 17:38   #7
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Re: Battery Cable Fusing

Not sure exactly how your wiring is configured with 3 and 3. I have 6 GC2s, 4 in one location and 2 in the bow. The connecting tie cable (#2 AWG) is just fused on both ends (150A ANLs).

My experience is all this business about bank wiring must be reverse return connected and perfectly balanced is much ado about nothing. I tap the fwd bank directly for the windlass and bow thruster. The BT pulls about 200 amps from the fwd bank and about 100 amps from the aft bank. Sure for some period of time the system is unbalanced, but if I check all three banks after a little thrusting they are voltage equalized in less than 30 minutes. Typically get 5 or 6 years out of a set of Sam's best batteries.
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Old 10-07-2018, 17:55   #8
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Re: Battery Cable Fusing

You'd get more otherwise, but NBD for you, long as everything works and is safe.

Some try a split bank with batts costing many thousands, and longevity then becomes more important.
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Old 10-07-2018, 19:49   #9
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Re: Battery Cable Fusing

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Originally Posted by Frankly View Post
My experience is all this business about bank wiring must be reverse return connected and perfectly balanced is much ado about nothing. I tap the fwd bank directly for the windlass and bow thruster. The BT pulls about 200 amps from the fwd bank and about 100 amps from the aft bank. Sure for some period of time the system is unbalanced, but if I check all three banks after a little thrusting they are voltage equalized in less than 30 minutes. Typically get 5 or 6 years out of a set of Sam's best batteries.

Hi Frank,
In industrial energy storage facilities, battery imbalance is a very big deal. Great pains and expense are taken to maintain the balance between the battery groups.
But here we are talking about 10’s of thousands of Ahr’s worth of capacity.

Until a few years ago, I did not know this was also the case in a marine installation where a 1000Ahr bank is considered large. My first exposure was when I read about Ben Ellison’s, FireFly install over on Panbo. Supplemental info has came from Maine Sail and Smartgauge’s websites. Also both Trojan and Lifeline have “white papers” on the subject.

In a nut shell, all are saying the best connection method is to have the feeds on opposite ends of the bank.

Now in all fairness, I kinda agree with you, on our small banks, does it really make a noticeable difference. I don’t know. But I did connect my split bank that way. Hence why I asked about the fusing issue with the long interconnect. At MSRP my FireFly’s cost almost $3k. If there is any thing I can do to help promote good battery health, I’m going to do it.
Even if it means not fusing the interconnect. But there again, it is totaly protected in not only the PVC tube but also in the two battery compartments. So the chance of it shorting out is very very small.
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Old 10-07-2018, 20:01   #10
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Re: Battery Cable Fusing

How does the pvc tube help...is it flame retardent?
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Old 11-07-2018, 03:56   #11
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Re: Battery Cable Fusing

Not using OCPD is not an option for a compliant installation.

Even if you are measuring each of your conductors to extremely close tolerance and then crimping the ring terminals with a crimp tool that provides the same crimping force for each crimp than the resistance of each circuit is already different to a small degree.

Let's try this: If you use the same MRBF fuse holder and fuses at each of your divided banks and torque the connecting conductors to the same torque, the resistance that the fuses add will be equal.

And BTW, the MRBF that was shown in the Defender link in Post #2 is the first generation type and has been been replaced for about six years.
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Old 11-07-2018, 04:46   #12
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Re: Battery Cable Fusing

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Originally Posted by er9 View Post
How does the pvc tube help...is it flame retardent?
Even though the PVC tube is the gray electrical conduit used in commercial applications...it is still plastic!
So it still melts and burns.

It was installed mostly for abrasion protection.
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Old 11-07-2018, 04:48   #13
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Re: Battery Cable Fusing

Quote:
How does the pvc tube help...is it flame retardent?

PVC is inherently flame retardant, even the white plumbing grade but the grey PVC conduit is also sunlight stable and usually thicker wall. Note, most plastics will burn if exposed to enough heat, in the industry there are several levels of flame retardantcy. BTW, most wiring jacket is PVC for this same reason.
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Old 11-07-2018, 05:05   #14
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Re: Battery Cable Fusing

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Originally Posted by CharlieJ View Post
Not using OCPD is not an option for a compliant installation.

Even if you are measuring each of your conductors to extremely close tolerance and then crimping the ring terminals with a crimp tool that provides the same crimping force for each crimp than the resistance of each circuit is already different to a small degree.

Let's try this: If you use the same MRBF fuse holder and fuses at each of your divided banks and torque the connecting conductors to the same torque, the resistance that the fuses add will be equal.

And BTW, the MRBF that was shown in the Defender link in Post #2 is the first generation type and has been been replaced for about six years.
Charlie, of course you are 1000% correct about OCPD and compliancy. Even the rule exceptions only lengthen the distance to the source.
And I do agree with the rule.

But the original question is not being fully answered. Nor have I seen it addressed in any of the ABYC rules that I have seen.

First off, I and others may just be making a mountain out of a mole hill with all of this "balance battery" talk. So this may just be a solution looking for a problem.

But anytime you have a parallel circuit with a resistance in the middle, the electricity will take the path of least resistance. Meaning half of the circuit will be under-used.

By installing the fuses on the long pos interconnect, you have basically done the same thing. The only way I can see to get around this is to also install fuses inline with the long neg interconnect. Which seems kinda silly to me. Hence my original question.
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Old 11-07-2018, 05:10   #15
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Re: Battery Cable Fusing

Quote:
Originally Posted by CharlieJ View Post
And BTW, the MRBF that was shown in the Defender link in Post #2 is the first generation type and has been been replaced for about six years.
Interesting, there are at least three styles of the Bussmann MRBF that differ in the detail of the insulator. I had not noticed the changes.







They are on page 76 of Bussmann's catalogue.
http://www.cooperindustries.com/cont...878.1531310657

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