Fuses and circuit breakers for LiFePO4

[s/v Jedi]

Originally Posted by Sailing Paul

But I don't want to run thick wire all the way to the busbar, which is why I was proposing a 40A breaker, or fuse, to protect the 50A wire. Perhaps that is silly though? Maybe I should just take the 200A wire all the way to the busbar?

Yes, either run the 200A circuit to the busbar or do a 60A circuit ans simply pop a 60A fuse in. No breaker in either case.

[Sailing Paul]

Originally Posted by CaptainRivet

Very simple highly recommend to use a 63AH NH 0 fuse+fuse holder, use a 16mm cable and you are good. That leave you a bit room to add more stuff, 16mm2 is still cheap and easy to route. You can also root it to your MPPT and other stuff as their terminal typically take up to 16mm2. So you simplfy your install having 1 cable size, lug size and tools for it.

N0 fuse is good for 80kA short curcuit, cheap and works also as disconnect switch as you can simply pull the fuse, also under full load. This your Lithium bank battery fuse (only class T or NH fusses have the short curcuit rating you need) and also protects the cable to the busbar.

Thanks, I have been looking into NH fuses. Good up to 125,000A! As you say, cheaper than T fuses and more readily available. In which case, why would anyone want to use a T fuse?

Edit: you mentioned earlier that an MRBF should be fine. Have you changed your mind on that?

[Sailing Paul]

One other question is what is the recommended place to connect the charging cable? Should it be charge-fuse-battery or charge-switch-fuse-battery?

For my lead, the charging goes straight to the batteries, with no fuse, but that wiring is 20 years old and lead may be different to lithium in any case.

[more]

Originally Posted by CaptainRivet

Thats incorrect when lead was installed...you need in min a LFP battery fuse that can withstand 20kA, which can be also the cable fuse when properly chosen due to current rating for the existing cable.
Sure he can use the existing cables and midi fuse.

you are incorect totaly,what you talking.

fuse protecting only cable, fuse have only 1 purpose To protect the wire and prevent it from starting fires.

in my house fuse 10A on other source 1/3 all europa power generation maybe bilion bilion amp short hapen sometime when i work. and i dont have burnet cable. some in boat.
boat have 1 big NH dc Fuse 200-500A depend how big is cable from battery to nh fuse. from NH fuse go to eletric board busbar. . from board go to user over termal fuse switch ussualy older fuse.
you have 100 megawathh litium battery or 100ah some fuse,some cable ecc

[more]

Originally Posted by Sailing Paul

One other question is what is the recommended place to connect the charging cable? Should it be charge-fuse-battery or charge-switch-fuse-battery?

For my lead, the charging goes straight to the batteries, with no fuse, but that wiring is 20 years old and lead may be different to lithium in any case.

never direct, only direct alowed is bilge pump but with ovn fuse switch
also your location is importat are you in Europa or USA

[Sailing Paul]

Originally Posted by more

never direct, only direct alowed is bilge pump but with ovn fuse switch
also your location is importat are you in Europa or USA

Europe

[CaptainRivet]

Originally Posted by more

you are incorect totaly,what you talking.

fuse protecting only cable, fuse have only 1 purpose To protect the wire and prevent it from starting fires.

in my house fuse 10A on other source 1/3 all europa power generation maybe bilion bilion amp short hapen sometime when i work. and i dont have burnet cable. some in boat.
boat have 1 big NH dc Fuse 200-500A depend how big is cable from battery to nh fuse. from NH fuse go to eletric board busbar. . from board go to user over termal fuse switch ussualy older fuse.
you have 100 megawathh litium battery or 100ah some fuse,some cable ecc

You are wrong: for LFP ISO and ABYC require a main battery fuse that must be dimensioned according to the max short curcuit current your bank can produce. 4x280AH cells are at around 15kA internal short curcuit cuurent. So you use NH or class T because of the short curcuit current rating and dimension its fuse rating according to the battery cable you use. So the fuse has both functions protect battery or better the installation upstream of the battery and cable.
I burned a car down 17 years ago where a 400AH bank shorted internally due to i assume permanent overload and a 400AH class T fuse got arced. That fuse cannot protect the cells itself whixh won't burn but isolates the battery from the rest of the install...

@Paul: 125kA is AC rating, 80kA is typically the DC rating of NH. NH is more common in Europe and is more bulkier in size then class T. Class T is more common and cheaper in US while europe is NH more common as used widespreaded in grid power stations and factories to fuse heavy machinery.
Additionally you get fake class T fuses but no fake NH fuses. You can actually use used Nh fuses and holders as long as the blown indicator shows its ok, they are super sturdy and the cooper parts are silver coated...if they get black use silver cuttlery polisher and the look like new in 5min. But thats optical as the black surface corrosion doesn’t harm its electric capabilities.
But for 1 piece of NH 00 size you need i would buy new as cheap, get 3 fuses (2 as replacement) and 1 NH00 holder.

Dieffent if you need NH3 630A like i do...there used make sense as only holder cost 250EUro and fuse 100Euro per piece...i paid 50Euro for a whole unsorted box on ebay from a ripped down factory (80% where new unused spares) with 3 630A fuses, one NH3 holder and 30 fuses and 12 holder in different sizes from 63A till 630A.

Order:
NH fuse at terminal then switch, main battery cable to your 50A midi fuse and main busbar. The 50A midi is not really need but doesn’t hurt either so keep it.
NH works also as switch but dor 63A a switch is so cheap that it is more convenient to operate it then pulling the NH fuse.

[Sailing Paul]

Just a thought, if I am going for NH instead of MRBF I can put the isolation switch between the battery and fuse. Is that better, or should it be battery-fuse-switch?

[CaptainRivet]

Originally Posted by Sailing Paul

Just a thought, if I am going for NH instead of MRBF I can put the isolation switch between the battery and fuse. Is that better, or should it be battery-fuse-switch?

It must be battery fuse switch order.
MRBF would have been fine on a 100AH LFP but not anymore on a 280AH bank.
Connect the NH fuse with a 10-15cm long cable and screw it down on the battery box so the force of the long battery cable goes onto the very sturdy NH fuse and not onto the battery terminal. Place the disconnect switch after the NH fuse wherever in the main battery cable its comfortable to reach as its just a convenience shut off.
If you leave the boat longer pull the NH fuse additionally.

[more]

Originally Posted by CaptainRivet

You are wrong: for LFP ISO and ABYC require a main battery fuse that must be dimensioned according to the max short curcuit current your bank can produce. 4x280AH cells are at around 15kA internal short curcuit cuurent. So you use NH or class T because of the short curcuit current rating and dimension its fuse rating according to the battery cable you use. .

again you are wrong EU made boat last 25 year have main battery fuse ussualy NH. normaly sizing cable and max combinet user. ussualy on production boat 100-300A cabel can manage +- 1000A

[CaptainRivet]

Originally Posted by more

again you are wrong EU made boat last 25 year have main battery fuse ussualy NH. normaly sizing cable and max combinet user. ussualy on production boat 100-300A cabel can manage +- 1000A

Not wrong ISO and ABYC require this...the NH fullfills both so piece of mind.
In ISO its under mobil use requirement means its valid for cars, trucks,RV,boat...everything thats moving and i am also a certified automotive surveyor (cars,RV,Caravans, truck till 7.5t) beside serval other educations.
They simplified it by requiring to use NH or class T fuse for LFP as cable fuse so people just use current rating for cable and fuse location as close possible to terminal also fullfills automatically the main battery fuse requirement.

That rules was always there and Normally thats not an issue but with boats are. when LFP is not on from factory the whole installation is according to Lead specs and a lead battery whith peukert effect and high internal resistance limiting itself in current so as short curcuit protection for the bank all cable fuse types avaliable automatically also fullfill the requirement as main battery fuse short curcuit protection...different with LFP.

[Sailing Paul]

To summarise then

1) LFP batteries should have a fuse with IC rating appropriate for the battery. MRBF up to 100Ah, NF or T above that. Is 100Ah the right cut off point?

2) A battery isolator should come next, followed by connection to a busbar.

3) Charging and load connections, with appropriate fuses for any reduction in wire size, should go to the busbar. To disconnect a load a switch should be inserted between the busbar and load.

Does that sound right?

What is best practice for the negative? Is it just everything to a busbar?

[s/v Jedi]

Originally Posted by Sailing Paul

To summarise then

1) LFP batteries should have a fuse with IC rating appropriate for the battery. MRBF up to 100Ah, NF or T above that. Is 100Ah the right cut off point?

2) A battery isolator should come next, followed by connection to a busbar.

3) Charging and load connections, with appropriate fuses for any reduction in wire size, should go to the busbar. To disconnect a load a switch should be inserted between the busbar and load.

Does that sound right?

What is best practice for the negative? Is it just everything to a busbar?

Lets say you have two 100Ah batteries in parallel. You could install a MRBF fuse on each battery and then a jumper to a power post. The negative terminals a small jumper to a second power post.

Then from the positive power post, a small jumper (which will carry twice the current…) to the isolation switch.

Similarly, from the negative power post, a single cable goes to the shunt for the battery (bank) monitor.

If you do this, remember the power post connections are straight between the crimped terminals, so first put a jumper from battery 1 on the post, then the big cable and then the jumper from battery 2. This balances it and achieves minimum resistance.

If there’s more than two batteries, use a busbar instead.

[CaptainRivet]

Originally Posted by s/v Jedi

Lets say you have two 100Ah batteries in parallel. You could install a MRBF fuse on each battery and then a jumper to a power post. The negative terminals a small jumper to a second power post.

Then from the positive power post, a small jumper (which will carry twice the current…) to the isolation switch.

Similarly, from the negative power post, a single cable goes to the shunt for the battery (bank) monitor.

If you do this, remember the power post connections are straight between the crimped terminals, so first put a jumper from battery 1 on the post, then the big cable and then the jumper from battery 2. This balances it and achieves minimum resistance.

If there’s more than two batteries, use a busbar instead.

Norhing to add..perfect write up.

[Sailing Paul]

Originally Posted by s/v Jedi

Lets say you have two 100Ah batteries in parallel. You could install a MRBF fuse on each battery and then a jumper to a power post. The negative terminals a small jumper to a second power post.

Then from the positive power post, a small jumper (which will carry twice the current…) to the isolation switch.

Similarly, from the negative power post, a single cable goes to the shunt for the battery (bank) monitor.

If you do this, remember the power post connections are straight between the crimped terminals, so first put a jumper from battery 1 on the post, then the big cable and then the jumper from battery 2. This balances it and achieves minimum resistance.

If there’s more than two batteries, use a busbar instead.

Interesting thanks, that makes sense.

I am not bothering with the shunt for the moment. I have ordered a JBD BMS which comes with Bluetooth. I will see what that tells me and how accurate it is before I invest in additional bits of kit.

If I ever buy a second battery I was thinking of using it as a reserve. Like our water tanks, use one until it dries, then swap over. That way if one blows up, I would still have the other. I do already have 2 smaller LFP batteries, 60Ah and 50Ah, that I use with an electric outboard/trolling motor. I have set the boat up so I could quickly attach these for domestic/nav use should I need to, but hopefully would never need to as our lead batteries will be the first option should the LFP fail.