Fuses for LiFePO4

[Col_T]

The boat was fitted with 5 * 110Ah sealed, lead acid batteries, for the domestic supply, when we bought it. I am now contemplating replacing those with a single Fogstar 300Ah LiFePO4 battery. My instinct is that I will not need to alter the fuses as part of this upgrade - is that correct?

 

As a supplementary question, the intention is to charge from our solar panels, initially, at least, so I’m thinking of swapping out my Epever MPPT solar controller for a Victron SmartSolar MPPT, to take advantage of the built-in lithium charging algorithm. I am wondering about altering the full-charge cutoff value in the MPPT to be 0.05v lower than that specified for the BMS in the battery, the idea being that the BMS cutoff would only trigger as a last resort, rather than being a daily occurrence. Good idea, or an unnecessary complication?

[nicknorman]

The fuse rating in terms of amps will not need changing. What might be a consideration is the type of fuse. A fuse in fact has 2 ratings, the amount of current at which it will blow, but also the maximum current it can interrupt. A 200A fuse will blow when presented with 5000A from a lithium battery short circuit, but having blown will it interrupt the current or will the current simply arc over the blown part of the fuse in a plasma and continue on its way? This is where a Class T fuse is appropriate, since it has a much higher maximum interrupt current. All that said, the BMS within the battery should protect against a short circuit so there is probably no need to change the type of fuse.

Regarding the MPPT, I think what you propose is a good idea in principle but I think 0.05v is too small a margin since these things are not particularly precise. I would set it at least 0.1v below and maybe a bit more, eg 0.2v. You will not lose any significant amount of capacity from the battery.

[wakey_wake]

1 hour ago, Col_T said:

The boat was fitted with 5 * 110Ah sealed, lead acid batteries, for the domestic supply, when we bought it. I am now contemplating replacing those with a single Fogstar 300Ah LiFePO4 battery.

Capacity-wise, 5*55 = 275 vs 300, seems a fair choice.

 

For peak current, e.g. for a large inverter, check that the BMS will comfortably handle it. Some larger-capacity (e.g. 300Ah) LFP batteries have a BMS which cannot handle a 1C discharge or C/2 charge (e.g. 50A charge/100A discharge). In any case I think it's wise to aim well under the nominal rating here.

 

1 hour ago, Col_T said:

My instinct is that I will not need to alter the fuses as part of this upgrade - is that correct?

 

Exactly as Nick says, you need to be sure your fuse will clear a worst case fault current, such as if the BMS has already died without your noticing, and has become permanently "on".

 

If you like details or flying sparks, Will Prowse on YT did some tests shorting large 48V banks into various fuses with an assortment of results, but I didn't find it very conclusive or thorough. In several cases the BMS seems to react before the fuse had reached its I²t.

 

My solution was one of these "for protection against overload up to a short-circuit current of 7.5 kA" per 12.8V 100Ah unit,

https://www.mouser.co.uk/ProductDetail/651-1019992 good to 60V DC. You can't just use any old AC breaker for this, you have to check the DC specs.

 

I believe there are two classes of "right answer",

one where you believe you can tell the BSS or insurance company that it's done competently / to manufacturer's instructions / etc.

and the other where you believe you've learned all that's relevant and taken reasonable steps to prevent it burning your boat. I went for the latter and hope it will satisfy the former when the time comes.

 

1 hour ago, Col_T said:

I’m thinking of swapping out my Epever MPPT solar controller for a Victron SmartSolar MPPT, to take advantage of the built-in lithium charging algorithm.

I don't know much about either, but I would be surprised if the Epever couldn't be told to stop at a voltage of your choosing.

 

1 hour ago, Col_T said:

I am wondering about altering the full-charge cutoff value in the MPPT to be 0.05v lower than that specified for the BMS in the battery, the idea being that the BMS cutoff would only trigger as a last resort, rather than being a daily occurrence. Good idea, or an unnecessary complication?

This was my original plan, although I was going to aim maybe 0.3V lower, but 👇

 

It sounds like you already reached the sensible conclusion that the BMS should not be relied upon for charge termination - it's a safety device, and one prefers it to be a second line of defence.

 

After this thread (link jumping in at a mention of Fogstar) I believe it's a sensible battery life and possibly safety choice to aim quite a lot lower. The loss in capacity is minimal, as can be seen looking at the area under the curve.

 

[jonathanA]

I'd ignore the techno babble ^^.

 

Wot Nick said about fuses. I'd certainly think swapping to victron smart solar is worthwhile.

 

ISTR the gen2 frogspawns have a 250A BMS so will handle a reasonably  meaty inverter. 

[Col_T]

Thanks for the feedback - time to look into class T fuses.

[GUMPY]

14 hours ago, Col_T said:

As a supplementary question, the intention is to charge from our solar panels, initially, at least, so I’m thinking of swapping out my Epever MPPT solar controller for a Victron SmartSolar MPPT, to take advantage of the built-in lithium charging algorithm. I am wondering about altering the full-charge cutoff value in the MPPT to be 0.05v lower than that specified for the BMS in the battery, the idea being that the BMS cutoff would only trigger as a last resort, rather than being a daily occurrence. Good idea, or an unnecessary complication?

My Victron MPPT system charges to 14.2v stays at that for 30 minutes then drops back to 13.5v. the BMS cut off is 14.6v so never triggers from Solar.  MPPT  charges to 100% then once the loads have run for a short while drops back to 99% and stays there whilst the sun is out.

The same happens with the mains charger and they both talk to each other to prioritise the charge source.

At present we are on shore power as it's free and the sun has not been that good.

 

[Russ T]

You don't say which model of Epever MPPT you have, but some models have user defined charge parameters that may be suitable for LFP batteries. It would certainly be worth checking before replacing it for a victron. Though I must admit, I am a big fan of victron stuff.

Class-T fuse, as Nick said is the best option. Placement of the fuse is also worth considering, as is buying a good quality fuse and holder.

[blackrose]

2 hours ago, Russ T said:

You don't say which model of Epever MPPT you have, but some models have user defined charge parameters that may be suitable for LFP batteries. It would certainly be worth checking before replacing it for a victron. Though I must admit, I am a big fan of victron stuff.

 

 

Yes, the MT50 remote meter plugs into most Epever MPPTs and should allow you to set user defined parameters or certainly make it a lot easier than trying to do it on the MPPT itself. They only cost about 30 quid which is a lot cheaper than a new Victron unit.

 

 

https://www.amazon.co.uk/Monitor-Solar-Controller-Display-Monitoring/dp/B06XG3TNNW

 

 

Edited May 3 by blackrose

[Col_T]

3 hours ago, Russ T said:

Placement of the fuse is also worth considering, as is buying a good quality fuse and holder.

At present, all fuses are located within the battery box, and I was thinking of putting the class T fuse in there. Is there a better location?

[Russ T]

23 minutes ago, Col_T said:

At present, all fuses are located within the battery box, and I was thinking of putting the class T fuse in there. Is there a better location?

I think you want the fuse to be as close to the source of power as possible. Ideally within 7 inches if the cable is not in a conduit. For batteries that gas, I believe, unless the fuse is ignition protected, they are better off outside the battery box, but within 7 inches. It is worth checking the BSS documentation (I don't think it stipulates a distance), and of course, LFP batteries don't gas as LA batteries do.

 

Will you be keeping a standard LA in the same enclosure?

Edited May 3 by Russ T

[Paringa]

Can i ask about MRBF fuses and their suitability please?

 

Mounted on the battery so no unprotected cable. Higher interrupt current than a Mega Fuse (but not as high as a T) and cheaper than class T.

 

And is there any merit on having a Class T for the "meltdown" but a lower rated Mega fuse or MRBF for first defence? Or does this add extra unnecessary complication?

 

I appreciate class T is the gold standard so will probably go that way with a lithium install but worth asking.

 

Many thanks

[Col_T]

51 minutes ago, Russ T said:

Will you be keeping a standard LA in the same enclosure?

That was the plan, yes. The present idea is ‘the lazy way’, moving as little as possible, so the starter would stay in its current location, with the lithium separate from the LA by 3 or 4 inches - is this going to be a problem?

[Russ T]

Some good info here (Not necessarily BSS specific). 

 

https://marinehowto.com/battery-banks-over-current-protection/

[pearley]

39 minutes ago, Paringa said:

Can i ask about MRBF fuses and their suitability please?

 

Mounted on the battery so no unprotected cable. Higher interrupt current than a Mega Fuse (but not as high as a T) and cheaper than class T.

 

And is there any merit on having a Class T for the "meltdown" but a lower rated Mega fuse or MRBF for first defence? Or does this add extra unnecessary complication?

 

I appreciate class T is the gold standard so will probably go that way with a lithium install but worth asking.

 

Many thanks

I have used them in my domestic bank of 2 x 200 amp LiFePo. My understanding is that they are ok for smaller batteries but Class T is Gold Standard. 

Branded fuses if course, not cheap eBay ones.

[Russ T]

1 hour ago, Col_T said:

That was the plan, yes. The present idea is ‘the lazy way’, moving as little as possible, so the starter would stay in its current location, with the lithium separate from the LA by 3 or 4 inches - is this going to be a problem?

I don't think the BSS explicitly bans fuses from being inside battery enclosures, so it is likely it won't be a problem as far as that is concerned but could be open to interpretation by examiners.

 

ETA. I just asked the question of notebook LM using the BSS examination checking procedure as source material. It interprets the results that fuses should not be placed inside battery enclosures, so as I said earlier, probably best to check with a BSS examiner first.

 

 

Edited May 3 by Russ T

[alias]

29 minutes ago, Russ T said:

ETA. I just asked the question of notebook LM using the BSS examination checking procedure as source material. It interprets the results that fuses should not be placed inside battery enclosures, so as I said earlier, probably best to check with a BSS examiner first.

 

Can you do me a favour and ask NotebookLM where specifically in the BSS documentation it is finding the information that led it to reach that conclusion?  

 

I haven't been able to find that in the checking procedures, and the last four BSS examiners that I have used have each passed an installation with a couple of fuses in the battery box.

[Jen-in-Wellies]

2 minutes ago, alias said:

 

Can you do me a favour and ask NotebookLM where specifically in the BSS documentation it is finding the information that led it to reach that conclusion?  

 

I haven't been able to find that in the checking procedures, and the last four BSS examiners that I have used have each passed an installation with a couple of fuses in the battery box.

Me either. Why outsource reading to a machine with a tendency to make stuff up, when you can do it yourself? The reading part, not the making stuff up part! The sections on batteries and fuses in the private boat requirements are not long.

[Russ T]

8 minutes ago, alias said:

 

Can you do me a favour and ask NotebookLM where specifically in the BSS documentation it is finding the information that led it to reach that conclusion?  

 

I haven't been able to find that in the checking procedures, and the last four BSS examiners that I have used have each passed an installation with a couple of fuses in the battery box.

Yes. I would tend to ignore it.

In response to my question "Does the document advise against a fuse being located inside a battery enclosure?"

 

It gave the following response

 

Quote

The sources do not contain a single prescriptive rule explicitly forbidding fuses inside battery enclosures, but they provide several requirements and safety principles that effectively **advise against it** and would likely lead to a failure during a BSS examination.

### **1. Requirement for Visibility and Accessibility**
The document requires that fuses and circuit breakers protecting battery charge or load circuits must be located where they can be **seen** [1-3].
*   According to the checking procedures for DC charge and load circuits, if a fuse cannot be found in a place where the circuit can be seen, the Examiner must mark the checklist as a **fail** [1-3].
*   The document notes it is "**extremely unlikely**" that a fuse would be located where the circuit is hidden [1-3].
*   "Accessible for inspection" is defined as being **capable of being seen and reached** [4]. A fuse located inside a closed battery box would typically be considered "hidden" and not readily visible for assessment.

### **2. Minimising Explosion Risks**
A core General Requirement is that all electrical systems must be installed in a way that **minimises the risks of explosion** [5, 6].
*   The sources state that unsealed or open-vented batteries must be stored in ventilated spaces to prevent the accumulation of **hydrogen gas** [6, 7].
*   Hydrogen readily forms an explosive mixture with air, and even **very small sparks** are capable of igniting these mixtures [8].
*   Fuses are potential ignition sources (particularly if they blow or have poor connections), so placing them inside a space designed to contain potentially explosive gases would be contrary to the requirement to minimise explosion risks.

### **3. Definition of a Battery Space**
The document defines a "battery space or box" specifically as a space or box **containing one or more batteries** [4]. This implies the enclosure is intended for the batteries themselves rather than for distribution equipment like fuse panels.

### **Summary of Restrictions**
While you might find a fuse "in a box" (such as a dedicated fuse box or consumer unit), placing that fuse or its box **inside the battery enclosure** is advised against because:
*   It would likely be considered **"hidden"**, leading to an automatic failure [1-3].
*   It introduces a **source of ignition** into a space where explosive hydrogen gas can accumulate [7, 8].
*   Cylinder lockers (which have similar gas-tightness requirements to prevent vapour entry) are explicitly prohibited from opening into **battery spaces** to keep ignition sources and batteries separate [9, 10]. 

 

5 minutes ago, Jen-in-Wellies said:

Me either. Why outsource reading to a machine with a tendency to make stuff up, when you can do it yourself? The reading part, not the making stuff up part! The sections on batteries and fuses in the private boat requirements are not long.

Because notebook LM uses the source material it is presented with. If the source material is incorrect, it is also going to be wrong. For me, I find it easier to use and interrogate than sifting through a 195 page document.

 

Although, I would agree I make stuff up too.

[Jen-in-Wellies]

I download rhe latest BSS document as a pdf and do a search for an appropriate word. Fuse in this case and scan the sections that come up. Quick and simple. No wading through all 195 pages, or trusting some LLM. 

[alias]

1 hour ago, Jen-in-Wellies said:

I download rhe latest BSS document as a pdf and do a search for an appropriate word. Fuse in this case and scan the sections that come up. Quick and simple. No wading through all 195 pages, or trusting some LLM. 

 

Me too. For those who wish to use a LLM it is also probably the quickest way to verify whether the results are reasonable.  LLMs "try" to be helpful but are often unable to deal with nuance and context in specialised subjects.  

 

For example in the quote above I doubt a BSS inspector would regard an engine room board above a battery box that can be lifted without tools as making the contents inaccessible or concealed.  Also the context of a BSS test is intended to be answering a set of explicit questions rather than seeking to infer what additional questions might logically need to be introduced by the Inspector (or an LLM).  

[David Mack]

3 hours ago, Paringa said:

Can i ask about MRBF fuses and their suitability please?

 

Mounted on the battery so no unprotected cable. Higher interrupt current than a Mega Fuse (but not as high as a T) and cheaper than class T.

 

And is there any merit on having a Class T for the "meltdown" but a lower rated Mega fuse or MRBF for first defence? Or does this add extra unnecessary complication?

 

I appreciate class T is the gold standard so will probably go that way with a lithium install but worth asking.

 

Many thanks

With my domestic bank being a pair of LFPs in parallel, should I fuse each battery separately, or just have a single fuse on the common positive cable?

The former is probably easier with MRBF fuses on each terminal, but for the latter a single Class T on the common cable is easier, and the cost is not too different to 2 x MRBFs.

 

I note that the cover provided with MRBF terminals is said to be inclined to pop off when a fuse is fitted, and anyway it doesn't cover the battery connection lug, so some other form of terminal cover is required.

[wakey_wake]

1 hour ago, David Mack said:

With my domestic bank being a pair of LFPs in parallel, should I fuse each battery separately, or just have a single fuse on the common positive cable?

My impression is that there is not yet any accepted correct answer, so it's more about having reasons for what you choose to do. Fusing the bank seems to be acceptable.

 

I have 5x 100Ah in parallel and I gave each an MCB (as ☝️).

My reasoning is that after seeing what happens to a bank of paralleled LA batteries when one cell gets an internal short (💥) I would not want that to happen with LFPs, and I prefer not to trust the BMSes alone to prevent it. I can't be sure that per-battery OCP will protect the faulty battery, but it seems more likely.

 

Last week I found another fragment of old battery casing, with part of a 'Trojan' label stuck to it, hiding behind the weed hatch riser. Been there 5+ years, from before I bought it.

 

1 hour ago, David Mack said:

The former is probably easier with MRBF fuses on each terminal, but for the latter a single Class T on the common cable is easier, and the cost is not too different to 2 x MRBFs.

Remember you don't only need one fuse to run on, but a spare one in case it pops. Or two spares, in case you don't correctly diagnose the cause first time round.

[David Mack]

13 minutes ago, wakey_wake said:

 

I have 5x 100Ah in parallel and I gave each an MCB (as ☝️).

And what was your reason for going for MCBs rather than fuses?

[wakey_wake]

19 minutes ago, David Mack said:

And what was your reason for going for MCBs rather than fuses?

Plan A had been to rely on the BMS for short circuit protection. Possibly BSS compliant but definitely not sensible!

If the FETs get zapped by a voltage spike the BMS can become unable to switch off.

 

Then I looked at what OCP is available.

Many say "class T fuse" and I agree they're good.

Some say MRBF but I dislike the design of them.

 

However when looking to buy class T holders, fuses and spares at 5x then it starts to come up towards the cost of the batteries... and rises if protection is triggered. Maybe it's a cost of LFP batterires? But I continued looking.

 

I knew the DC breakers offered on Amazon are probably junk. Maybe interesting to play with, not good for protecting a boat from fire.

7 hours ago, Russ T said:

https://marinehowto.com/battery-banks-over-current-protection/

Examples shown there - damage due to trip failure, damage due to nuisance trips well under the rating.

 

Once I looked at reputable electronic suppliers, the parts selector found me those Phoenix Contact "TMC 71C 63A" (a series in different ratings) and I picked the lowest that meets my bank demand, 5x63A. They have proper datasheets, a reputable supplier, 7500AIC, good to 60VDC, they're cheaper than a good fuse, and they can be reset or switched off for maintenance of individual units. What's not to like?

 

Will current be shared exactly equally? No, but it's cabled in star form so it should be close.

Will there be nuisance trips? Possibly, but not yet and anyway they're resettable.

Is the AIC enough? It may be hard to tell because the data coming with the batteries is sparse, but it meets the MHT docs above and is supported by calculations based on the resistance of a length of 16mm² which I used to parallel them. (Sorry, don't think I kept those.)

 

I've mentioned the setup before and nobody has suggested anything wrong with it... except

22 hours ago, jonathanA said:

I'd ignore the techno babble ^^.

which isn't meaningful or useful criticism.

[Russ T]

11 minutes ago, wakey_wake said:

 

I've mentioned the setup before and nobody has suggested anything wrong with it... except

How does that circuit breaker of yours compare to a reputable T-class fuse in terms of tripping time, and how important is that?

Edited May 3 by Russ T