LFP Battery and Battery Bank Fusing

[luna]

In your internal short circuit scenario, a fuse on the parallel leads also protects the shorted battery from additional current.

When you chose to use a MRBF on a LFP battery, what part of the then current Standard did you use to substantiate the choice?

 

[Frosty]

Yes, I've been trying to follow along also. Based on this thread I think I would like to change out to the MBRF fuses on my batteries, I have 2 - 100a batteries and I've been trying to decipher which size fuses I should install. Actually, I only have one battery right now but will be adding a second one.

I was also wondering if I should be sizing the fuse based on the potential battery output or based upon my projected load plus a healthy margin? We are not a power-hungry boat.

With 100ah batteries, you shouldn't have more than 5,000 amps going into a short circuit. So the AIC rating of the MRBF is ample (it's 10,000 amps).

So then for the size of the fuse, you want to protect the cabling. This would be the large cabling that comes off your batteries. Of course when that cabling gets smaller (as it branches out to loads) then you re-fuse with smaller fuses to protect that smaller cabling. You no longer have to worry about AIC rating on those because you have taken care of that with the initial MRBF.

So you can look at tables for the ampacity of the wire. I use the ones put out by ABYC (found easily for free on the web). To find out what ampacity you should be aiming for you look at at 4 things:

1) What temperature is your cabling rated for? I typically use Ancor wire and it is rated to 105ºC. I *think* all marine cable would be, but you can find out by reading the printing on the cable insulation.

2) What size is the cable (4AWG, etc.)

3) Is it bundled with other wires? If so there are various de-rating schemes (which you can also find in the tables). By that I mean that you may, eg, have to multiply the ampacity of the cable by say, .75. In other words, the ampacity goes down when bundled due to heat. That said, your primary battery cables are unlikely to be bundled.

4) Are these wires in an engine space? This again has to do with heat. There is a separate column in the chart for this (I would use this column whenever cables will be in a hot area).

There is nothing wrong with using a smaller amp size fuse than needed. Some say to use the 75ºC column's ampacity for a built in "fudge factor." On the other hand, I don't see any benefit to going crazy with reducing the fuse size. The wire can handle what its published to handle.

I seem to have trouble getting images to go where I want them in a post, so I will post examples of the tables in a post right after this one.

I am using 200A MBRF fuses on all 4 of my batteries. I am thinking of making them smaller. Plus a 400A T fuse for the whole bank. Each battery is rated, 150A max continues discharge and 200A max charge. My charger/Inverter only puts out 120A charging to all four batteries.

I can't see any benefit to going lower than a 200a fuse given what you have stated above. I mean, presuming your battery cables are at least 1AWG (or larger, so 1/0, 2/0, or 4/0).

As far as the 400a Class T fuse for the whole bank. That depends on how large the whole bank is. How many amp hours is your entire bank?

Cable size is another factor in all this, but. This all gets confusing and gives me a headache trying to read this whole thread or the topic. So I based the fuses more on the load then what the batteries can be rated for. In other words. Small fuses = more protection in my mind. Maybe I am wrong?

See above in this post for description of tables you can use to figure this out. I'll attach the tables to a subsequent post.

 

[ksanders]

@ksanders
But the BMS is supposed to shut down when you description occurs. That should isolate the offending battery from the others.
Yes a lot of new possibilities from lead

Yes lots of possibilities.

 

[Frosty]

Here are some ampacity tables. You simply read across. You need to know:

1) Load size

2) Cable size*

3) Cable temp rating (marked on cable; likely 105ºC for marine cable)

4) Is it in an engine space (or other hot space) or not?

5) Is it bundled with other wires?

*Sometimes you will find out that your cable size is not large enough to be able to properly fuse it (not enough ampacity). Then you need to go up in cable size. I usually "circle around" the charts and numbers a couple times to make sure it all matches up. That said, if you have a 12-volt system, avoiding too much voltage drop usually means your cable size is larger than required for proper fusing, but not always.

 

[Marco Flamingo]

do you have different data substantiating your post?

Yes. It is repeatedly stated throughout this thread.

I was also wondering if I should be sizing the fuse based on the potential battery output or based upon my projected load plus a healthy margin? We are not a power-hungry boat.

The answer is "both." Overcurrent protection must address both issues. Basic overcurrent amperage (what we generally refer to as the fuse's rating) and amperage interrupt capacity or AIC (what we generally don't refer to).

The "projected load plus a healthy margin" is only data point #1 for selecting overcurrent protection. That's the one we are most familiar with. Based on a standardized wire gauge (AWG in the US), the NEC has adopted a table of acceptable amperages for a given cable size (the source of data point #1). This standard has been uniformly accepted by other regulatory bodies, cable manufacturers, fuse manufacturers (like Blue Seas), and even advisory bodies like ABYC. Those tables are easy to locate and understand. If you need to power XXX, you can figure the cable size, the fuse size, and even the cable length to keep the voltage drop within 3%. That protects the cable under certain conditions.

Data point #1 addresses the "we are not a power hungry boat" issue. My main feeds are fused at 250A and I can't even imagine a scenario where I would have a continuous load over that. I do have a approximate 500A load for several milliseconds when starting the engine.

All of that is fine for data point #1. For many applications, there is no need to think about data point #2 and it can be ignored. In fact, some can't even see it. But it has become an issue with a new generation of batteries now available to the public. Batteries that can produce a ****load of power, i.e., what you have accurately called fusing for "potential battery output." I think it is generally refered to as short circuit capacity or SCC. That is data point #2 that a fuse, breaker, or overcurrent protection must contend with. It wasn't much of an issue with flooded lead acid battery banks, as recreational boats didn't have tens of thousands of amps for their potential battery output.

It is possible to ignore data point #2. To date, there are not common tables like with data point #1 that instruct the public on how to deal with it. But as with data point #1, it is best to compute SCC "plus a healthy margin" in choosing an overload protection device. The worst case scenario is, should a mainline positive cable fall off and drop on the engine block or another perfectly grounded surface, what is the total possible SCC (plus a healthy margin). The #1 data point fuse will certainly pop, but can the fuse handle data point #2?

One answer would be I don't actually know and neither does the ABYC. Therefore, I'm okay. Another answer would be based on what the battery manufacturer says is the SCC rating for the battery. They don't say? Well then again, I don't know and neither does the ABYC. Therefore, I'm okay. Also possible would be to test the battery and obtain its actual SCC. Dangerous, expensive, and probably not good for the battery. I'm not comfortable with those scenarios, so I went with general industry knowledge and guidelines for approximating the SCC of a LFP battery bank, whether or not those guidelines are yet to be adopted by the ABYC. The emerging consensus is that an LFP battery can produce a SCC of 5,000 amps for each 100Ah of capacity. That is likely an SCC padded with a "healthy margin," just like the AWG tables for computing data point #1.

It is still possible to claim that I don't really "know" the exact SCC of my batteries in an imperical sense (and therefore data point #2 can be ignored). In that case, an MRBF fuse rated at an AIC of 10,000 amps can handle the SCC because I don't know the actual SCC. I don't have the data. I can't figure it out on my own and the ABYC has not yet told me what to do. I could just fixate on data point #1 and claim that is good enough. Turns out that upgrading to handle the computed SCC (likely padded with a healthy margin) was easy and painless for my install.

 

[ksanders]

In your internal short circuit scenario, a fuse on the parallel leads also protects the shorted battery from additional current.

When you chose to use a MRBF on a LFP battery, what part of the then current Standard did you use to substantiate the choice?

To be perfectly honest I did not refer to the standard when I did my installation. I have been working as a professional engineer specializing in the utility industry in the areas of protection and automation engineering for my working career, including DC systems as well.

But... earlier in this thread there were tables and references to the standard, that spell out the AIC rating of the MBRF, and the literature for Blue Seas clearly states that the MBRF is AYBC approved for large battery banks, so I went with that.

In short i did not get overly concerned. The data that I was able to gather all added up, and I spent my life doing this for a living, so I completed my installation and went on to more interesting things.

Online forums attract a concept I'll call "paralysis by analysis" This subject falls into that category.

 

[Frosty]

... the literature for Blue Seas clearly states that the MBRF is AYBC approved for large battery banks, so I went with that.

Just as preamble, I'm happy with my fusing and it sounds like you are content with yours. So there's that.

But that said, I find Blue Sea's assertion that MRBF is ABYC approved for "large battery banks" to be really weird. I mean, what is "large"? How is there not a number? What if the bank is 4k ah? I mean, that's surely large. Clearly an MRBF would not be remotely suitable for that bank. It barely clears a single 200ah LFP battery (not based on anyone's rating, but just based on how many amps that battery can throw into a short circuit, just physics). So even if a person's "large" is much smaller than my hyperbolic example above, it's still not going to be an accurate assertion.

I just don't understand how Blue Sea can put out a "number" called "large" and have anyone take it seriously. It's not a number. It's totally subjective. My "large" bank might be your piddly, way-too-small bank.

Fuses don't know about adjectives. Just numbers.

 

[CharlieO.]

This has been a very informative discussion, thank you for the contributions and the replies to my question. I look forward to getting back to my boat and making some adjustments based on what I've learned.

 

[luna]

But... earlier in this thread there were tables and references to the standard, that spell out the AIC rating of the MBRF, and the literature for Blue Seas clearly states that the MBRF is AYBC approved for large battery banks, so I went with that.

Fair enough.........but I don't think that E-11's Table 3B (nor anything published by ABYC) spells out the AIC rating of a MRBF. That pleasure is up to the manufacturer.

Blue Seas is (as is the case for many things that bear their label) just a marketer of these MRBFs. Their brand does not even hide it as the fuse bears the word BUSS, just like the ones made by the real manufacturer Bussmann, part of Eaton.
Blue Seas also does not state that ABYC "approves" the MRBF because ABYC does not "approve" anything, they write Standards.

What Blue Seas, being a good marketer does state is:

• High Interrupt Rating satisfies ABYC requirements for DC main circuit protection on large battery banks

Ok, what kind of battery and how large?

Eaton calls it a bit different they say:

• Application Full range circuit protection for automotive and marine applications. Break in capacity meets the requirements of conventional vehicle batteries and 42V electrical networks

So, what exactly is a conventional vehicle battery? Is that your 300 Ah LFP house battery or is it a vehicle's start battery?

 

[SteveK]

I found this comment from Marine how to interesting.

Below are the specifications for a blue Sea systems Cass T fuse. Notice the fully encased metal body and the 20,000 A interrupt capacity. Also note that these fuses are rated at 20,000A at 125 V. Tthe higher the voltage the tougher it is to meet in AIC rating. Compare this to a typical ANL fuse which only has a 6000 amp AIC rating at 32V. a class T fuse would have a significantly higher AIC rating@12 V if it was tested at this point because it meets 20,000 AIC it 125 V there was no sense in spending the money to tested at a lower voltage.

This suggests that a class T fuse could have 50,000 AIC, who knows until someone does the test. Maybe the MRBF 100A fuse was tested but the 300A was not, who knows for sure it is limited to 10K AIC.

 

[Iggy]

Do you have 4-200Ah batteries for a total of 800Ah?

Yes. I also am using 2/0 cable to the buss bars. From there I think there 1/0. I say think, they are larger by at least 3/8" to 1/4" But no markings and was there when I bought the boat.

 

[SteveK]

Yes. I also am using 2/0 cable to the buss bars. From there I think there 1/0. I say think, they are larger by at least 3/8" to 1/4" But no markings and was there when I bought the boat.

OK, IMO the MRBF fuses by themselves are good enough, then the class T is insurance to keep loads under 400A.
Your class T 400A fuse will support a 2/0 wire but I wonder if you do have 1/0 maybe go down to 300A. Needs some research.
You are not having problems as your needs do not draw near the 400A fuse limit and are using below where the wire would show an effect.

 

[RobertLeatch]

I found this comment from Marine how to interesting.

This suggests that a class T fuse could have 50,000 AIC, who knows until someone does the test. Maybe the MRBF 100A fuse was tested but the 300A was not, who knows for sure it is limited to 10K AIC.

Actually one of the more appealing features of the MRBF product line is that it's specs seem well documented and accessible to the average punter. For example MRBF Terminal Fuse - 300A - Blue Sea Systems

One of the reasons this topic is so susceptible to analysis paralysis is because so much of the actual data knowledge you need to have available to make an informed decision is hidden in multiple obtuse and often inaccessible specs and standards.

 

[Iggy]

OK, IMO the MRBF fuses by themselves are good enough, then the class T is insurance to keep loads under 400A.
Your class T 400A fuse will support a 2/0 wire but I wonder if you do have 1/0 maybe go down to 300A. Needs some research.
You are not having problems as your needs do not draw near the 400A fuse limit and are using below where the wire would show an effect.

My mistake! It must be 3/0 or 4/0. As I mentioned it is bigger than the 2/0

The house bank was used for the bow thruster. But I went to 24V for that and a AGM bank.

 

[ksanders]

Actually one of the more appealing features of the MRBF product line is that it's specs seem well documented and accessible to the average punter. For example MRBF Terminal Fuse - 300A - Blue Sea Systems

One of the reasons this topic is so susceptible to analysis paralysis is because so much of the actual data knowledge you need to have available to make an informed decision is hidden in multiple obtuse and often inaccessible specs and standards.

Yep, says right there that it meets the interrupt rating of even large battery banks.

Now... When the new PROPOSED standard from AYBC goes into effect, if it is not changed, then installations from that point forward will need to meet the NEW specification.

 

[twistedtree]

Now... When the new PROPOSED standard from AYBC goes into effect, if it is not changed, then installations from that point forward will need to meet the NEW specification.

Coincidentally this recently came up in an ABYC meeting, and we were directed to the "Scope" section which is boiler plate other than the actual date. It reads:

"Based on ABYC's assessment of the existing technology, and the problems associated with achieving the goals of this standard, ABYC recommends compliance with this standard for all systems and associated equipment manufactured and/or installed after July 31, 20XX. (one year from publication)"

So it seems very clear that new versions of the standard only apply to equipment built or installed after the effective date. There is zero retroactive applicability, however if you install new stuff in your boat, that new stuff should meet current standards. That all seems very reasonable to me.

I think this is a great thing to keep in your back pocket next time you are interviewing a prospective surveyor. From the stories we all hear, it sounds like this is not understood by most surveyors.

 

[SteveK]

Yep, says right there that it meets the interrupt rating of even large battery banks.

Now... When the new PROPOSED standard from AYBC goes into effect, if it is not changed, then installations from that point forward will need to meet the NEW specification.

Geez Kevin, I waited so long and finally get to disagree. A surveyor will go by new standards and not what TT posted. We may have argued OEM installation, but doubt if you can say done before the standard goes into effect to a surveyor.

 

[twistedtree]

Geez Kevin, I waited so long and finally get to disagree. A surveyor will go by new standards and not what TT posted. We may have argued OEM installation, but doubt if you can say done before the standard goes into effect to a surveyor.

Hence the importance of discussiing this before hiring them. Ask what the evaluation/judgement criteria is. The answer will probably be ABYC if you are in North America. Then show them that "Scope" statement as ask if that is how they apply ABYC in evaluating a boat?

 

[ksanders]

Coincidentally this recently came up in an ABYC meeting, and we were directed to the "Scope" section which is boiler plate other than the actual date. It reads:

"Based on ABYC's assessment of the existing technology, and the problems associated with achieving the goals of this standard, ABYC recommends compliance with this standard for all systems and associated equipment manufactured and/or installed after July 31, 20XX. (one year from publication)"

So it seems very clear that new versions of the standard only apply to equipment built or installed after the effective date. There is zero retroactive applicability, however if you install new stuff in your boat, that new stuff should meet current standards. That all seems very reasonable to me.

I think this is a great thing to keep in your back pocket next time you are interviewing a prospective surveyor. From the stories we all hear, it sounds like this is not understood by most surveyors.

this is the same issue we have for land base systems when we're dealing with the nec

At some point, you have to move forward with your project because proposed standards aren't always set in stone.

 

[ksanders]

Geez Kevin, I waited so long and finally get to disagree. A surveyor will go by new standards and not what TT posted. We may have argued OEM installation, but doubt if you can say done before the standard goes into effect to a surveyor.

I look forward to buying you a beer when we get up to BC this fall. You're probably right about the surveyor, but we'll have to see how it goes.

I'm also pretty sure that if there's a larger requirement for AIC on the MBRF fuses that blue seas is going to re-engineer the fuse or the holder or both.

 

[BDofMSP]

To me this seems to go to a point that was made earlier. The standard isn't the ceiling, it's the floor. So I could certainly choose components that meet the current standard, and then convince my surveyor that I comply. But at the end of the day I'm really more concerned about my safety, and compared to the cost of most things we discuss here, a fuse and a fuse holder seems to be a place where over compliance is an easy choice.

That said, I'm not about to refit all of my systems every time there's something that might be "safer". In this particular situation, since I'm planning new work, I'll work with my installer to engineer this robustly. I'm looking into the NH2 fuses as the first layer of protection, but it's frustrating how difficult the research is for non-experts. I get the sense that AIC ratings will become a very strong marketing angle as these LiFePO4 systems become more and more common. And to Kevin's point, the products will follow.

 

[Delta Riverat]

I agree.

The MBRF "style" with the battery post mount is a very elegant and easy to implement solution the protects the wire right at the source which is the major intent eh?

 

[Frosty]

I agree.

The MBRF "style" with the battery post mount is a very elegant and easy to implement solution the protects the wire right at the source which is the major intent eh?

Yes. But there's a "but": If the MRBF doesn't meet the AIC rating you need, it can just arc through/melt/whatever and it's like you have no fuse at all. So the intent is foiled. So on an LFP battery over 200ah, better to use a Class T or one of the newfound alternatives.

I'm practically a Blue Sea fanboy, and have an entire "Blue Sea Tub" in my boat garage full of Blue Sea goodies. Plus they are all over my boat. But I don't buy their concept that "MBRF is fine for 'large' LFP battery banks." "Large" is meaningless. My "large" bank might be your tiny bank. And we don't need to use subjective terms; we have numbers. It's actually kind of shocking to me that Blue Sea would say something so nebulous on something so critical.

I don't mean to beat a dead horse, but that "Okay for 'large' LFP banks' seems to be one of those internet "truths" that keeps on giving....

Of course each individual can make their own decision. I'm not typing this to convince anyone who has a setup to change it. Or who is deadset on MRBF's. But rather because I'm guessing people who are new to LFP will be reading threads like this to find information.

 

[BDofMSP]

Interesting that when I put this question to AI about what fuse for a 460ah LiFePO4 battery, considering AIC, it went to Class T immediately. But when I said "what if that isn't enough?" it basically said "well now you're into industrial stuff, and you're on your own. Find a pro and good luck"

AI gets it's answers from the internet. And there just isn't a lot of recent stuff out there for it to train on.

BD

 

[SteveK]

Interesting that when I put this question to AI about what fuse for a 460ah LiFePO4 battery, considering AIC, it went to Class T immediately. But when I said "what if that isn't enough?" it basically said "well now you're into industrial stuff, and you're on your own. Find a pro and good luck"

AI gets it's answers from the internet. And there just isn't a lot of recent stuff out there for it to train on.

BD

Ai is out of step. This forum has already found the 460A Epoch has a 50,000 AIC built in fuse, yet Ai did not repeat. Maybe it has to hear it three times.
I notice my searches start with an Ai answer

 

[luna]

My rather weak understanding of what ABYC says in their Standards is that, in the case of a circuit breaker, the breaker itself does not need to be UL listed, but it must meet the requirements of the applicable UL standard, which for a Molded Case Circuit Breaker (MCCB) is UL 489, and/or it's CSA equal.

This begs the question: How would anybody know if any CB meets the requirements of UL489 without UL testing it?

The real question I have is does this kind of language exist within the ABYC Standards with respect to Main Battery Fuses and UL-248 or any other UL Standard?

Anybody know?

 

[Marco Flamingo]

The MBRF "style" with the battery post mount is a very elegant and easy to implement solution the protects the wire right at the source which is the major intent eh?

Ahhh, nope. Easy to implement, maybe. Elegant, if it works. Protect the wire at the source, if you are lucky. Protecting the wire as the major intent? That is the purpose of the fuses' general amperage rating, not the AIC rating. What about when it doesn't work because it is a dinky little fuse with a dinky AIC rating? The "protected" wire turns into a filament. As was noted above, the common little glass tube 5A fuses have the same 10k AIC rating as MRBF. It is not that difficult to do better.

I use MRBF on my C-Dory. As per Blue Sea, it is adequate to protect my "large" battery bank, which is two Group 21 lead batteries. That is about the extent of "protection" that I would trust with MRBF fuses. Man, are they elegant and easy for that purpose. But this thread is about LFP batteries and, specifically, LFP battery banks. I don't know if Blue Seas considers my trawler battery bank small or large, but can't see where MRBF offers me very much with four 280Ah LFP batteries, two to a bank, so 560Ah in each bank.

As has been repeated over and over in this thread, for LFP battery banks AIC is equally as important as the fuse's nominal breaking amperage. A 250 amp fuse doesn't do anything if it can't hold back the following 28,000A arc generated by my 560Ah LFP bank. Relying on MRBF is a gamble (or a joke). Granted, it is an elegant and easy to implement gamble/joke.

As noted above, some fuse AIC ratings are given at 150V DC with the understanding that their AIC is likely much larger at lower DC voltages. Fine. But a MRBF's 10,000 AIC is rated at 14V DC. You are never going to get better than that. The AIC drops as the voltage rises. In fact, since we are talking LFP, it is possible that an LFP battery's voltage might be higher than 14V, meaning that the effective AIC could be lower than 10kA. So an MRFB is right at the edge for a "large" battery bank of four 50Ah LFP batteries in parallel if not at full charge. MRBF is cheap, easy to install, stocked at West Marine, and therefore people like them. Elegant if they work.

One might go bear hunting with a .22. It would be cheap and easy. Cool if it works. Good luck. Seriously, look closely at whether MRBF is really providing you with what you need.

 

[Delta Riverat]

I'm referring to one fuse per battery, not bank.

 

[Frosty]

I'm referring to one fuse per battery, not bank.

That could work. Especially if you have, say, 100ah individual batteries. This was pretty common some years ago with eg. Battleborn having 100ah units.

Just seems that nowadays many are going with larger ah per battery. Even my relatively "small" 200ah batteries were right on the edge for the MRBF fuses *per battery*. I went with Class T. Larger, more expensive, and less tidy looking; but I didn't want to feel like I was right on the edge.

In future (especially if I had even larger batteries in my bank), I would look into the option of the MCCB that @Marco Flamingo researched and implemented. I don't know all the ins and outs of those right now, but they look interesting (and at least one mfgr, Lithionics, is using something that I think is similar (going by memory) on their 320ah internal BMS battery.

 

[Frosty]

Geez, sorry. I'm having trouble getting my posts to go through, then .... they show up twice. (I don't see a way to delete one.)