LFP Battery and Battery Bank Fusing

[twistedtree]

11.10.1.2 tells us that OCP's are to be "readily accessible." We also know from info. upthread what the definition is for "readily accessible."

Then the Exceptions come into play and this list of things only need to be "accessible." I have no idea if the text of E-11 further defines "accessible" so let's just use the definition in Websters, which is: capable of being reached, used, or understood; available; or easy to approach.

That seems like a rather insignificant change in the actual text and in the meaning of that text that I would think a lot of people simply won't recognize. Specifications or Standards reading is never easy and you really need the entire text.

Regardless, of the Exceptions, Note 3. seems to apply to fuse holders and fuses as I would think in order to comply to E-11.5. a guard on the fuse holder would be required.
If I have this right, the guard requirement of E-11.5. results in the requirement to remove a fuse assembly from the "readily accessible" group and the Exception clause places it in the slightly different "accessible" group.

Unless it's noted elsewhere this "accessible group" is not required to be capable of being reached quickly & safely for effective use in emergency conditions without the use of tools, like the "readily accessible" group.

That being the case, a Class T fuse inside a battery (isn't that an electrical device?) may well comply with ABYC.

BTW, you can send in formal "Requests for Interpretation" (RFI) and ABYC will respond. It may take a while, but it will happen.

I believe the exception about fuses that are internal to electrical device is intended to cover internal fuses commonly found in inverters, autopilots, etc. But you would need to send in an RFI to get ABYC's take on it.

 

[Datenight]

Please forgive what may be a foolish question. Does sheathed mean the wire from the battery is in a conduit or split/spiral loom?
Thanks,

Rob

 

[twistedtree]

Please forgive what may be a foolish question. Does sheathed mean the wire from the battery is in a conduit or split/spiral loom?
Thanks,

Rob

Yes, among other things. Someone mentioned electrical tape, which I think technically complies.

 

[Datenight]

Thank you TwistedTree..
Rob

 

[luna]

I posted the definition in Post #24

Ya, how foolish of me! Despite reading your Post# 24 numerous times, your first sentence escaped me. I'm human.

So now one has to ponder whether the cover of a battery would be considered "permanent boat structure."
That word "permanent" causes me to envision a chainsaw or worse.

Batteries get replaced, are they permanent? Although maybe LFP with good care will change all of that.

Even if the permanent thing prevailed and the cover of a battery that concealed a high current fuse ruled out acceptance, the "within an electrical device" clause surely is not challengeable even though it may have been targeted to low current fuses internal to your inverter or auto pilot. Perhaps a rewording of the Standard is in order.

Just as an aside, is it now that ABYC is wading into the design requirements of devices like inverters and refrigerators?
My microwave has at least one fuse in it. It would be a real pain, that would require removal of numerous Brad nailed panels (are they permanent?) to gain access to the fuse buried deep inside the nuker. Does that mean I would fail the ABYC test?

 

[Don L]

I don't understand what the battle is here, or really wants going on. The fuse on the battery cabling is to protect the cable. The requirements have been around a while.

I personally didn't change any fuses when I changed my acid batteries to LFP. It isn't like the cable changed.

I am sure there will be people to tell me how wrong I am.

 

[SteveK]

From E-11 2008:

11.4.24 Sheath - A material used as a continuous protective covering, such as overlapping electrical tape,
woven sleeving, molded rubber, molded plastic, loom, or flexible tubing, around one or more insulated
conductors.

Has this changed? Yes a new number in 2023

11.4.32 Sheath - a material used as a continuous protective covering, such as overlapping electrical tape, woven sleeving, molded rubber, molded plastic, loom, or flexible tubing, around one or more insulated conductors.

 

[Frosty]

]The fuse on the battery cabling is to protect the cable. The requirements have been around a while.

I personally didn't change any fuses when I changed my acid batteries to LFP. It isn't like the cable changed.

I am sure there will be people to tell me how wrong I am.

Not here to tell you you are wrong (because that all depends on what batteries and fuses you have). But here is why even though the cable may not change, the fuses may need to.

It has to do with the balance between AIC rating (Ampere Interrupt Capacity) and the internal resistance of the battery (how many amps can pour out quickly into a short). AIC rating has nothing to do with the amp size of the fuse, but rather the robustness of the body of the fuse/holder itself. If you have a short, so amps come rushing out of the battery --- and the AIC rating is too low, then the fuse/holder can just weld together and not act like a fuse at all (unrelated to the amp size of the fuse). Bad deal.

Different types/sizes of batteries can "pour out" different amounts of amps in a short circuit situation (Short circuit current rating, which not all battery mfgrs publish, but some do - thank you, Lifeline.)

I'm pulling the first part of these numbers (flooded cell) out of a hat, but let's say you had a bank of two flooded cell batteries which had a short circuit current rating of 2,500 amps apiece. You have a pair, so that's 5,000 amps of potential short circuit current. If you had say an ANL fuse for that bank, they are rated at 6,000 amps AIC. So fine.

BUT, let's say you put in a pair of Lifeline 31XT's (AGM). They are rated at around 4,250 amps apiece short circuit current, if memory serves. So now you'd have 8,500 amps of short circuit current coming out into a fuse/holder rated at 6,000 amps AIC. Not fine.

I don't see short circuit current ratings for many LFP batteries, but I would think they might be higher than AGM due to low resistance? Or at least not lower. I believe ABYC has just come out with a guideline of you should have 5,000 amps of AIC rating for every 100 ah of LFP battery. So now your 250 ah bank that you have changed to LFP (again, just making an example) would need a fuse/holder with a 12,500 amp AIC rating. That puts you into Class T territory (20,000 amp AIC rated).

(Some people fuse each individual battery now with LFP, but then that would still mean that you'd change your fusing scheme even though/if the wire hadn't changed.)

That's just an example about how even if the wire hasn't changed, and you are protecting the wire with the fuse.... the fuse type might need to change with different batteries.

 

[Marco Flamingo]

When you have a lot of DC current blowing a fuse, it is similar to this video of a knife switch. The fuse element melts away, eliminating the direct electrical connection, but the air space distance may not be enough to stop the arc. An ANL fuse might have a distance of a centimeter. Good enough to stop 6k amps, i.e., enough for a single 150Ah LFP. If there is more current, an arc develops and the short continues despite the fuse having blown.

Here is a YouTube explanation of the thermo-magnetic DC breakers I installed. The contacts pass through an arc suppression chamber. Good to interupt 50k DC amps.

 

[Don L]

I don't see any of that changing the current rating of the cable, which is what the fuse it protecting. What difference is the short circuit capacity of the battery have to do with it? The fuse does nothing to stop a battery from internally shorting.

 

[Marco Flamingo]

A 250 amp fuse can protect 2/0 cable ONLY IF IT CAN HANDLE THE SHORT CIRCUIT AMPERAGE OF THE POWER SOURCE. A 250A ANL fuse can do that for many lead acid batteries. It erroneously appears that it can do that for a bank of LFP batteries if the AIC is ignored. That's what makes the battery(s) capacity an issue.

Fuses don't stop an internal short, as happens with lead acid batteries, but that isn't relevant to the external fuse's AIC.

 

[Don L]

Is there an ABYC fuse requirement based on battery size??? An 8D battery is a lot bigger than 4 G31s in parallel. But the main cable fusing is the same and based on the cable. There is no fusing required on the 4 G31s unless they are far apart.

 

[luna]

Don L.

The concept here is that the current can still flow through the fuse even thought your 300 Amp fuse has blown, similar in some ways to the current that jumps a spark plug gap.
This usually only happens when huge amounts of current are unleashed from the battery by way of an external short circuit. If this short circuit was allowed to continue the battery cables and connected wiring would get very hot, very quickly.

Large capacity fuses are rated in their ability to stop this current flow when the fuse is already blown by what is known as their Amperage Interruption Capacity, or AIC. Common Class T fuses are AIC rated at 20,000 Amps.

Lithium based batteries have the ability to unleash significantly more current than that of an equivalent sized lead acid battery.
As a result even though you are protecting the wire, when you select the fuse type you need to take into account what is the size of the threat? How much external short circuit current can the new battery deliver?

In your example, changing from LA to LFP batteries likely increased the threat to the wires. You should at least review the specs on your fuse and replace it if you feel uncomfortable.

 

[Marco Flamingo]

Is there an ABYC fuse requirement based on battery size???

Yes. More importantly, there is a common sense requirement. The "battery size" at issue is the short circuit capacity of the battery(s), which differs considerably based on battery chemistry. A fuse or other current protection device, in order to work, must be able to break during a dead short at the rated capacity (say 250A). 250A the easily understood part. It's printed right on the fuse. It pops open and protects the 2/0 cable.

Assuming that an arc doesn''t develop. How would an arc develop? From high current capacity, as is the case with LFP batteries. That's why fuses have two (or more) ratings. One is the common amperage (250A in this example). That protects the cable until it doesn't. When doesn't it? When the available short circuit current exceeds the interupt rating (AIC). The ABYC assumes that each 100A battery can produce 5,000 amps of short circuit current. It doesn't take much of an LFP battery bank to exceed the AIC rating of the fuses that were used for lead acid banks.

A 250A MEGA fuse can blow at 250A, protecting 2/0 cable. But a MEGA fuse has an AIC of only 6,000 amps. If the available current is in excess of 6k amps (as could be the case with a single 150Ah LFP) the MEGA fuse could turn into a MEGA light show. And the 2/0 cable (and other things) would not be protected despite the 250A fuse.

 

[BDofMSP]

Simplified:

• If there's not a fuse, the cable melts and starts a fire.
• Limited current blows a fuse and prevents that.
• Massive current MELTS the fuse and it's like not having a fuse.
• Some batteries can put out massive current and other can't.
• Some fuses can handle massive current and others can't.

So if you change batteries you may need to change the fuse to one that can handle massive current. Or it could end up being like no fuse at all.

 

[SteveK]

I’ve read most of the threads on LFP battery upgrades and now that I have decided on my battery selection, I am still a little confused on fusing options, between MRBF and class T. I will have 3 LiTime 230AH batteries in a single bank (they all fit in my starboard battery box) connected to a 600A busbar then going to the battery switch. Since my inverter is a Multiplus 12/3000, the maximum possible current draw is between 250-300A. So if we say house draw should not exceed the 50 amp panel breaker, then the max current should be less than 350 amps. The only exception to that is where the start battery bank needs help and the emergency parallel solenoid is engaged. At that point the upper limit would be driven by the BMS at 200A and the three in parallel at 600A coming from the bus. Options:

1. Class T 300A on each battery and nothing on the bus bar. (Allows most amps for emergency start)
2. 250A MRBF on each battery and 400A class T on bus bar.
3. 250A Class T on each battery and 400A MRBF on bus bar.
4. TBD

Tom

@twistedtree said I would lean towards #3 as well, keeping in mind the principals.

I have gone back/forth between 2 & 3. I do not grasp the reason for changing the AMP of the two fuses between 2&3. This morning it came to me that the #3 is better for the order of the fuses, but that the amp rating should be the same with the rating of the cable. If 250A protects the cable then why have a second fuse at 400A regardless of the type of fuse.
I also wondered why have two fuses in series. It occurred to me both fuses are protecting the same wire downstream but having one fuse at lower amps will protect the second fuse.
So class T 400A followed by MRBF 250A or 300A makes sense. The MRBF is cheaper and will trip on the slightest short protecting the wire and second class T fuse. The class T remains to stop a runaway surge up to 20,000AIC which 400A LFP appears to have.

 

[Don L]

provide a link for ABYC with battery cable fusing depending on battery type and capacity

This a power boat forum where ther are large batteries to start engines. How many her have those fused? I bet few because they will say ABYC says we don't have to. That of course passes no "common sense" (yes I had mine fused)

 

[Don L]

I am going to just leave this thread. I find most lithium threads attract over thinking.

 

[NoRain]

Don, you are confusing two terms. The size of fuse, and the class. The size is what you’re referring to and should be appropriate to the wire it is protecting, as you said. But the Class is related to current generating capacity of the source of current, in this context the battery. I’m going to pick a couple numbers out of the air here because I don’t have time to go dig up the actuals, but your FLA batteries had the ability to generate say 1000A of current if shorted. A pair of group 31 AGM may be able to supply 10,000A. A couple of 200AH lithiums might be able to supply 20,000 - 40,000 amps. So you need to use a fuse of the correct class to protect against arc-welding the contacts or plasma-arcing across them, as per the videos above. The size of that fuse would then be whatever is appropriate for your cabling. eg a Class T, 200A if you have 1/0, or Class T, 400A if you have 4/0 wiring connected. (Repeat, those are SWAG ratings, check before diving in).

 

[twistedtree]

Not here to tell you you are wrong (because that all depends on what batteries and fuses you have). But here is why even though the cable may not change, the fuses may need to.

It has to do with the balance between AIC rating (Ampere Interrupt Capacity) and the internal resistance of the battery (how many amps can pour out quickly into a short). AIC rating has nothing to do with the amp size of the fuse, but rather the robustness of the body of the fuse/holder itself. If you have a short, so amps come rushing out of the battery --- and the AIC rating is too low, then the fuse/holder can just weld together and not act like a fuse at all (unrelated to the amp size of the fuse). Bad deal.

Different types/sizes of batteries can "pour out" different amounts of amps in a short circuit situation (Short circuit current rating, which not all battery mfgrs publish, but some do - thank you, Lifeline.)

I'm pulling the first part of these numbers (flooded cell) out of a hat, but let's say you had a bank of two flooded cell batteries which had a short circuit current rating of 2,500 amps apiece. You have a pair, so that's 5,000 amps of potential short circuit current. If you had say an ANL fuse for that bank, they are rated at 6,000 amps AIC. So fine.

BUT, let's say you put in a pair of Lifeline 31XT's (AGM). They are rated at around 4,250 amps apiece short circuit current, if memory serves. So now you'd have 8,500 amps of short circuit current coming out into a fuse/holder rated at 6,000 amps AIC. Not fine.

I don't see short circuit current ratings for many LFP batteries, but I would think they might be higher than AGM due to low resistance? Or at least not lower. I believe ABYC has just come out with a guideline of you should have 5,000 amps of AIC rating for every 100 ah of LFP battery. So now your 250 ah bank that you have changed to LFP (again, just making an example) would need a fuse/holder with a 12,500 amp AIC rating. That puts you into Class T territory (20,000 amp AIC rated).

(Some people fuse each individual battery now with LFP, but then that would still mean that you'd change your fusing scheme even though/if the wire hadn't changed.)

That's just an example about how even if the wire hasn't changed, and you are protecting the wire with the fuse.... the fuse type might need to change with different batteries.

Those are all excellent examples

 

[twistedtree]

When we think of fuses, we think about the current rating, and presume that any current over the rating (let's say 100A for this discussion) the fuse will open. Simple, right? I certainly thought so until just a few years ago, and I have multiple engineering degrees, including electrical engineering.

But it's not that simple. We aren't born knowing this stuff, and often never have a need to learn it. In reality that 100A fuse will interrupt a current that is over 100A, but ONLY if it's ALSO below say 5,000A. That last part is the fuses AIC rating, and it means that if the current is over 5,000A, the fuse may not actually open, and hence not provide any protection at all. WTF, right?

In most cases nobody has to worry about this, but with batteries that have very large short circuirt current capability, it actually matters quite a bit. So when picking a battery fuse, you need to pay attention to TWO specs, not just one. You need to look at the current rating, which is based on the capacity of the cables that it's protecting, and you need to look at the AIC rating which is based on the short circuit current rating of the batteries. This has been in E-11 for as long as I've been exposed to it, and is why you often see Class T fuses in certain locations - it's because they have a very high AIC rating, typically 20,000A.

In small lead banks AIC is not much of concern. In larger lead banks, it can be. And in LFP banks, it definitely is a concern because they have very high short circuit currents. The same is true for Thin Plate Pure Lead batteries. Because of this, the new draft of E-11 due out this summer revises this section, and adds guidelines for AIC fuse ratings for different types and capacities of batteries.

And just when you think you have reached the bottom of what you need to know about fuses, keep in mind that voltage ratings are very important, and typically have a direct relationship with the AIC rating. A fuse may have an AIC rating of 8,000A at 12V, but only 5,000 at 24V, and 3,000 at 48V. So fusing that is great for a 12V bank may be completely inadequate for a 24V bank. So read the specs very carefully.

Getting back to Tom's original question, the reason I prefer option 3 is because it places the high AIC fuses immediately down stream of the batteries. Each battery fuse needs to have the AIC capacity required for a single battery. Once in place, it solves the AIC problem for everything downstream. With option 2, the MBRF fuse likely doesn't have the AIC capacity for a single battery. And now the AIC problem at the buss bar is a three-battery AIC problem (3 times the required AIC vs one battery) and much harder to solve.

 

[luna]

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.

 

[tiltrider1]

In the 80’s and 90’s you rarely saw a house bank greater than 400 amps. It wasn’t until 2000’s that inverter/chargers could use larger banks. We started to see 1400amp banks, this is when AIC hit the map for most of us. Still it’s not 1 in 100 who understand this. It’s more like 1 in 10,000.

 

[SteveK]

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.

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.
It was the 250A increased to 400A that puzzled me. Why not have the same or have a higher A closer to the battery.

 

[Frosty]

I am going to just leave this thread. I find most lithium threads attract over thinking.

That's kind of a bummer. You asked a question about why would anyone need to use a different fuse to protect cable that hasn't changed (but batteries have). That was a good question.

Several people took the time to type out thoughtful responses, but.... now you are "going to just leave" because we are "over-thinking"?

I mean, why ask the question then?

Well, then I hope that someone (or ones) else will find this information useful. Because as several posters above pointed out, we are used to only thinking about fuse amp size as relates to the ampacity of the wire, but nowadays (with current sources that can pump tens of thousands of amps into a short) it's also important to think about AIC ratings.

This a power boat forum where ther are large batteries to start engines. How many her have those fused? I bet few because they will say ABYC says we don't have to. That of course passes no "common sense" (yes I had mine fused)

Mine are. I have less than 250 amps of starting current, so I could (not sure if maybe big diesels simply can't due to too much starter amp load?). I also chose a fuse type with a decently long trip delay.

ABYC are basically (usually) minimum good standards. So even though they don't "require" fusing a start circuit, a boat owner can still choose to. It's fine to do better.

 

[Don L]

.... now you are "going to just leave" because we are "over-thinking"?

just staying out of your way

short circuit of a G312 small battery is 5000a, wonder what an 8D is. not that it matters

I need to do a better job unwatching a thread. I don't know anything anyway and the only boat I have is in a picture on the wall.

 

[Frosty]

just staying out of your way

I have no idea what you mean. "Staying out of my way"?

You asked a question. I (and others) responded politely, took the time to look up some numbers, and tried to write clearly.

I made a point to mention that I wasn't saying you or your setup is wrong (for one thing, I have no idea what your setup is -- or was if you don't have a boat at the moment).

Since you mentioned having fuses, that made it sound like you cared whether or not your setup was electrically safe. Otherwise you could just have had wire. Plus.... you asked. Figured if nothing else you wanted to understand.

 

[twistedtree]

That's kind of a bummer. You asked a question about why would anyone need to use a different fuse to protect cable that hasn't changed (but batteries have). That was a good question.

Several people took the time to type out thoughtful responses, but.... now you are "going to just leave" because we are "over-thinking"?

I mean, why ask the question then?

Well, then I hope that someone (or ones) else will find this information useful. Because as several posters above pointed out, we are used to only thinking about fuse amp size as relates to the ampacity of the wire, but nowadays (with current sources that can pump tens of thousands of amps into a short) it's also important to think about AIC ratings.



Mine are. I have less than 250 amps of starting current, so I could (not sure if maybe big diesels simply can't due to too much starter amp load?). I also chose a fuse type with a decently long trip delay.

ABYC are basically (usually) minimum good standards. So even though they don't "require" fusing a start circuit, a boat owner can still choose to. It's fine to do better.

Again, all good points. I have to keep reminding myself that ABYC covers small boats with outboards, along with trawler-type boats. An outboard power cabling expects to connect directly to a battery. That's one example of why such exemptions exist. A constant background question when creating these standards is "is it overly disruptive", and "is there a demonstable problem being solved". That said, I'm all in favor of a fuse on the start batteries if you can do it and not have the starter surge blow it out. On many boats, it's very doable.

 

[Frosty]

I posted a dup and don't know how to remove it. Please see next post.

 

[Frosty]

@twistedtree

Good to know ABYC keeps all types of boats in mind.

And yes, that "outboard only has a single cable coming out" is one thing that kept me from going LFP for at least 5 years longer than I otherwise might have (but hey, they got less expensive, so it wasn't all bad). And the irony is that with a smaller, more weight-sensitive boat, LFP made even more sense!

But so yeah, you've got maybe a 35-amp "alternator" (which isn't even as robust as a real alternator) and you can't separate the load of starting from the supply of charging.

When the Victron Orion TR Smarts came out it helped. But you had to hope they had an amp size that worked out (otherwise a thirsty LFP bank could suck your alternator dry in no time). The Orion XS is the real winner, to my mind. You can set the number of amps it will pull all the way down to nearly nothing, or up to 50 amps. Whew, problem solved!

I would always read with envy about folks with TWO positive wires; one for the starter and one for the (real) alternator. And ways to control the alternator too. With "smaller" outboards, one can only dream of such things.