BMS capacity vs fuse size

[GILow]

I’m sure it has been discussed already, but I can’t find it.

I have a 24 volt, 304 AH (7.2 kWh) LiFePO4 battery bank, built from 8 EVE 304AH prismatic cells. This is managed by a JK BMS (JK B2A8S20P) which is rated for a continuous 200A charge/discharge and 350 A “peak” output, which I assume is some kind of momentary surge capability.

My main loads for the bank are a pair of inverters, one rated at 4000W (surge 8000W) and one rated at 2000W (surge 4000W). The other loads are trivial stuff like a pair of DC to DC converters powering the 12v AGM house bank, plus the solar inputs from the various solar controllers.

We will have an all-electric galley, using three single burner Nuwave plates, each rated at 2kW max and a 1.8kW convection microwave/grill.

Now, to save you from reaching for the calculator or pen and paper (or even the mental maths), both of those inverters maxed out would need a nominal 250 amps, more than the BMS can supply and probably more than the battery bank should be asked to supply. However, the intention is not to run them both at once but instead to choose the inverter most suited to the needs at the time. Basically the smaller inverter will run the coffee machine, toaster, kettle etc over on the starboard butler station with an outlet to plug in one of our three portable induction plates should we want to, while the other will supply the main galley with outlets for the portable induction plates and the convection microwave.

We are happy to manually manage the loads, we’ve done so for years with the existing FLA setup on the old boat.

But I’m lost on the fuse setup.

I’m assuming my best bet will be a Class T fuse between the BMS and the main distribution bus (with an isolator between the fuse and the bus). If so, how should I size it? To the nominal peak capacity of the BMS (350A) or should I jump to the next size which appears to be 400 A?

Next, how should I fuse each of the inverters? Class T again or will ANL be ok? Fused for their surge or continuous ratings?

Over to the experts.

Matt

[Statistical]

Fuse protects the wire. So what does your wire support?

So if you go with a 400A fuse ensure your have wiring to handle 400A. If your wiring can handle 350A or 300A or 250A then fuse it accordingly. You can go up on fuse if there is no specific fuse available.

The fuse protects the wire (and by extension you and your family). Don't overthink it. Don't go well this wire can only handle 200A but the BMS will shutoff so I can use a 400A fuse. At that point you have replace a BMS for a fuse when it comes to safety. BMS can fail. Fuses are about as simple of a device as you can make and so the risk profile is a lot more mundane.

If you want specific numbers in terms of wire vs fuses how far is it from the battery to main busbars round trip?

For example if it is 15 feet round trip to the busbar and you are using good boat wire (105C insulation rating) and this is not inside the engine space then:

1/0 AWG is good for 195A continuous so would be fused with no more than 200A fuse.
2/0 AWG is good for 225A continuous so would be fused with no more than 250A fuse.
3/0 AWG is good for 260A continuous so would be fused with no more than 300A fuse (or 250A to be conservative)
4/0 AWG is good for 300A continuous so would be fused with no more than 300A fuse.

A lot depends on how much you want to upgrade wiring for this 350A surge capacity. Personally I would just plan on 200A and so wire to support 200A and fuse at 200A.

[wholybee]

The BMS is going to act faster to protect anything than the fuse. Keep that in mind. You really don't need to protect the BMS, as it protects itself, and will do so faster than a fuse.

The fuse is there to protect the wire. You can go as small as you want with the fuse as along as it supports the load you intend to run, and as long as it is small enough to protect the wire. Assuming your wire is large enough for the 250A the inverters need, the fuse should be 250A. If the wire is smaller than required for 250A, then use a smaller fuse, and manage your usage to not blow the fuse.

[GILow]

A couple of thoughts….

1. The day I rely on a Chinese BMS to act as advertised and shut off due to excess current you can bury me with a spade.

2. A 200 amp fuse WILL get destroyed by the first current surge. I’m not planning to spend the next few years plugging in $70 fuses each week.

[more]

Originally Posted by GILow

A couple of thoughts….

1. The day I rely on a Chinese BMS to act as advertised and shut off due to excess current you can bury me with a spade.

2. A 200 amp fuse WILL get destroyed by the first current surge. I’m not planning to spend the next few years plugging in $70 fuses each week.

you totally wrong.
FUSE PROTECT CABLE AND ONLY CABEL fuse sizing is in corelating with cabel size and lenght

2/all this what you call 70$ fuse rated 200A easy short time in second can got to 600A and more.all this fuse is hot realy hot. if you want use
2P DC MCCB DC Solar Molded Case Circuit Breaker MCCB Overload Protection Switch Protector
https://www.aliexpress.com/item/1005...Cquery_from%3A


more this in china is installed and produced ws USA,EU,Australia and rest world.

if you have money go victron lynx. nice design,fuse not expensive. again fuse protect cable not you kitchen habit use.

next time say i have cabel 85mm2 3 meter long what fuse use.

[GILow]

Originally Posted by more

you totally wrong..

Ok then.

[GILow]

Can I put some minds at rest?

I’ll put appropriate cable in place. And yes, the fuses have to protect the damn cable. But what if I put 1000 amp cable in? Then do I install a 1000 amp fuse?

No, that scenario is stupid.

I’m trying to get my head around the likely currents in this system so that I can install fuses (and bl—dy cables) to suit. I was hoping there’d be some analysis of the system I describe based on prior experience.

Incidentally, the heavy duty cable runs are in the order of less than 300 mm to the bus bar. The joy of starting with a clean slate.

[Audeamus]

If you haven't come across it Blue sea systems have a handy online cable size calculator which could be helpful for working out fuse requirement.

[Statistical]

Originally Posted by GILow

A couple of thoughts….

1. The day I rely on a Chinese BMS to act as advertised and shut off due to excess current you can bury me with a spade.

2. A 200 amp fuse WILL get destroyed by the first current surge. I’m not planning to spend the next few years plugging in $70 fuses each week.

I think it is a lot more likely it shuts off at lower current then not shutting off.

Still if you want a 350A. 400A. even 500A link then fine the wire should handle that current and then be fused apropriately.

Designing a wire connection that can handle 200A and then plopping in a 400A fuse is just begging to burn your boat down. The only thing less safe would be no fuse at all.

[Statistical]

Originally Posted by GILow

Can I put some minds at rest?

I’ll put appropriate cable in place. And yes, the fuses have to protect the damn cable. But what if I put 1000 amp cable in? Then do I install a 1000 amp fuse?

No, that scenario is stupid.

It is less stupid then putting a 1000A fuse on a wire capable of 500A. If the wire can support 1000A then up to 1000A would be compliant.

I’m trying to get my head around the likely currents in this system so that I can install fuses (and bl—dy cables) to suit. I was hoping there’d be some analysis of the system I describe based on prior experience.

It supports 200A continuous. I would ignore the marketing wank hard to use and limited duration 350A surge capacity. Wire the connection to handle 200 and fuse accordingly.

Note that a 200A fuse will not blow at 201A for 1 millisecond. Check the specs on the specific fuse you buy but a current graph is probably something like this. At least 5 minutes at 150% of rating (300A), at least 1 minute at 200% of rating (400A), at least 1 second at 300% of rating (600A).

[more]

Originally Posted by GILow

Can I put some minds at rest?

I’ll put appropriate cable in place. And yes, the fuses have to protect the damn cable. But what if I put 1000 amp cable in? Then do I install a 1000 amp fuse?

No, that scenario is stupid.

I’m trying to get my head around the likely currents in this system so that I can install fuses (and bl—dy cables) to suit. I was hoping there’d be some analysis of the system I describe based on prior experience.

Incidentally, the heavy duty cable runs are in the order of less than 300 mm to the bus bar. The joy of starting with a clean slate.

The recommended main fuse and cable size is based off the sum of all amperages + 125% overcurrent for the largest branch (or 250% for the largest motor).

[CaptainRivet]

Well that install sounds and is so wrong in many ways.

1) 1st the BMS is rated to low what you installed.
2nd a 200A JK BMS can do 150A continious and around 250-300A peak, even rated for 200A. These 150Euro Chinese BMS and JK is best of the lot, have unmatched Mosfets to take care of the load/charge and due to this its one of 12 mosfet that always the weakest which gets the full load in first milliseconds and will die slowly and unnoticed. So one of 12 mosfets gone without you noticing it, next 11 and so on and that means current capability lowers and BMS is dead and with your load that will happen sooner then later.
So max load must be 75% of its rating. If you paralell them that doesn't change but the probability you have 2 weak mosfets sharing the load is high, thats why 2x200A BMS is only 250-300A cont., 3x 200A parallel is only 400A cont. And 4x200A is only 500A cont. More then 4 there is no raise in current capability anymore, you can parallel more for capacity but not for current capability.

2) you overload the cells, 304AH EVE with 6kw inverter power (efectivly 6x1.41=>9Kw on/off in milliseconds as its AC) is too much for one 1p8S with 304AH. You need a 2nd battery with a 2nd BMS, that also solves your issues with BMS not enough current capability, 2x JK 200A BMS in parallel can do 250A-300A continuous, so exactly what you need.
3) Each battery also gets a BMV 700 or 712 Battery monitor and and cut off relay in case the mosfet short, and as mosfet like to short close the BMS cannot block the power anymore when it needs to. So you need to install independent last resort method to disconnect the battery and the victron BMV 700 or 712 have a relay output that steers this disconnect relay.
4) each of the batteries you fuse with 150A class T or NH superfast fuse. That is a cable and BMS and battery protect fuse in one. Yes the cable must be speced to run min. 150A, due to voltage drop you can and should go higher with diameter, its short way so costs are marginally going up. I always use here 70sqmm or 95sqmm due to voltage drop, thats gauge 2/0 or 3/0.
So the order is battery terminal/fuse/shunt/ disconnect relay. The disconnect relay goes last so you can switch on the power with the BMV 700 or 712.
5) for the inverter the manufacturer should gave you a fuse rating to use as well as a cable diameter to use. A 4kw cont uses on 24V=167A plus efficency assume 85% 192A so a 200A fuse would be right. You can use ANL or i would recommend NH fuse. The each inverter needs a disconnect switch, i recommend to get a off/1/2 switch, on contact 1 you connect a precharge resistor means a 10 till 20W resisitor with anything between 100 and 1kohm. thaT resistor connects the contact 1 of the switch to contact 2. On contact 2 you connect plus cable of inverter. So you disconnect by switching off, switching on first 1 and count slowly till 10 and then switch to 2. Like this you don't kill the mosfets in your BMS with a huge surge of the semiconductors in the inverters. The 4kw inverter i would use 95sqmm or gauge 3/0 with 200A fuse, the 2kw 50sqmm or gauge 1/0 and 100A fuse.

[CaptainRivet]

Originally Posted by more

The recommended main fuse and cable size is based off the sum of all amperages + 125% overcurrent for the largest branch (or 250% for the largest motor).

This partly again is wrong.
The fuse specs tell you how you have to spec it.
What you stated is Megafuse or ANL fuse in cable or standard rating , called also GG. The are slow or fast acting fuse

For inductive loads there is a special fuse type and this should be used with motors like windlass and if you fuse your starter, both should always be a breaker that you can re-engage quick as its life depending equipment.

Class T are always superfast acting due to their size and if you use them for inverters with surge loads (which i don't recommend) you need to go higher to 140% so the fuse doesn't blow when you switch on the inverter or you spec 125% but need a pre-charge resistor.

NH-fuses should be speced to 100% as they are build to run at 100% rating for 7/24/365 with the standard fuses GG or cable spec. They are optimised to have the lowest voltage drop possible as they are originally developed for grid stations and commercial customers like factories and the loss in 0.5% size can count up to 1000s of euros/$ a month. You can see this also on their size and amount of ceramics used.

Just also understand a fuse is a resistor and they create a voltage drop and the bigger the fuse the bigger the voltage drop&loss plus heat emited.
So my recommendation is to use NH with the big loads and also as main battery fuse.

[CaptainRivet]

Originally Posted by more

you totally wrong.
FUSE PROTECT CABLE AND ONLY CABEL fuse sizing is in corelating with cabel size and lenght

2/all this what you call 70$ fuse rated 200A easy short time in second can got to 600A and more.all this fuse is hot realy hot. if you want use
2P DC MCCB DC Solar Molded Case Circuit Breaker MCCB Overload Protection Switch Protector
https://www.aliexpress.com/item/1005...Cquery_from%3A


more this in china is installed and produced ws USA,EU,Australia and rest world.

if you have money go victron lynx. nice design,fuse not expensive. again fuse protect cable not you kitchen habit use.

next time say i have cabel 85mm2 3 meter long what fuse use.

Sorry thats wrong, different fuses protect differnt things.
And also one fuse can act for different purposes simultaneously and need to speced accordingly.

Best example for that is the needed battery fuse for lithium batteries.
In contrary to lead with its high resistance thats limiting current in a short a lithium with internal resistance close to a short not so it needs by ISO, ABYC and also healthy common thinking a fuse to disconnect the battery.
That fuse must have a short curcuit rating higher then the capability of the lithium with can be up to 400C...so min 20kA, better 50kA is required.
Simultaneously the same fuse needs to also protect the battery cable, so the cont rating of the fuse is what the cable can max do eg 250A on a gauge 2/0 or 95sqmm.
Next you can also use that identical fuse to eg protect from overcurrent so if the cells can only do 1C in a 200AH battery then the rating to chose is 200A, even the cable can do 250A.
4th simultaneously the same fuse can also protect a mosfet BMS, in that case the fuse type need to be supperfast to act quick and real current rating, so here 150A cont of the JK BMS.

So to sum up you need a fuse with following specs:
1) short curcuit rating 20k, better 50kA for lithium
2) cable can carry max 250A, so a max. 250A rating for cable or less
3) overcurrent protection of cells requires 200A rating
4) BMS protection needs superfast acting type with 150A rating.

So you need a 150A Class T (with 20kA or better 50kA spec) or 150A NH fuse superfast (they all have 80kA DC rating) in this case, 4 functions in one!

Parallel mosfet BMS always!!! Need superfast acting battery fuse speced to real const current carring of one BMS as the balance between the 2 parallel batteries is not always 50% and eg short peaks will be supplied mainly by the battery that has the lowest total internal resistance and to protect the BMS from high current killing the mosfets you need a superfast acting fuse speced to real rating of the BMS...a mosfet BMS that cost less then 350Euro always use 75% of the given rating as below that threshold the mosfets cannot be calibrated and given rating is too high.

Inverters have an internal equipment protecting fuse so here you only fuse the cable that connects the inverter, so you only spec it to cable current capability, nothing else. Using a class T is wrong, best is NH in GG followed by ANL fuse.

Regarding victron Lynx be careful, the case melted when i pulled 650A for the lynx distributor...its rated for 1000A but if you look very carefully you find thats peak load and real current capability is 400A.
It carries a 8mmx30mm busbar copper tin plated which due to busbar calculator good for 384A contimious. I use it till 300A, above that i make my own made from silver plated 99.8% copper busbars in 9x45mm and NH 2 and 3 fuses.
But yes the lynx for most its a neat and safe way to install distribution and fuses acting as main busbar.

[more]

Originally Posted by CaptainRivet

This partly again is wrong.
The fuse specs tell you how you have to spec it.
What you stated is Megafuse or ANL fuse in cable or standard rating , called also GG. The are slow or fast acting fuse

For inductive loads there is a special fuse type and this should be used with motors like windlass and if you fuse your starter, both should always be a breaker that you can re-engage quick as its life depending equipment.

Class T are always superfast acting due to their size and if you use them for inverters with surge loads (which i don't recommend) you need to go higher to 140% so the fuse doesn't blow when you switch on the inverter or you spec 125% but need a pre-charge resistor.

NH-fuses should be speced to 100% as they are build to run at 100% rating for 7/24/365 with the standard fuses GG or cable spec. They are optimised to have the lowest voltage drop possible as they are originally developed for grid stations and commercial customers like factories and the loss in 0.5% size can count up to 1000s of euros/$ a month. You can see this also on their size and amount of ceramics used.

Just also understand a fuse is a resistor and they create a voltage drop and the bigger the fuse the bigger the voltage drop&loss plus heat emited.
So my recommendation is to use NH with the big loads and also as main battery fuse.

soon is voting for EU parliament, go in your country on list,win on voting. make club in EU parliament. try all this your idea make law and standard. and after 20-30 year your idea be standard. Or born again go in school, finish marine school, go on ship 1 year,return from ship and go 6 month college 6 month work on ship, and after 12 year finish colege.
but for now
The recommended main fuse and cable size is based off the sum of all amperages + 125% overcurrent for the largest branch (or 250% for the largest motor).

again fuse protecting cable, this is good decise and lawmaker.

main fuse from source to branch.
branch fuse from panel to user.

soon (4-40 year) coming new law and CE standard for low voltage DC on all type vechile