Lead is dead

[CaptTom]

Originally Posted by Fuss

Can I add your name to the list of posters that contributed to CaptTom making such a poor decision. It’s only a small amount so he can replace his new lead acids with Lifepo4.

Just make the check out to "cash." Full disclosure though, it won't be going to replacing my shiny new FLA batteries anytime soon.

[sailorboy1]

Originally Posted by CaptTom

Just make the check out to "cash." Full disclosure though, it won't be going to replacing my shiny new FLA batteries anytime soon.

Heck if I could get 10 years out of them like you I would have kept getting the golf cart batteries. So I understand and don't understand why others don't or why it matters so much to them.

[Dave9111]

Originally Posted by Statistical

Will test is good but he is testing over but still manageable loads. For example on a 100A system he was hitting it with 120A or 180A. This would replicate that through user misuse the user is drawing form various loads to high of an ampacity. Yes many (but not all) of BMS disconnected as expected. That is much different than a dead short where the BMS will be hit with potentially 10,000A+.

Using an LFP battery without a fuse is dangerous and violates ABYC and probably just about every standard out there. There is no BMS or drop in battery manufacturer which advises not using a fuse because the BMS and fuse have different roles. Similarly using a LFP with a fuse that has insufficient AIC is feel good security. It will work until the one time you really need it to disconnect and then it won't. The current will exceed the AIC, the fuse will trip but the current won't stop. The wires will get hot enough to melt and catch fire and your boat will burn down. Yes it will work fine all the lower current scenarios and then won't the one time you really need it.

Just use a use and one with proper AIC. The cheapest easiest near zero effort protection you can do on a boat electrical system.

Some people go "what if my fuse blows and I have to keep replacing it". Well one cool thing you can do is upsize your wiring and use a larger fuse. For example if you have a BMS with a 200A limit. Then size the main battery cables to handle 300A and use a 300A Class T fuse. Since the BMS will trip at 200A the fuse should never blow short of a dead short scenario or other massive overcurrent which overwhelms the BMS. The BMS handles "routine" overcurrent and the fuse protects you up to 20,000A. The fuse protects the wire and as such there is no require the fuse have the same current limit as the BMS.

Hi Statistical,

So your premise is to always install a fuse, and I agree with that in most circumstances. However you state that you should use a T class fuse because the currents are "huge" but you fail to provide any information on what those short circuit currents are. So the "safe" thing is to install the highest AIC rated fuse you can find.

What?? Who taught you this? Ask for your money back!

Many people can't afford a T fuse. Not everyone has "boat bucks to spend" because of what "might be" needed. So instead of going with a lesser rated fuse they will simply do without. That's the wrong message.

I am a EE with 40+ years of experience.

You don't guess at what short circuit currents "might" be. You try and find some evidence of what they are. Again, this isn't rocket science. Lifepo4 batteries don't provide magical currents that defy physics. Lead Acid can also provide huge short circuit currents. I've seen a short melt lead acid battery terminal off the battery.

From my experience in watching things fail and operate at thousands of amps I could tell that there is no way you are going to get more than a few thousand amps of short circuit current out of a 12 v, 100 amp hour battery. It won't happen.
For one thing, at a thousand + amps, for a few seconds, the BMS will self destruct. The traces will burn right off the board. Also, the internal wiring will heat up and at least begin to melt. All of this will add to the internal resistance and limit the short circuit current of the battery. This is resistance in addition to the internal resistance of the cells themselves.

Doing some simple research, I find this.
The author of this paper uses what appears to be a CATL 160ah LIFEPO4 cell and does a short circuit test on the raw cell. He also tests other type of cells for short circuit current.

https://www.researchgate.net/publication/316171277

What he finds is that the raw cell creates a max of about 950 amps of short circuit current. If this cell was part of a 12 volt Lifepo4 battery and it was used without a BMS, the short circuit current would still be about 950 amps. That is because the internal resistance (besides the test gear) would be the only thing limiting the current.

Add in internal wiring and a BMS and the short circuit current will be less than this. This was for a 160ah cell.

An MBRF fuse is limited to 10,000 amps aic. (Amps Interrupting Capacity) .

Now why would anyone put a Class T fuse in this application?

You act like you are being logical, when in fact you appear to simply be scared of this stuff. Yes, this stuff can cause fires. But filling your boat with gasoline improperly can also start a fire.

One more thing: You mentioned that if you put a big fuse at your battery you can upsize the cable being fed from the fuse. Huh?
Why would you do that? Change the boat wiring to accommodate the fuse?

The fuse needs to be sized to protect the cable. You want the fuse to blow before the cable gets hot in a short circuit situation. Also branch circuits off the battery cable must have their own fusing or breakers to protect the smaller downstream conductors. The fuse is sized based on the conductor size being fed. The conductor is sized based on the maximum load.

Again, this isn't rocket science, not much here is new, but it does require thought above and beyond just buying the highest rated AIC fuse you can find and then telling everyone else to do the same. You are giving out bad advice.

Regarding the ABYC; that group tends to be self serving and caters to the boat building industry. That doesn't mean that what they say is entirely wrong, but they don't seem to handle change well and their first priority is to their customers, the boat building industry. Not the end users.

[Statistical]

Originally Posted by Dave9111

Many people can't afford a T fuse. Not everyone has "boat bucks to spend" because of what "might be" needed. So instead of going with a lesser rated fuse they will simply do without. That's the wrong message.

Seriously? You can afford a boat and the ongoing maintenance, repairs, and upgrades AND a large LFP bank and all the additonal changes requires for charging but you can't afford a $40 fuse?

You don't guess at what short circuit currents "might" be. You try and find some evidence of what they are. Again, this isn't rocket science. Lifepo4 batteries don't provide magical currents that defy physics. Lead Acid can also provide huge short circuit currents. I've seen a short melt lead acid battery terminal off the battery.

From my experience in watching things fail and operate at thousands of amps I could tell that there is no way you are going to get more than a few thousand amps of short circuit current out of a 12 v, 100 amp hour battery.

Maybe not but what about a 300 Ah battery or a paid of 300Ah batteries in paralle?

For one thing, at a thousand + amps, for a few seconds, the BMS will self destruct. The traces will burn right off the board. Also, the internal wiring will heat up and at least begin to melt.

You know what else will be melting the cells themselves and now you have an internal short circuit one that no fuse can protect you from.

One more thing: You mentioned that if you put a big fuse at your battery you can upsize the cable being fed from the fuse. Huh?
Why would you do that? Change the boat wiring to accommodate the fuse?

You missed the entire point. You got all angry and righteous and didn't read.

I said you should have a larger (as in rated amps not AIC) fuse than the BMS. This will ensure the fuse will never blow other than a dead short. Of course using a larger fuse requires using a larger cable.

As an exmaple if you have a BMS capable of 200A max and you size your wire and fuse for 200A it is possible the fuse will blow. Remember you are the one who thinks losing a $40 fuse will financially cripple someone so we probably want to avoid that.

However if you have wiring and fuse capable of 300A then other than dead short there should be no scenario where the fuse blows. This staggering financial investment that

Again, this isn't rocket science, not much here is new, but it does require thought above and beyond just buying the highest rated AIC fuse you can find and then telling everyone else to do the same. You are giving out bad advice.

Your right you should compute the max possible current using the internal resistance of the cells used. For 12V only small house banks a MBRF fuse is likely sufficient. The issue is going from likely sufficient to confirmed sufficient is finding the data from the manufacturer on either the internal cell resistance (which determines maximum current) OR the manufacturer provides a max short circuit current. ABYC even recommends using that to determine the AIC. The problem is most manufacturers do not provide this.

For example my previous boat had two EVE 304Ah cells in parallel. Internal resistance is 0.5 mOhms so fully charged at 3.4V per cell that is a max current of 6,800A with two in parallel that is >10k. Even including resistance of the cell busbars it is still >10k amps and certainly greater than 5,000A. I would add that for 24V applications MRBF is rated at only 5,000A.

If you want to use the cheapest fuse you can find after spending thousands to upgrade the house bank on your boat which is worth tens of thousands or hundreds of thousands then go ahead. The reality is you will probably be ok. Personally though I work hard and invest my money so that I don't have to take out a mortgage to buy a $40 Class T fuse.

I mean I could see if you went on this rant because I was "demanding" people use $1,200 isolation transformer we are talking a $40 fuse. To be clear though the only place Class T fuses are "needed" (or maybe I should say advised so you won't start foaming at the mouth and throwing off personal attacks again) is on the batteries themselves. From the primary busbar and certainly the branch circuits the additional resistance of the wiring between the battery terminals and the possible point of any fuse means short circuit current will be lower potentially a magnitude or more lower.

Blue Sea (actually Busman a solid brand rebranded as Blue Sea) 300A class T fuse ($38)
https://www.amazon.com/Blue-Sea-Syst.../dp/B000XBGT3S

Blue Seas 300A MBRF fuse ($18)
https://www.amazon.com/Blue-Sea-Syst...dp/B0017SJOH4/

So your freak out was based on someone recommending a $38 part over an $18 part. A part which if done properly (ensuring it is higher than than the max current allowed by the BMS) is likely a once in a lifetime part short of an emergency.

[emilecantin]

https://www.youtube.com/watch?v=y3gzhep8w4A

This guy tried it and couldn't even measure the current with his expensive Fluke meter.


In this case the MBRF fuse did the job, but I'm honestly a bit uneasy about the form factor and having the cable so close to the terminal should anything come loose. That's why I recommended a Class T fuse up-thread.


Full disclosure: my boat currently has a MEGA fuse on the battery; I ordered a Class T after researching it but it won't show up until Friday. It'll be my project for the weekend.

[Cheyne]

I have two of the new Victron Class-T Power In on pre-order for our Epoch upgrade that we are doing. I have four of the Epoch 460's. Two batteries per Power In with a Class T per battery. We currently have a Lynx Shunt with a ANL fuse in it, but that is what Victron spec'd when we build the orginal system.

https://www.victronenergy.com/dc-dis...n#pd-nav-image

Also the Epoch's each have a Class T fuse inside the battery. Picture stolen from Panbo.

[Statistical]

Originally Posted by Cheyne

I have two of the new Victron Class-T Power In on pre-order for our Epoch upgrade that we are doing. I have four of the Epoch 460's. Two batteries per Power In with a Class T per battery. We currently have a Lynx Shunt with a ANL fuse in it, but that is what Victron spec'd when we build the orginal system.

https://www.victronenergy.com/dc-dis...n#pd-nav-image

The ANL at the Lynx Shunt is not an issue. Keep in mind a fuse only protects things "downstream of it". So even if a short happened directly after the fuse the resistance of the short circuit includes the busbar and the main battery cable, battery switch, battery fuse, etc everything "upstream".

If someone wanted to use Class T fuses for things beyond the battery they certainly could but the potential current drops significantly the further from the battery you get. Class T provides the most benefit as a fuse to protect the battery and main battery cables.

[emilecantin]

Originally Posted by Cheyne

I have two of the new Victron Class-T Power In on pre-order for our Epoch upgrade that we are doing. I have four of the Epoch 460's. Two batteries per Power In with a Class T per battery. We currently have a Lynx Shunt with a ANL fuse in it, but that is what Victron spec'd when we build the orginal system.

https://www.victronenergy.com/dc-dis...n#pd-nav-image

Also the Epoch's each have a Class T fuse inside the battery. Picture stolen from Panbo.

Wow, how did I not see that Class-T Power In? Looks like a very nice product! It'll need to be mounted pretty close to the batteries to satisfy ABYC's 7" rule, though.


Good on Epoch for including such a fuse directly inside the battery, makes for a very safe installation. Looks like CAN communication (or anything other than Bluetooth, really) is reserved to their biggest model, but I guess it makes sense.


@Dave9111 if you don't want to buy fuses, I found a handy replacement chart for you :

[BlueH2Obound]

Originally Posted by Delta sailor

I'm very interested in your electrical set up. I'm at a crossroad of either installing a small generator or reworking my electrical for the sole purpose of AC. This boat is a freshwater boat that we spend long weekends on during the summer. My 12v system handles everything but we have no AC option.

My existing setup is a mere 200ah AGM set up with a 2kw inverter along with a 65amp original altenator. Boat is a 38ft Beneteau first 38 from the mid 80's so space is critical.

Question- what size AC can you run/ how long/ etc? And did you research yourself or hire outside contractor to help design/install?

We can run either of our AC units on solar during full sun light (not both) and still put a charge into the batteries. The Salon AC is a 16500 btu unit, and the master stateroom is 9000 btu's.

As the solar gets less productive as the day goes on, we switch over to the 9000 btu unit in the master stateroom, running from decreasing solar input, and inverter from the LiFePO4 house battery bank. We typically turn off the 9k btu unit around 9:00 pm or so, so run it exclusively off the batteries for 5 or 6 hours. The remaining house battery bank will cover all the overnight current draws on the two freezers, fridge, icemaker, radio, chart plotter (for the anchor alarm), lights, etc. As stated in a prior post, our boat (and power profile) is/are energy hogs!
We generally wake with about 40% SOC in the AM if we've used the AC in the evening/overnight. That will drop to say 35% SOC from coffee, microwave, Starlink, etc before the solar begins to charge the batteries again.

Regarding outside contractors for design/install of my electrical system . . . . over about 3 months, and speaking to 7 or 8 "experts" in LiFePO4 and Solar installs, it rapidly became apparent that those same "experts" were pretty clueless with regard to what it would take to design/install a safe and efficient system.
Even the two ABYC electricians I consulted wanted to do the work on a "Cost Plus" basis, with no estimated final price going into the project. Yeah, THAT wasn't going to happen . . . .
It was pretty apparent that all the “professionals” I spoke wanted me to fund their education into how to design and install a LiFePO4 system. They claimed to know what they were talking about, but it was obvious they didn't have a clue, as I stated above. One of them even asked “What is a BMS?” Seriously? Even when I explained what it was, it was pretty obvious he wasn't familiar with what a BMS was, or what it did . . . . This was from a “professional”.

So, my background, I'm an FAA rated Airframe and Powerplant Mechanic, and have taught Aircraft Systems and Turbine Engines for Embry Riddle Aeronautical University, so I have a passing knowledge of electrical systems. Enough that with a lot of studying, and assistance from several other people knowledgeable with boat electrical systems, I was able to design, source, and install an electrical system that meets or exceeds ABYC requirements for LiFePO4 batteries, as well as solar panel installation.

Best of luck as you go forward!

[Dave9111]

Originally Posted by Statistical

Seriously? You can afford a boat and the ongoing maintenance, repairs, and upgrades AND a large LFP bank and all the additonal changes requires for charging but you can't afford a $40 fuse?




Maybe not but what about a 300 Ah battery or a paid of 300Ah batteries in paralle?



You know what else will be melting the cells themselves and now you have an internal short circuit one that no fuse can protect you from.



You missed the entire point. You got all angry and righteous and didn't read.

I said you should have a larger (as in rated amps not AIC) fuse than the BMS. This will ensure the fuse will never blow other than a dead short. Of course using a larger fuse requires using a larger cable.

As an exmaple if you have a BMS capable of 200A max and you size your wire and fuse for 200A it is possible the fuse will blow. Remember you are the one who thinks losing a $40 fuse will financially cripple someone so we probably want to avoid that.

However if you have wiring and fuse capable of 300A then other than dead short there should be no scenario where the fuse blows. This staggering financial investment that





Your right you should compute the max possible current using the internal resistance of the cells used. For 12V only small house banks a MBRF fuse is likely sufficient. The issue is going from likely sufficient to confirmed sufficient is finding the data from the manufacturer on either the internal cell resistance (which determines maximum current) OR the manufacturer provides a max short circuit current. ABYC even recommends using that to determine the AIC. The problem is most manufacturers do not provide this.

For example my previous boat had two EVE 304Ah cells in parallel. Internal resistance is 0.5 mOhms so fully charged at 3.4V per cell that is a max current of 6,800A with two in parallel that is >10k. Even including resistance of the cell busbars it is still >10k amps and certainly greater than 5,000A. I would add that for 24V applications MRBF is rated at only 5,000A.

If you want to use the cheapest fuse you can find after spending thousands to upgrade the house bank on your boat which is worth tens of thousands or hundreds of thousands then go ahead. The reality is you will probably be ok. Personally though I work hard and invest my money so that I don't have to take out a mortgage to buy a $40 Class T fuse.

I mean I could see if you went on this rant because I was "demanding" people use $1,200 isolation transformer we are talking a $40 fuse. To be clear though the only place Class T fuses are "needed" (or maybe I should say advised so you won't start foaming at the mouth and throwing off personal attacks again) is on the batteries themselves. From the primary busbar and certainly the branch circuits the additional resistance of the wiring between the battery terminals and the possible point of any fuse means short circuit current will be lower potentially a magnitude or more lower.

Blue Sea (actually Busman a solid brand rebranded as Blue Sea) 300A class T fuse ($38)
https://www.amazon.com/Blue-Sea-Syst.../dp/B000XBGT3S

Blue Seas 300A MBRF fuse ($18)
https://www.amazon.com/Blue-Sea-Syst...dp/B0017SJOH4/

So your freak out was based on someone recommending a $38 part over an $18 part. A part which if done properly (ensuring it is higher than than the max current allowed by the BMS) is likely a once in a lifetime part short of an emergency.

Wow, who is angry?? Yeah, Its not me.

I guess I got under your skin just a bit.

Question: When you take out your boat and you need to put on a life jacket, do you put on one, and then a second one, and then a third one just to be extra, extra safe? Or are you ok with just one?

You stated that everyone should always use a T class fuse because lithium batteries can produce oodles of short circuit current.

You stated that without any references at all.

Obviously that is wrong.

MRBF fuses have a lot of advantages since they are right at the battery terminal. But you need to stay within the capabilities of the fuse.

MRBF fuses have a place as do Class T fuses.

The user needs to figure out what to do. The battery manufacturers aren't providing much help.

But telling everyone that a Class T fuse is needed when all they want is to use a single 12 volt, 100 ah battery in their rubber raft so they can get to the dock with their trolling motor, doesn't make any sense.

If the battery costs $160 and the trolling motor costs $120, who wants to pay $50+ dollars for a T class fuse and holder that is entirely unnecessary. Plus its big. An MRBF fuse would be much more appropriate. And even that could be considered an overkill for that application.

Regarding other larger lithium battery banks, they need to be looked at individually for voltage, max short circuit current available at each point in the system, etc.

Here are some videos for those of you want to see what these things look like.

https://www.youtube.com/watch?v=z_2FoJHw0CA

https://www.youtube.com/watch?v=NAInCXHONwk

https://www.youtube.com/watch?v=Q54HRkXhL5Y

>
For example my previous boat had two EVE 304Ah cells in parallel. Internal resistance is 0.5 mOhms so fully charged at 3.4V per cell that is a max current of 6,800A with two in parallel that is >10k. Even including resistance of the cell busbars it is still >10k amps and certainly greater than 5,000A. I would add that for 24V applications MRBF is rated at only 5,000A. <

I'm curious as to why you would have this setup? What voltage was your battery? Paralleling cells like that can be a bad idea unless they are closely matched.

If you read the paper that I quoted you would realize that you are likely way off in your calculations. The cell in that paper was a 160ah and its short circuit current was a bit less than 1000 amps. If you extrapolate his test results, your two 304 ah cell based battery should produce less than 4000 amps of short circuit current. Add in any wiring, which has to be present to make the cells useful and the short circuit current falls further. So I'm at less than 4000 amps and you are at more than 10K amps. You should know that your battery terminals won't withstand anything close to 10K amps. 10K amps simply isn't happening. I've been around a lot of high power AC and DC systems and you don't just pump 10K amps through small conductors. It just doesn't happen.

Still why were you using raw cells in your boat? Did you put these into some type of battery box? I would hope so. It can be difficult to make something like that safe. Plus obviously you would need to add a BMS along with an enclosure to be safe.

[emilecantin]

Originally Posted by Dave9111

Wow, who is angry?? Yeah, Its not me.

I guess I got under your skin just a bit.

Question: When you take out your boat and you need to put on a life jacket, do you put on one, and then a second one, and then a third one just to be extra, extra safe? Or are you ok with just one?

You stated that everyone should always use a T class fuse because lithium batteries can produce oodles of short circuit current.

You stated that without any references at all.

Obviously that is wrong.

MRBF fuses have a lot of advantages since they are right at the battery terminal. But you need to stay within the capabilities of the fuse.

MRBF fuses have a place as do Class T fuses.

The user needs to figure out what to do. The battery manufacturers aren't providing much help.

But telling everyone that a Class T fuse is needed when all they want is to use a single 12 volt, 100 ah battery in their rubber raft so they can get to the dock with their trolling motor, doesn't make any sense.

If the battery costs $160 and the trolling motor costs $120, who wants to pay $50+ dollars for a T class fuse and holder that is entirely unnecessary. Plus its big. An MRBF fuse would be much more appropriate. And even that could be considered an overkill for that application.

Regarding other larger lithium battery banks, they need to be looked at individually for voltage, max short circuit current available at each point in the system, etc.

Here are some videos for those of you want to see what these things look like.

>
For example my previous boat had two EVE 304Ah cells in parallel. Internal resistance is 0.5 mOhms so fully charged at 3.4V per cell that is a max current of 6,800A with two in parallel that is >10k. Even including resistance of the cell busbars it is still >10k amps and certainly greater than 5,000A. I would add that for 24V applications MRBF is rated at only 5,000A. <

I'm curious as to why you would have this setup? What voltage was your battery? Paralleling cells like that can be a bad idea unless they are closely matched.

If you read the paper that I quoted you would realize that you are likely way off in your calculations. The cell in that paper was a 160ah and its short circuit current was a bit less than 1000 amps. If you extrapolate his test results, your two 304 ah cell based battery should produce less than 4000 amps of short circuit current. Add in any wiring, which has to be present to make the cells useful and the short circuit current falls further. So I'm at less than 4000 amps and you are at more than 10K amps. You should know that your battery terminals won't withstand anything close to 10K amps. 10K amps simply isn't happening. I've been around a lot of high power AC and DC systems and you don't just pump 10K amps through small conductors. It just doesn't happen.

Still why were you using raw cells in your boat? Did you put these into some type of battery box? I would hope so. It can be difficult to make something like that safe. Plus obviously you would need to add a BMS along with an enclosure to be safe.

I think you're missing the point here. By your own calculations, a short could potentially run up to the 4000 amp range. Do you really want to rely on a MEGA fuse that's only rated to interrupt 5000A ? That's cutting it a bit too close for me.


An MRBF fuse (10,000AIC) is probably sufficient, though: we've seen it work in some youtube videos, and as you mention they're reasonably priced. My only issue with them is the form factor. They can be convenient, true, but it's easy to imagine a failure mode where the battery cable comes loose from the fuse, and contacts the battery post directly. It's far-fetched, yes, but eliminating a possible failure mode is worth it in my opinion, so I went with Class T on my boat.

[Statistical]

Originally Posted by emilecantin

I think you're missing the point here. By your own calculations, a short could potentially run up to the 4000 amp range. Do you really want to rely on a MEGA fuse that's only rated to interrupt 5000A ? That's cutting it a bit too close for me.


An MRBF fuse (10,000AIC) is probably sufficient, though: we've seen it work in some youtube videos, and as you mention they're reasonably priced. My only issue with them is the form factor. They can be convenient, true, but it's easy to imagine a failure mode where the battery cable comes loose from the fuse, and contacts the battery post directly. It's far-fetched, yes, but eliminating a possible failure mode is worth it in my opinion, so I went with Class T on my boat.

One other thing I like about Class-T fuses is the holders tend to be pretty chonky busbars. Wire heating under high load is not constant across the wire. The connector tend to get hotter. The fuse itself also produces heat due to resistance. So the junction of wire to lug to stud to fuse holder to fuse is a hotspot. It will get hotter than the middle of the wire for any given load. Class-T fuse holders tend to be thick busbars with lots of surface area. This by conduction and then radiation keeps the fuse and wire cooler. There is a reason they are the standard for industrial high power applications well beyond the boating world.

I would add that MRBF are only 5kA AIC on 24V+ systems. That is a lot but as you point out not that much headroom on a large system even being optimistic about max current in a dead short. Contrary to the prior poster I never said 100% of people 100% of time must use Class-T. I said for those with large systems or higher voltages they should use a Class-T. For those that have smaller 12V banks MRBF is likely fine although upgrading to a Class-T can provide peace of mind and the cost difference is negligible.

I would add that the prior estimate of no more than 4 kA in a dead short is based on a study where the tester observed ~1kA however that cell had higher internal resistance. Also while the study (yes I read it) didn't lay out their testing method the test photos show the short was created by connecting two wires to the cell terminals and then shorting the wires. The wires themselves add resistance. The internal resistance of LFP cells is so low that even a few feet of wiring can be more potentially multiples more resistance than the cells. As a result a dead short across the battery terminals could be much higher.

Am I saying MBRF is a deathtrap? No. In fact given how terrible most wiring is on most boats and the fact that even with all kinds of dangerous mistakes they mostly don't burn down the odds are likely one would be fine with MBRF or even MEGA fuse. The question though is why?

For most people converting to lithium is quite expensive. They are spending hundreds or thousands on the batteries, likely need DC to DC charger to handle starter battery, may need new AC chargers, or upgrade to externally regulated alternators, new battery monitoring system, often going with new wiring too because things needs to be moved. If you aren't doing the work yourself you have dozens of hours of boatyard labor on top of that. Despite the term "drop in replacement" it isn't as simple (in most cases) as buy a $300 LFP battery and connect it. If spending thousands of dollars is the place to save $20 really the primary battery fuse? I would say no.

Class-T (or MBRF) are really only needed at the battery itself. Once you get to bussbars or branch circuits the increased resistance from the wiring between the battery means that for those additional downstream fuses peak current is much lower and as such like ANL or MEGA (would recommend ANL) with lower AIC is fine.

So it isn't like the recommendation is replace every fuse on your boat with a class-T fuse it is use Class-T for the primary battery fuse meaning for most boats we are talking 2 or 3. If sized properly and using a BMS it is likely a one time cost.

[Dave9111]

>
I think you're missing the point here. By your own calculations, a short could potentially run up to the 4000 amp range. Do you really want to rely on a MEGA fuse that's only rated to interrupt 5000A ? That's cutting it a bit too close for me.


An MRBF fuse (10,000AIC) is probably sufficient, though: we've seen it work in some youtube videos, and as you mention they're reasonably priced. My only issue with them is the form factor. They can be convenient, true, but it's easy to imagine a failure mode where the battery cable comes loose from the fuse, and contacts the battery post directly. It's far-fetched, yes, but eliminating a possible failure mode is worth it in my opinion, so I went with Class T on my boat.
<

This thread is running pretty long and it might be confusing, but I never recommended a MEGA fuse. So I'm not sure what you are talking about.

My only point is that a T class fuse is not always required. In fact for most marine applications I suspect that it is an overkill.

If the actual maximum short circuit current is 4000 amps, then using a fuse that has a rating of 5000 AIC, is not "cutting it close" at all.

The 5000 AIC rating of a fuse is the MINIMUM AIC of the fuse. It can likely interrupt a LOT more than 5000 amps. This is just how fuses are rated.

The other issue is who made your fuse? Was it made by a reputable company who actually makes a quality product or is it a fake?
Did you buy it off Amazon or Ebay because it was "really" cheap?

There are fuses out there that are made by companies that are not quality devices.

If you have a fake, you may not have a fuse at all.

If you are interested in all of this, then here is more reading.

https://diysolarforum.com/threads/li...tection.14343/

One other thing I want to point out is that there are some drawbacks to Class T fuses as well that are not obvious. But you can read about those in the link above.

[Dave9111]

Also, one more point.
I don't know how everyone runs their boat, but I keep spares onboard. If I install a fuse on my boat, I will need at least two fuses, not one.

[Statistical]

Originally Posted by Dave9111

Also, one more point.
I don't know how everyone runs their boat, but I keep spares onboard. If I install a fuse on my boat, I will need at least two fuses, not one.

Oh no not an extra $25 on that thousand dollar electrical system upgrade.

https://shop.pkys.com/Blue-Sea-5119-...mp_p_1668.html

https://shop.pkys.com/Blue-Sea-5190-...mp_p_1638.html