LFP Battery and Battery Bank Fusing

[RobertLeatch]

I don't know if it is material, but if option 3 is utilized the bus bar mounted MRBF will be limited by product availability to only 300 Amps.

If 400 Amps is required then another class T would likely do the job.

One thing I've noticed is that the AIC rating for MRBF fuses are all the same regardless of the more commonly referenced fuse Amp rating. For example all the BlueSeas MRBF fuses (from 30 to 300 Amps) have an interrupt rating of 10000 Amp. Not sure how AIC is physically implemented in the fuse, but it's clearly independent of the fuse Amp rating.

Marine Rated Battery Fuses - Blue Sea Systems

Space-saving ignition protected fuse for 30 to 300 Amp loads. Must use with Terminal Fuse Block.

www.bluesea.com

 

[Delta Riverat]

It has to do with arc suppression. When the fuse blows you get a conductive mist of the metal that was the fuse element before it got melted/vaporized. This "residue" is conductive and if there is enough and the voltage is high enough an arc can form and do much additional damage (think arc welding heat)

Suppression can take various forms, distance between the "burnt ends", chemicals added to extinguish the arc. AC arcs are easier to deal with than DC arcs as the voltage and current go to zero 120 or 100 times a second as well as change direction. Higher voltages are harder to suppress than lower, thus the AIC rating decreasing with increasing voltage.

 

[twistedtree]

One thing I've noticed is that the AIC rating for MRBF fuses are all the same regardless of the more commonly referenced fuse Amp rating. For example all the BlueSeas MRBF fuses (from 30 to 300 Amps) have an interrupt rating of 10000 Amp. Not sure how AIC is physically implemented in the fuse, but it's clearly independent of the fuse Amp rating.

Marine Rated Battery Fuses - Blue Sea Systems

Space-saving ignition protected fuse for 30 to 300 Amp loads. Must use with Terminal Fuse Block.

www.bluesea.com

Yes, exactly. The AIC rating is more about construction materials and spacing between parts, and is largely independent of fuse rating. But the AIC rating will typically be different for different voltages. Their MRBF 100A fuse has an AIC of 10,000A @14V, 5,000A @32V, and 2,000A @58V

 

[tpbrady]

I think I need to explain the thought process. The 250A class T on each battery was both to provide AIC current protection, but limit each battery to 250A ( max continuous current is 200A). The MRBF on the bus bar was to limit current going to all the boat systems to 300A (I have reduced that to 300). That is what the cabling can handle. I don’t expect to go over that even running the inverter at the “firewall”.

Tom

 

[luna]

who are you replying to. This is proposed option 3 by OP
3. 250A Class T on each battery and 400A MRBF on bus bar.

I'm replying to the OP who originally specified a 400 Amp MRBF fuse that simply does not exist as the highest current carrying capacity of a MRBF fuse is 300 Amps.

I see now that the OP has revised the requirement to 300 Amps, that is doable.

 

[RobertLeatch]

I think I need to explain the thought process. The 250A class T on each battery was both to provide AIC current protection, but limit each battery to 250A ( max continuous current is 200A). The MRBF on the bus bar was to limit current going to all the boat systems to 300A (I have reduced that to 300). That is what the cabling can handle. I don’t expect to go over that even running the inverter at the “firewall”.

Tom

Putting aside the fuse capacity (250a or 300a etc) isn't the question really "does each individual battery fuse need a 10000A AIC (MRBF) or 20000A AIC (class T) rating?". If 10000A per battery is OK then the MRBF solution will be cheaper and also neater at the battery post.

 

[J.T kearney]

Putting aside the fuse capacity (250a or 300a etc) isn't the question really "does each individual battery fuse need a 10000A AIC (MRBF) or 20000A AIC (class T) rating?". If 10000A per battery is OK then the MRBF solution will be cheaper and also neater at the battery post.

Robert, If your battery(s) are at or under 200 amps each your AIC would be 10000 and with in proposed ABYC allowance for MRBF from what I read. Anything over that amperage is at least a class T fuse
Cheers J.T

 

[twistedtree]

I think I need to explain the thought process. The 250A class T on each battery was both to provide AIC current protection, but limit each battery to 250A ( max continuous current is 200A). The MRBF on the bus bar was to limit current going to all the boat systems to 300A (I have reduced that to 300). That is what the cabling can handle. I don’t expect to go over that even running the inverter at the “firewall”.

Tom

I think that all makes sense.

 

[twistedtree]

Robert, If your battery(s) are at or under 200 amps each your AIC would be 10000 and with in proposed ABYC allowance for MRBF from what I read. Anything over that amperage is at least a class T fuse
Cheers J.T

Here's the new language for figuring out the AIC requirements for LFP batteries where the short circut current isn't specified.

11.10.1.3.2.2 If the short circuit current of a battery is unknown, then it shall be calculated using 3kA for every 100Ah of capacity for lead-acid batteries other than thin-plate pure lead (TPPL) and 5kA for every 100 Ah of capacity for lithium ion batteries and thin-plate pure lead (TPPL) batteries, or

The "or" pertains to voltages over 125VDC

So Tom's 230ah batteries need fusing with AIC of 11,500A. MRBF tops out at 10,000, so not sufficient.

 

[Frosty]

11.10.1.3.2.2 If the short circuit current of a battery is unknown, then it shall be calculated using 3kA for every 100Ah of capacity for lead-acid batteries other than thin-plate pure lead (TPPL) and 5kA for every 100 Ah of capacity for lithium ion batteries and thin-plate pure lead (TPPL) batteries, or

Thank you so much for posting that. Very helpful.

Also interesting. I did a calculation for my 200ah LFP battery by looking up a formula (mfgr, does spec the resistance so I inserted that) and I thought I had it figured out at just about 10,000 amps short circuit current per one battery.

Then I dug further into the calculating weeds and wasn't totally sure I was right (like maybe I was in way over my mathematical head). But I used a Class T per 200ah battery anyway, just to not worry about being on the edge (MRBF was tempting, of course, as they are so compact, and so are the spares).

So it's interesting to see ABYC's guideline coming in right at 10,000 amps for 200ah. Glad I went with the Class T.

Actually, it's interesting to me that the MRBF have as high an AIC rating (and 300 amp top rating) as they do. The metal contact area looks so small. Obviously there's more to the MRBF design than meets my amateur eye.

 

[SteveK]

so then MRBF is limited to every 200A LFP, more than that will need a class T fuse. Also class T will handle every 400A LFP
That will make it simple to calc.

 

[ksanders]

I have the exact setup the OP described.

300AH LiFeP04 batteries (3)
600a buss bar

What I did was to use a 200A MBRF on each of the three battery positive terminal

From the 600A buss bar i have two 4/0 cable runs.

one going to my OEM main DC distribution
one going to my Victron inverter/chargers.

If and when i see a real ABYC standard, not maybe, not someday, but a real published standard that specified the AIC rating needed for a LiFeP04 battery, I'll change out my solution for something different.

 

[RobertLeatch]

so then MRBF is limited to every 200A LFP, more than that will need a class T fuse. Also class T will handle every 400A LFP
That will make it simple to calc.

I'm reminded of the old saying that "if all you have is a hammer then everything looks like a nail".

I suspect we're all so used to a few specific types of fuses (ANL, Mega, MRBF, Class T) common on boats that we look only to them as options. Given the size of some LFP banks (think large off grid homes etc) I find it hard to believe that the best solution they've come up with is putting an individual Class T fuses on each LFP battery > 200 amp. Absolutely not an expert in this, but for example a few quick googles turned up NH Fuses as an interesting option (eg Mersen FWP NH DC-rated, up to 100kA AIC). Given the size of our emerging LFP banks (I have 6 x 330 amp batteries) maybe we're past the point where we can just wire a few new fuse holders into the circut and sleep well. Just saying.

 

[BDofMSP]

I'm reminded of the old saying that "if all you have is a hammer then everything looks like a nail".

I suspect we're all so used to a few specific types of fuses (ANL, Mega, MRBF, Class T) common on boats that we look only to them as options. Given the size of some LFP banks (think large off grid homes etc) I find it hard to believe that the best solution they've come up with is putting an individual Class T fuses on each LFP battery > 200 amp. Absolutely not an expert in this, but for example a few quick googles turned up NH Fuses as an interesting option (eg Mersen FWP NH DC-rated, up to 100kA AIC). Given the size of our emerging LFP banks (I have 6 x 330 amp batteries) maybe we're past the point where we can just wire a few new fuse holders into the circut and sleep well. Just saying.

And they mount directly to the battery, like MRBF? Or so it appears. Very interesting. Man they are heavy. I wonder what they cost?

 

[Marco Flamingo]

I'm reminded of the old saying that "if all you have is a hammer then everything looks like a nail".

And if all you have in your toolbox is the same old junky hammers, one tends to think that's all there is. How can I make these old hammers do what I need? That's essentially the first post here.

The solar people and the electronic vehicle people must read these posts and laugh. Class T fuse? How quaint. That's what Grandpa F used in his shop (for AC equipment). Marine Rated Battery Fuse (MRBF)? That must be for especially for boats or something? With a 300A max and 10K AIC limit? Especially for little boats?

Turns out there are new tools. But you won't find them at West Marine.

 

[SteveK]

There was a battery that had a built in fuse Will Prowse reviewed, which apparently will not meet ABYC standards for readily accessible. It was different from the known hammer.
evolution.
What fuses were on the 8D batteries used for the house needs? I do not recall any.

 

[twistedtree]

Thank you so much for posting that. Very helpful.

Also interesting. I did a calculation for my 200ah LFP battery by looking up a formula (mfgr, does spec the resistance so I inserted that) and I thought I had it figured out at just about 10,000 amps short circuit current per one battery.

Then I dug further into the calculating weeds and wasn't totally sure I was right (like maybe I was in way over my mathematical head). But I used a Class T per 200ah battery anyway, just to not worry about being on the edge (MRBF was tempting, of course, as they are so compact, and so are the spares).

So it's interesting to see ABYC's guideline coming in right at 10,000 amps for 200ah. Glad I went with the Class T.

Actually, it's interesting to me that the MRBF have as high an AIC rating (and 300 amp top rating) as they do. The metal contact area looks so small. Obviously there's more to the MRBF design than meets my amateur eye.

The guy who came up with the guidelines did a whole Spice analysis with what everyone thought were good assumptions not just about the battery, but about some minimal amount of connector and cabling resistance. Anyone can make different assumption, but I think his were as good or better that anything else, and you have to pick something....

The MRBF fuses are impressive, and very widely used. Same with Mega fuses. But the LFP AIC problem is tough to solve, and I expect that many, many installations don't actually comply. It also sucks that pretty much nobody specified the short circuit current for their batteries, and as a result nearly all LFP batteries are also technically non-compliant. But everything is getting pushed in the right direction, and actual field experience says it's all pretty safe if you follow E-13 as closely as possible.

Speaking of high AIC fuses, there are also NH2 fuses which have very high voltage ratings and 50kA AIC rating. I've never used them, but they look inpressive and I have heard they are less expensive than Class T fuses. Also physically bigger which might pose a problem.

 

[twistedtree]

so then MRBF is limited to every 200A LFP, more than that will need a class T fuse. Also class T will handle every 400A LFP
That will make it simple to calc.

Assuming 12V. MRBF drops off fast with higher voltages. I think I posted the details earlier.

 

[twistedtree]

If and when i see a real ABYC standard, not maybe, not someday, but a real published standard that specified the AIC rating needed for a LiFeP04 battery, I'll change out my solution for something different.

It was published in 2022,

 

[twistedtree]

And if all you have in your toolbox is the same old junky hammers, one tends to think that's all there is. How can I make these old hammers do what I need? That's essentially the first post here.

The solar people and the electronic vehicle people must read these posts and laugh. Class T fuse? How quaint. That's what Grandpa F used in his shop (for AC equipment). Marine Rated Battery Fuse (MRBF)? That must be for especially for boats or something? With a 300A max and 10K AIC limit? Especially for little boats?

Turns out there are new tools. But you won't find them at West Marine.

Very true about being captive to our own experience. It's our greatest asset, and biggest liability.

There has also been a back and forth "borrowing" of tech between boats on land applications for the nearly 30 years I have been messing with this stuff, and the "givers" and "borrowers" keep switching places. In the late 90's, all teh off-grid people were taking tech from the marine world. My first inverter was a Heart Interface Freedom 10, stolen fair and square from the marine world, but installed in a house. Then Trace became King of the Hill, mostly selling into off-grid installations, but also used in many boats. Grid-tied solar caused an explosion in inverter companies and panels companies, and panel prices plummeted from about $5/W to sub 1$/W. The vast majority of products were then being sold into land applications. But now companies like Victron have added important new features for marine and mobile applications like power boost, and that's now getting sucked up by the off-gridders.

But it all says the same thing. Look outside your tool box, not just inside it. One of the things I enjoy the most about going aboard other boats is that I ALWAYS see some clever new idea. And I always see cool things in other people's tool boxes.

 

[heysteve]

On a somewhat related issue, Will Powers and others have mentioned that some newer LFP batteries have internal fire extinguisher modules. Is that an ABYC thing or just marketing?

 

[twistedtree]

What fuses were on the 8D batteries used for the house needs? I do not recall any.

The same rules have always applied. I see a lot of Class T fuses on bigger banks. Also ANL fuses.

 

[twistedtree]

On a somewhat related issue, Will Powers and others have mentioned that some newer LFP batteries have internal fire extinguisher modules. Is that an ABYC thing or just marketing?

Not an ABYC thing

 

[SteveK]

It was published in 2022,

Is this it? Last amended 2023

2. This table is relevant to conventional battery technology (lead acid, AGM, Gel) and is not applicable to technologies such as lithium ion and thin -plate pure lead (TPPL), which may require significantly higher AIC ratings.

AMPERE INTERRUPTING CAPACITY (AIC)
The Ampere Interrupting Capacity (AIC) refers to the maximum short circuit current that an overcurrent protective device can safely interrupt.

Overcurrent protection devices often need to break high levels of current. Without a proper Ampere Interrupting Capacity, the internal points of the device can melt and fuse together, rendering the device unable to open the circuit.

The required AIC rating of the DC overcurrent protection device is determined by the size of the battery bank and, unlike current rating, is not dependent on the conductor size.

When used as the main battery overcurrent protection device, the following requirements should be met:

• For batteries or battery banks with a rating of 2200 CCA (Cold Cranking Amps) or 500 amp-hours or less, the overcurrent protection device should have a minimum AIC rating according to TABLE 3B.

• For batteries or battery banks rated higher than 2200 CCA or 500 amp-hours, the overcurrent protection device should have a minimum AIC rating as follows:
o At least equal to the battery manufacturer's short circuit rating, or
o 20 kA at 125 VDC or higher, if the battery manufacturer's short circuit rating exceeds 10 kA.

For batteries in series/parallel configurations the short circuit current of the battery bank is calculated by adding the short circuit current values of the batteries connected in parallel, and the short circuit rating of the batteries connected in series is not added for this calculation (e.g., six 6 VDC batteries are connected in series/parallel to create 12 VDC battery bank; if short circuit rating of a single battery equals 2400 A, the total calculated short circuit rating of the battery bank equals 7200 A).

For additional information please refer to ABYC E-11.

 

[twistedtree]

Is this it?
View attachment 163456
AMPERE INTERRUPTING CAPACITY (AIC)

The Ampere Interrupting Capacity (AIC) refers to the maximum short circuit current that an overcurrent protective device can safely interrupt.

Overcurrent protection devices often need to break high levels of current. Without a proper Ampere Interrupting Capacity, the internal points of the device can melt and fuse together, rendering the device unable to open the circuit.

The required AIC rating of the DC overcurrent protection device is determined by the size of the battery bank and, unlike current rating, is not dependent on the conductor size.

When used as the main battery overcurrent protection device, the following requirements should be met:

• For batteries or battery banks with a rating of 2200 CCA (Cold Cranking Amps) or 500 amp-hours or less, the overcurrent protection device should have a minimum AIC rating according to TABLE 3B.

• For batteries or battery banks rated higher than 2200 CCA or 500 amp-hours, the overcurrent protection device should have a minimum AIC rating as follows:
o At least equal to the battery manufacturer's short circuit rating, or
o 20 kA at 125 VDC or higher, if the battery manufacturer's short circuit rating exceeds 10 kA.

For batteries in series/parallel configurations the short circuit current of the battery bank is calculated by adding the short circuit current values of the batteries connected in parallel, and the short circuit rating of the batteries connected in series is not added for this calculation (e.g., six 6 VDC batteries are connected in series/parallel to create 12 VDC battery bank; if short circuit rating of a single battery equals 2400 A, the total calculated short circuit rating of the battery bank equals 7200 A).

For additional information please refer to ABYC E-11.

If it's from the published E-13, then yes. But the primary AIC info is in E-11 and has been around for a very long time.

 

[ksanders]

I have the exact setup the OP described.

300AH LiFeP04 batteries (3)
600a buss bar

What I did was to use a 200A MBRF on each of the three battery positive terminal

From the 600A buss bar i have two 4/0 cable runs.

one going to my OEM main DC distribution
one going to my Victron inverter/chargers.

If and when i see a real ABYC standard, not maybe, not someday, but a real published standard that specified the AIC rating needed for a LiFeP04 battery, I'll change out my solution for something different.

It was published in 2022,

Is this it?
View attachment 163456
AMPERE INTERRUPTING CAPACITY (AIC)

The Ampere Interrupting Capacity (AIC) refers to the maximum short circuit current that an overcurrent protective device can safely interrupt.

Overcurrent protection devices often need to break high levels of current. Without a proper Ampere Interrupting Capacity, the internal points of the device can melt and fuse together, rendering the device unable to open the circuit.

The required AIC rating of the DC overcurrent protection device is determined by the size of the battery bank and, unlike current rating, is not dependent on the conductor size.

When used as the main battery overcurrent protection device, the following requirements should be met:

• For batteries or battery banks with a rating of 2200 CCA (Cold Cranking Amps) or 500 amp-hours or less, the overcurrent protection device should have a minimum AIC rating according to TABLE 3B.

• For batteries or battery banks rated higher than 2200 CCA or 500 amp-hours, the overcurrent protection device should have a minimum AIC rating as follows:
o At least equal to the battery manufacturer's short circuit rating, or
o 20 kA at 125 VDC or higher, if the battery manufacturer's short circuit rating exceeds 10 kA.

For batteries in series/parallel configurations the short circuit current of the battery bank is calculated by adding the short circuit current values of the batteries connected in parallel, and the short circuit rating of the batteries connected in series is not added for this calculation (e.g., six 6 VDC batteries are connected in series/parallel to create 12 VDC battery bank; if short circuit rating of a single battery equals 2400 A, the total calculated short circuit rating of the battery bank equals 7200 A).

For additional information please refer to ABYC E-11.

OK

If this is the current published standard the MBRF is compliant it looks like for a 300AH battery.

According to this standard you need 5000 AIC and the MBRF is rated at 10,000 AIC.

I must be missing something, because this looks just fine for the MBRF used as intended on each parallel battery.

 

[RobertLeatch]

Is this it?
View attachment 163456
AMPERE INTERRUPTING CAPACITY (AIC)

The Ampere Interrupting Capacity (AIC) refers to the maximum short circuit current that an overcurrent protective device can safely interrupt.

Overcurrent protection devices often need to break high levels of current. Without a proper Ampere Interrupting Capacity, the internal points of the device can melt and fuse together, rendering the device unable to open the circuit.

The required AIC rating of the DC overcurrent protection device is determined by the size of the battery bank and, unlike current rating, is not dependent on the conductor size.

When used as the main battery overcurrent protection device, the following requirements should be met:

• For batteries or battery banks with a rating of 2200 CCA (Cold Cranking Amps) or 500 amp-hours or less, the overcurrent protection device should have a minimum AIC rating according to TABLE 3B.

• For batteries or battery banks rated higher than 2200 CCA or 500 amp-hours, the overcurrent protection device should have a minimum AIC rating as follows:
o At least equal to the battery manufacturer's short circuit rating, or
o 20 kA at 125 VDC or higher, if the battery manufacturer's short circuit rating exceeds 10 kA.

For batteries in series/parallel configurations the short circuit current of the battery bank is calculated by adding the short circuit current values of the batteries connected in parallel, and the short circuit rating of the batteries connected in series is not added for this calculation (e.g., six 6 VDC batteries are connected in series/parallel to create 12 VDC battery bank; if short circuit rating of a single battery equals 2400 A, the total calculated short circuit rating of the battery bank equals 7200 A).

For additional information please refer to ABYC E-11.

Are we sure that the listed AIC in this table is per 100amp capacity of the battery (as previously mentioned) or is it just the value listed for the range. For example, does a 300a battery (so in the 256 - 500 line) require a fuse with 5000 AIC or 15000 AIC (ie. 3 x 5000).

 

[twistedtree]

Are we sure that the listed AIC in this table is per 100amp capacity of the battery (as previously mentioned) or is it just the value listed for the range. For example, does a 300a battery (so in the 256 - 500 line) require a fuse with 5000 AIC or 15000 AIC (ie. 3 x 5000).

You need to be really careful here. We have a table quoted from the published versions of E-11 that does not take LFP into account, and text from E-13. In that context I think Kevin is correct, or at least I read it the same way.

But we have also been talking about the upcoming revisions to both E-11 and E-13, and this area has changed in both standards. That's where the new per-100Ah calculation comes from.

Snip-its from standards can be really dangerous because you lose their context, you lose definitions, and you lose other related and supporting material. So I would really encourage anyone diving into a significant electrical refit to spend the $200 to join ABYC, get the standards, and read them in their entirety. The cost will likely be one of the cheapest line items in your project.

 

[heysteve]

The full table with notes was referenced in a previous thread called "ABYC over current protection question"

Here's the table. "See note 3" means a class T fuse.

In that posting, "Note 2" says it does not apply to "lithium ion or thin plate pure lead" batteries.

 

[SteveK]

Are we sure that the listed AIC in this table is per 100amp capacity of the battery (as previously mentioned) or is it just the value listed for the range. For example, does a 300a battery (so in the 256 - 500 line) require a fuse with 5000 AIC or 15000 AIC (ie. 3 x 5000).

It is from ABYC, but what year? Posted on ABYC FacebookJune 27, 2023

But yes I interpret between 256-500 a fuse with 5000 AIC is required. I agree with KSanders that the 10,000 AIC MRBF should then work for up to 1,000Ah.

But it is suggested that each 100Ah LFP battery outputs 5,000 AIC in the upcoming 2025 revision.