LFP Battery and Battery Bank Fusing

[tpbrady]

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

 

[CPD]

In my case, which I believe to be the norm, I did not fuse the (very short) parallel connections between the House Bank batteries. Fused only the final connection of the House Bank to the 600-amp “Charging” busbar. The sequence of consideration should be your determination of worse-case current draw, which then should determine your wire gauge, and then fusing is with respect to the gauge of wire you have selected.

 

[tiltrider1]

This is a good question.

I watched a YouTuber rationalize that his BMS acted as a fuse and his inverter charger had built in circuit safety so no fuse was needed. However, I don’t know too many surveyors who will accept this logic.

As I interpret ABYC a class T after the buss bar is acceptable as long as the runs are within the max length. I have always wired a bypass switch that allows me to go direct from house bank to start bank by passing the fuse should I ever have a start emergency requiring the house bank.

Putting a T fuse on each battery is the more complicated & expensive route but potentially safer and more flexible.

When I say buss bar I am assuming the bus bar is only for paralleling the battery bank, not a house buss bar.

 

[Delta Riverat]

I'm running option 2 on my setup.

 

[Fleming]

I'm a fan of option 2.
I agree with Tiltrider1 that a BMS does not count as overcurrent protection.
Remember the 7-40-72 rule. If you're fuse is not within 7 inches of the positive terminal of the battery then it needs to be in a sheath and the fuse must be within 72 inches of the battery.

 

[J.T kearney]

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

Tom
I'm not a marine sparky, but you should be aware of the AIC limit on the MRBF fuse holder. There was mention of this on another thread. Check out Blue Seas spec's on this because of arching potential. It was mentioned that ABYC wanted 5000 AIC per 100 amps with lifepo4 batteries so you'll be over the MRBF rating. I also would not think off the BMS as a fuse ( even though some have incorporated them with a BMS).
Cheers J.T.

 

[tpbrady]

JT,

I’m leaning toward option 3 because of the AIC limits of MRBF fuses. If I do that the only way to meet the 7 inch rule with the class T fuse is to double side tape them to the top of the battery. The problem with standards is sometimes the industry hasn’t built something to meet that standard in the space normally available.

Tom

 

[twistedtree]

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

- A battery is essentially an unlimited current source, so has to be fused to protect the battery cable. This is what's behind the 7", or 72"+sheathing rule. I've never thought about what happens if you reach a buss bar before the distance limit, but I'd still be inclined to include a fuse just as though if you didn't meet the distance limit. Without fuses, the potential current and AIC requirements increase for every battery you land on the buss bar, so the problem just gets harder. My boat is built with on end of the fuses bolted to the buss bar, just as an example. This is all consistent with new language in E-11 that requires intermediate fusing when paralleling large banks in order to meet AIC requirements.

- And because the battery is essentially an unlimited current source, the battery fuses need to have sufficient AIC ratings to ensure that they actually open in the event of a fault.

- For the loads coming off the buss bar, fusing should be based on the capacity of the downstream conductor.

MRBF and mega fuses are a bit of a dilemma since they are widely used, but often not fully AIC compliant.

As for a BMS fulfilling fuse requirements, the answer is a flat-out No as far as ABYC is concerned. The new draft of E-11 allows for electronic circuit breakers, but they are restricted to branch circuits.

Using a BMS as the required battery disconnect device is evolving. In the published E-13 the battery disconnect must be separate from and in addition to the BMS disconnect. The current draft allows the BMS disconnect to double as the battery disconnect under certain circumstances. Key is that it must be a mechanical contactor, not solid state. It must be normally open and return to open if power is interrupted (normally open, monostable). The control to meet the E-11 battery disconnect requirements must directly interrupt power to the BMS contactor.

 

[J.T kearney]

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

- A battery is essentially an unlimited current source, so has to be fused to protect the battery cable. This is what's behind the 7", or 72"+sheathing rule. I've never thought about what happens if you reach a buss bar before the distance limit, but I'd still be inclined to include a fuse just as though if you didn't meet the distance limit. Without fuses, the potential current and AIC requirements increase for every battery you land on the buss bar, so the problem just gets harder. My boat is built with on end of the fuses bolted to the buss bar, just as an example. This is all consistent with new language in E-11 that requires intermediate fusing when paralleling large banks in order to meet AIC requirements.

- And because the battery is essentially an unlimited current source, the battery fuses need to have sufficient AIC ratings to ensure that they actually open in the event of a fault.

- For the loads coming off the buss bar, fusing should be based on the capacity of the downstream conductor.

MRBF and mega fuses are a bit of a dilemma since they are widely used, but often not fully AIC compliant.

As for a BMS fulfilling fuse requirements, the answer is a flat-out No as far as ABYC is concerned. The new draft of E-11 allows for electronic circuit breakers, but they are restricted to branch circuits.

Using a BMS as the required battery disconnect device is evolving. In the published E-13 the battery disconnect must be separate from and in addition to the BMS disconnect. The current draft allows the BMS disconnect to double as the battery disconnect under certain circumstances. Key is that it must be a mechanical contactor, not solid state. It must be normally open and return to open if power is interrupted (normally open, monostable). The control to meet the E-11 battery disconnect requirements must directly interrupt power to the BMS contactor.

Mr Twisted Tree
If there is room to mount T fuses inside of the battery box for each battery (mounted on the inside of the box)would that be considered ABYC compliant? If there was room preferably on the outside of the battery box!!
Cheers J.T

 

[twistedtree]

Mr Twisted Tree
If there is room to mount T fuses inside of the battery box for each battery (mounted on the inside of the box)would that be considered ABYC compliant? If there was room preferably on the outside of the battery box!!
Cheers J.T

I think if the fuse meets the AIC requirements, and the cable lengths and sheathing meet the requirements, then yes. But I don't speak for ABYC, just for myself.

I don't think ABYC says anything about it, but consider access both for inspection and replacement.

Charlie, what do you think?

 

[SteveK]

J.T. Are you asking from a safety viewpoint, what if the fuse blows that close to the battery.
There are plenty of discussions lately of batteries with built in fuse, of using MRBF on battery post.

 

[twistedtree]

I think if the fuse meets the AIC requirements, and the cable lengths and sheathing meet the requirements, then yes. But I don't speak for ABYC, just for myself.

I don't think ABYC says anything about it, but consider access both for inspection and replacement.

Charlie, what do you think?

Thinking about this some more, I checked and E-11 requires over current protection devices to be "readily accessible". And the definition for readily accessible is "capable of being reached quickly and safely for effective use under emergency conditions without the use of tools."

The few videos that I've seen of internal fuses in batteries I don't think would meet this requirement. Of course it would all depend on the construction of the battery, but the examples I've seen look like you have to remove screws to get the battery cover off, and I think you probably have to disconnect the battery cables as well to get the cover off.

 

[tpbrady]

The 7 inch requirement is really hard to meet and still use a battery box unless you only use half the box and mount the fuse on the side of the box. Just the bend radius of the cable makes it hard. If you don’t use a box and strap the batteries down, you then need some surface to mount the fuse holder on. With the fuse holder on top of the battery they are accessible by popping the straps holding the box down and removing the lid. It won’t be necessarily quick, but the purpose of the fuse is to take the quick out of it. Once the fuse does its job we are now into troubleshooting and repair.

Tom

 

[luna]

Thinking about this some more, I checked and E-11 requires over current protection devices to be "readily accessible". And the definition for readily accessible is "capable of being reached quickly and safely for effective use under emergency conditions without the use of tools."

I think the "readily accessible...............use of tools" clause should only apply the resettable circuit breakers.

Even though I can quickly and safely reach my Class T fuses, I can't envision any method where I would have active "effective use" of them or do anything meaningful to them, without the use of tools.

That does not mean I would accept that an internally mounted fuse as the only OCP on my batteries.

 

[heysteve]

I'm a bit confused by this whole Class T fuse placement issue for LFP. Are we saying that each LFP battery needs its own Class T fuse or the busbar needs a Class T?

My existing setup has an MBRF (10000 AIC) on each of four LFP batteries with individual wires to a busbar. The main DC feed then has an ANL (6000 AIC) heading to the panel and a Class T heading to the inverter/charger. Perhaps I should replace the ANL with a Class T but there doesn't seem to be much point in that if each battery needs a Class T.

I sometimes wonder if the requirements are just chasing vanishingly small risks. Realistically, what are the chances that the BMS circuit protection failing and the MBRF getting welded shut?

 

[SteveK]

Thinking about this some more, I checked and E-11 requires over current protection devices to be "readily accessible". And the definition for readily accessible is "capable of being reached quickly and safely for effective use under emergency conditions without the use of tools."

TT, please quote the E-11. This may be further interpretation of the written word issue. I cannot image how a class T fuse is "readily accessible" in any mounted location more so than another mounting location. Once it blows, tools are required.

I was about to relocate my Class T fuse from the current position to inside the box for the more protected location, now will wait to see how this plays out.

 

[J.T kearney]

I'm a bit confused by this whole Class T fuse placement issue for LFP. Are we saying that each LFP battery needs its own Class T fuse or the busbar needs a Class T?

My existing setup has an MBRF (10000 AIC) on each of four LFP batteries with individual wires to a busbar. The main DC feed then has an ANL (6000 AIC) heading to the panel and a Class T heading to the inverter/charger. Perhaps I should replace the ANL with a Class T but there doesn't seem to be much point in that if each battery needs a Class T.

I sometimes wonder if the requirements are just chasing vanishingly small risks. Realistically, what are the chances that the BMS circuit protection failing and the MBRF getting welded shut?

Heysteve
Definitely change that ANL fuse to a class T fuse between your buss bar and house panel. How many amps is your house bank?
Cheers J.T.

 

[J.T kearney]

J.T. Are you asking from a safety viewpoint, what if the fuse blows that close to the battery.
There are plenty of discussions lately of batteries with built in fuse, of using MRBF on battery post.

SteveK
I was trying to come up with a mounting solution for class T fuses within 7 inches of the lifepo4 battery, thinking inside and outside the box so to speak, to see if anyone has a solution or ideas
Cheers J.T.

 

[tiltrider1]

All my cabling between my inverter/charger, batteries and house bus bar is 4/0 wire. Makes it impossible to have any tight bends in-the first 7 inches. My T fuse is 36” away so all wires are sheathed.

Then again all my wires are sheathed, just a good practice.

 

[heysteve]

Heysteve
Definitely change that ANL fuse to a class T fuse between your buss bar and house panel. How many amps is your house bank?
Cheers J.T.

I have four SOK 206AH LFPs connected to a bus bar (not directly to each other). My only regret is they now sell for half of what I paid

 

[twistedtree]

I think the "readily accessible...............use of tools" clause should only apply the resettable circuit breakers.

Even though I can quickly and safely reach my Class T fuses, I can't envision any method where I would have active "effective use" of them or do anything meaningful to them, without the use of tools.

That does not mean I would accept that an internally mounted fuse as the only OCP on my batteries.

I think the access is as much to check to see if the fuse is blown or breaker is tripped, as it is ease of repair. Replacing a class T, ANL, Mega, etc. fuse will require tools.

 

[Marco Flamingo]

JT,

If I do that the only way to meet the 7 inch rule with the class T fuse is to double side tape them to the top of the battery. The problem with standards is sometimes the industry hasn’t built something to meet that standard in the space normally available.

Tom

I ran into this on my LFP installation. Going to LFP makes even more sense if you can take advantage of not needing routine access to the top of the batteries as with FLA. Using an MBRF fuse on each battery terminal (assuming smaller LFP batteries) still presents the problem of access and the need for tools. One can get away from the access issue if following the ABYC guidelines that allow the fuse within 72 inches if the cable is "sheathed." That still presents several issues. First, a class T fuse <72" away still requires tools. Second, the ABYC ampacity rating for a cable is reduced if it is in a bundle, conduit, or sheathing. Bundle or conduit needs no definition and the purpose can be deduced. Both allow heat concentration, and therefore derate the cable's ampacity by softening the insulation. Sheathing, however, can be for a single unbundled conductor and its purpose (I would think) is for protection of the cable. But the term "sheathing" can apply to a lot of different materials, including some that are both protective and ventilated. I deciphered the ABYC language as battery-to-fuse "less than 72 inches if a single conductor covered by a protective sheathing that doesn't trap heat." That's just a common sense interpretation and I don't actually know if common sense is allowed.

All that was left was figuring out a class T or better fuse that doesn't require tools. Those are called DC specific circuit breakers. I believe ABYC recommends 5,000 AIC for each 100Ah of LFP battery. My banks are 560Ah each, so I would need two DC breakers with a 28,000 AIC or better. I chose ones with 35,000 AIC that are about the same size as class T but provide beyond the necessary rating.

The industry (usually the EV market) has built the items necessary to harness a large LFP battery bank. The items are just not the norm (yet) and might be out of the ABYC's line of sight.

 

[twistedtree]

I'm a bit confused by this whole Class T fuse placement issue for LFP. Are we saying that each LFP battery needs its own Class T fuse or the busbar needs a Class T?

My existing setup has an MBRF (10000 AIC) on each of four LFP batteries with individual wires to a busbar. The main DC feed then has an ANL (6000 AIC) heading to the panel and a Class T heading to the inverter/charger. Perhaps I should replace the ANL with a Class T but there doesn't seem to be much point in that if each battery needs a Class T.

I sometimes wonder if the requirements are just chasing vanishingly small risks. Realistically, what are the chances that the BMS circuit protection failing and the MBRF getting welded shut?

Batteries are essentially an unlimited current source, so they need to be fused to keep all that current from getting out in an uncontrolled manner. That's why the cable from the battery or battery bank needs to be fused. The AIC rating is because in a fault condition there can be so much current that the fuse can't successfully interrupt the flow. So as you wire batteries together in parallel, you need to fuse them before their short circuit current rating exceeds the AIC rating of whatever fuse you plan to use. Class T fuses have high AIC ratings, so are usually selected.

With LFP batteries is takes less battery capacity to reach very high short circuit current capabilities, and hence demand a very high AIC fuse. Breaking up large battery banks has always required fusing them in groups to meet the requirement, but the upcoming E-11 now talks about this explicitly since it's a much more common need with LFP. Also, because very few LFP manufacturers specify their short circuit rating, even though it's required, there are now guidelines based on battery Ah capacity for sizing short circuit current.

The BMS keeps the battery safe, but it knows nothing about what's connected to the battery. The fuse keeps all the connected stuff safe. You can't count on either doing the other's job.

 

[twistedtree]

TT, please quote the E-11. This may be further interpretation of the written word issue. I cannot image how a class T fuse is "readily accessible" in any mounted location more so than another mounting location. Once it blows, tools are required.

I was about to relocate my Class T fuse from the current position to inside the box for the more protected location, now will wait to see how this plays out.

11.4.1 Accessible - capable of being reached for inspection, removal, or maintenance without removal of permanent boat structure.

11.4.37 Readily Accessible - capable of being reached quickly and safely for effective use under emergency conditions without the use of tools.

11.10.1.2 Overcurrent protection devices shall be readily accessible.

EXCEPTION: Following overcurrent protection devices are allowed to be accessible:
1. Overcurrent protection devices within an electrical device or appliance.
2. Overcurrent protection devices that are reset remotely such as digitally controlled devices.
3. Overcurrent protection devices requiring guarding of energized parts and protection against accidental contact in accordance with E-11.5.
4. Overcurrent protection devices requiring ignition protection if installed in spaces where ignition protection is required.

NOTE: Accessible overcurrent protection devices may require use of tools for replacement.

TT's note: The definitions talk about the device being "reachable", not about what's require to change or replace it.

 

[twistedtree]

The 7 inch requirement is really hard to meet and still use a battery box unless you only use half the box and mount the fuse on the side of the box. Just the bend radius of the cable makes it hard. If you don’t use a box and strap the batteries down, you then need some surface to mount the fuse holder on. With the fuse holder on top of the battery they are accessible by popping the straps holding the box down and removing the lid. It won’t be necessarily quick, but the purpose of the fuse is to take the quick out of it. Once the fuse does its job we are now into troubleshooting and repair.

Tom

I think everyone agrees that the 7" rule is tough to meet, and that's why you are allowed up to 72" if you sheath the cables. I don't think I have ever seen a fuse within 7", and instead always sheathed cable.

 

[NoRain]

11.4.37 Readily Accessible - capable of being reached quickly and safely for effective use under emergency conditions without the use of tools

I would be very interested in the thinking that resulted in this. What is the point of being able to * quickly reach* the Class T fuse assembly without tools if you are unable to *do anything* to it without tools. This strikes me as illogical.

 

[SteveK]

11.4.37 Readily Accessible - capable of being reached quickly and safely for effective use under emergency conditions without the use of tools

I would be very interested in the thinking that resulted in this. What is the point of being able to * quickly reach* the Class T fuse assembly without tools if you are unable to *do anything* to it without tools. This strikes me as illogical.

As time goes by I am beginning to understand the thinking and the words that result. In this case No tools should be needed to put your hand on the fuse. In other words tool free. Does not mean it cannot be in a box where the lid can be removed without tools. Does mean cannot be out of sight until you use tools to get to it. I am ESL so look at the written word literally often wondering like you, what were they thinking.
I love the sheating references as well, Ordinary electrical tape wound overlapping on the cable is ABYC approved sheating. But when you read or hear sheating that is the last thing that pops into your mind.

 

[luna]

11.10.1.2 Overcurrent protection devices shall be readily accessible.

EXCEPTION: Following overcurrent protection devices are allowed to be accessible:
1. Overcurrent protection devices within an electrical device or appliance.
2. Overcurrent protection devices that are reset remotely such as digitally controlled devices.
3. Overcurrent protection devices requiring guarding of energized parts and protection against accidental contact in accordance with E-11.5.
4. Overcurrent protection devices requiring ignition protection if installed in spaces where ignition protection is required.

NOTE: Accessible overcurrent protection devices may require use of tools for replacement.

TT's note: The definitions talk about the device being "reachable", not about what's require to change or replace it.

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.

 

[Iggy]

I used this for a Pos buss bar and protection. https://www.amazon.com/dp/B0753F35Q8?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_6&th=1

I know the buss bar is rated for 300A. I used 200Amp fuses in the block. Just in case I needed to use the house bank to start the engine. In 3 yrs I haven't thank God! The Max output per battery is 200a.

Don't know if this helps any one? But it seamed the simplest with protection.

 

[twistedtree]

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.

I posted the definition in Post #24

I don't know that I can shed much more light on this, but I think the goal of this stuff being accessible is to prevent it from being hidden behind screwed down panels that are hard to get to, and where nobody knows it's hiding there in the first place. We have all experienced stuff squirled away in unexpected, and difficult to access locations. And although I agree one still needs tools to change most large fuses, the accessibility requirements is as much about being able to inspect the fuse to see if it's blow (or if a breaker is tripped), as it is about resetting the breaker or replacing the fuse.