Fuses and circuit breakers for LiFePO4

[Sailing Paul]

Hi, I am planning to add a LiFePO4 battery to my boat to run domestic and Nav systems. There are no heavy loads such as inverters, thrusters or windlass. Max draw will be the fridge and autopilot. I was thinking of using a 200A MRBF fuse at the battery, then a 40-50A circuit breaker. I am planning on using 6mm cable to connect to the domestic circuit, good for 50A.

The BMS is rated at 200A, but I would like to have a backup, hence the MRBF fuse.


Will this be sufficient? What do other people do with fuses on LiFePO4?

[tanglewood]

The various standards (ABYC and ISO draft) require both a fuse/breaker and a manual disconnect switch. This is above and beyond any BMS.

The fuse has a pretty high AIC requirement. I don't recall the numbers off hand, but the bottom line is that a Class T fuse is what you need, or one with equivalent AIC rating.

[Sailing Paul]

MBRF have an IC rating of 10,000A at 14V. I am surprised that is not sufficient.

Agree on isolator switch, I am intending to incorporate one after the MBRF, or T fuse if that is really essential.

Edit: just looked up T class fuses and they are rated at 20,000A. They seem to be very expensive and bulky. I would definitely prefer MRBF if acceptable for the job.

[s/v Jedi]

Originally Posted by Sailing Paul

MBRF have an IC rating of 10,000A at 14V. I am surprised that is not sufficient.

Agree on isolator switch, I am intending to incorporate one after the MBRF, or T fuse if that is really essential.

Edit: just looked up T class fuses and they are rated at 20,000A. They seem to be very expensive and bulky. I would definitely prefer MRBF if acceptable for the job.

Your plan has a number of problems that can lead to disaster. If you use wiring that is good for 50A then you may not use a 200A fuse.

So you need a 50A fuse. If the battery is something like a 100Ah capacity and has an internal BMS then I think a MBRF fuse is okay. That said, look at the diameter of the battery terminal stud or bolt and compare that to the hole size of the MRBF fuse holder. If the fuse holder has a larger hole then you should not use it.

[Cpt Mark]

Originally Posted by Sailing Paul

MBRF have an IC rating of 10,000A at 14V. I am surprised that is not sufficient.

Agree on isolator switch, I am intending to incorporate one after the MBRF, or T fuse if that is really essential.

Edit: just looked up T class fuses and they are rated at 20,000A. They seem to be very expensive and bulky. I would definitely prefer MRBF if acceptable for the job.

Now is not the time to cheap out. Spend the money, do it right or don't do it at all. Your risk is fire which would not make you happy.

[Sailing Paul]

Originally Posted by s/v Jedi

Your plan has a number of problems that can lead to disaster. If you use wiring that is good for 50A then you may not use a 200A fuse.

So you need a 50A fuse. If the battery is something like a 100Ah capacity and has an internal BMS then I think a MBRF fuse is okay. That said, look at the diameter of the battery terminal stud or bolt and compare that to the hole size of the MRBF fuse holder. If the fuse holder has a larger hole then you should not use it.

It is a 280Ah battery pack, M8 studs and MBRF fuses are available in M8. 200A is because at some point I may want a 1kw inverter. The plan would be 200+ A wire to the isolator switch, 200A wire to the 40A circuit breaker, then 50A wire from the circuit breaker. There is already a 50A midi fuse protecting the house circuit, which would be at the other end of the new 50A wire.

There would actually be a second circuit from the isolator, through a fuse and to a AC/DC charger so I can charge whilst the battery is disconnected from the house circuit. So 200A wire to the fuse, 50A from the fuse to charger.

How does that sound?

[Sailing Paul]

Originally Posted by Cpt Mark

Now is not the time to cheap out. Spend the money, do it right or don't do it at all. Your risk is fire which would not make you happy.

I intend to do it right, but don't wish to over engineer this. The MBRF takes up less space, sitting on the battery, as well as being lower price.

10,000A at 14V implies 1.4mohm resistance. I am trying to find the internal resistance of the cells. If they are more than 0.35 mohm each then the battery could not deliver more than 10,000 A could it? Then there are the other resistance such as the BMS, connectors and internal wires.

[s/v Jedi]

Originally Posted by Sailing Paul

It is a 280Ah battery pack, M8 studs and MBRF fuses are available in M8. 200A is because at some point I may want a 1kw inverter. The plan would be 200+ A wire to the isolator switch, 200A wire to the 40A circuit breaker, then 50A wire from the circuit breaker. There is already a 50A midi fuse protecting the house circuit, which would be at the other end of the new 50A wire.

There would actually be a second circuit from the isolator, through a fuse and to a AC/DC charger so I can charge whilst the battery is disconnected from the house circuit. So 200A wire to the fuse, 50A from the fuse to charger.

How does that sound?

Not so good. The MRBF fuse with M8 is fine, no problem there. But why a path of battery-fuse-switch-breaker-house ? This doesn’t make sense. You must do battery-fuse-busbar and that’s it. Then for the breaker panel: busbar-fuse-breaker panel.

[Sailing Paul]

Originally Posted by s/v Jedi

Not so good. The MRBF fuse with M8 is fine, no problem there. But why a path of battery-fuse-switch-breaker-house ? This doesn’t make sense. You must do battery-fuse-busbar and that’s it. Then for the breaker panel: busbar-fuse-breaker panel.

It would actually be battery-fuse-switch-breaker-busbar. The busbar already exists and as you say it is then busbar-fuse-breaker panel. I thought I needed an isolation switch for the battery. Are you saying I do not? My lead acid house bank goes to an isolation switch and then straight to the busbar - no fuse at all!

With the engine on the house bank is connected to the starter battery through a charging relay. The plan is only to use the existing lead acid bank for house if the new LFP failed. It would never be connected at the same time as the LFP. It would still be used for the windlass though and as a backup for the starter battery. I definitely need a way to isolate the LFP should I use the lead acid for house.

[Sailing Paul]

Thinking about this again, I could go battery-fuse-isolator-busbar, which would mirror the existing lead-acid bank. For charging I would then go charger-fuse-battery, again mirroring lead-acid. However, I would not need 200A wire to the busbar or to the charger when 50A wire is fine, so I would want 40-50A fuses or circuit breakers to protect the thinner wire.

Which is better though? To have the charger connect to the isolation switch, or go straight to the MBRF fuse?

[s/v Jedi]

Originally Posted by Sailing Paul

It would actually be battery-fuse-switch-breaker-busbar. The busbar already exists and as you say it is then busbar-fuse-breaker panel. I thought I needed an isolation switch for the battery. Are you saying I do not? My lead acid house bank goes to an isolation switch and then straight to the busbar - no fuse at all!

With the engine on the house bank is connected to the starter battery through a charging relay. The plan is only to use the existing lead acid bank for house if the new LFP failed. It would never be connected at the same time as the LFP. It would still be used for the windlass though and as a backup for the starter battery. I definitely need a way to isolate the LFP should I use the lead acid for house.

When you have the fuse, you don’t need the breaker. It is battery-fuse-switch-busbar.

[Sailing Paul]

Originally Posted by s/v Jedi

When you have the fuse, you don’t need the breaker. It is battery-fuse-switch-busbar.

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?

[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.

[more]

Originally Posted by Sailing Paul

Hi, I am planning to add a LiFePO4 battery to my boat to run domestic and Nav systems. There are no heavy loads such as inverters, thrusters or windlass. Max draw will be the fridge and autopilot. I was thinking of using a 200A MRBF fuse at the battery, then a 40-50A circuit breaker. I am planning on using 6mm cable to connect to the domestic circuit, good for 50A.

The BMS is rated at 200A, but I would like to have a backup, hence the MRBF fuse.


Will this be sufficient? What do other people do with fuses on LiFePO4?

do you planing bulid new boat from scratch. or use on your Jeanneau Sun Odyssey 35.

if you planing use on your Jeanneau Sun Odyssey 35 he have all fuse,braker what CE certification ask ,also have shematic what eu law ask,you dont need nothing ad ,everthing what you add is worst

[CaptainRivet]

Originally Posted by more

do you planing bulid new boat from scratch. or use on your Jeanneau Sun Odyssey 35.

if you planing use on your Jeanneau Sun Odyssey 35 he have all fuse,braker what CE certification ask ,also have shematic what eu law ask,you dont need nothing ad ,everthing what you add is worst

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.