Safe to relly on LifePO4's BMS to protect the wire from battery to switch?

[LifesBetterWhenYoureBeating]

Hi folks,

I've recently learned that I need over current protection on the wire going from my house bank to my switch, and since that bank powers my windlass, watermaker and other house loads, I'd need at least a 200A fuse to avoid nuisance tripping. However, I've just switched my bank to a single 300ah LifePO4 with a BMS that limits continuous discharge to 200A, so I'm wondering if it'd be ok to rely on that to protect the wire from the battery to the switch?

[CharlieJ]

Every single B+ conductor on a boat, with the exception of the conductor supplying engine starters, must be protected with an appropriately sized overcurrent protection device (OCPD), i.e., fuse or circuit breaker.

This is regardless of any control and protection circuit/system.

[wingssail]

Originally Posted by CharlieJ

Every single B+ conductor on a boat, with the exception of the conductor supplying engine starters, must be protected with an appropriately sized overcurrent protection device (OCPD), i.e., fuse or circuit breaker.

This is regardless of any control and protection circuit/system.

CharlieJ, Your post should be modified thusly:

Every single B+ conductor on a boat, with the exception of the conductor supplying engine starters, to meet ABYC standards and to be safe from fire and electrical shorts, must be protected with an appropriately sized overcurrent protection device (OCPD), i.e., fuse or circuit breaker.

Many of us with older boats which never had fuses on all conductors, for example from the batteries to the battery switch, or from the battery switch to the distribution panel, or from the alternator to the battery switch, have elected to live with that, and we rely on breakers on the panel for every attached device, so "must" seems a bit presumptuous. It's not something I "must" have. Maybe "should" but not "must".

[CharlieJ]

As I tell my clients when they disagree with the Standards: "IYB...It's your boat."

And, by the way, this voluntary requirement in the USA is a legal requirement in those countries that abide by the ISO standards.

[boatpoker]

Originally Posted by CharlieJ

Every single B+ conductor on a boat, with the exception of the conductor supplying engine starters, must be protected with an appropriately sized overcurrent protection device (OCPD), i.e., fuse or circuit breaker.

This is regardless of any control and protection circuit/system.

Correct, however I've seen starter batteries blown because they were unprotected. I always recommed to go a step further and protect starting batteries too. If I remember correctly Rod Collins Has photos of such an incident and also recommends protection for starter batteries.

PS. I'm aware of the difficulties in protecting huge amp draw larger engines. I have a 300amp MBRF on the starter battery of my 55hp Westerbeke.

[sailingunity]

I've heard because of the instantaneous current that lithium batteries can provide that a class T fuse is needed, as they have a greater ability to stop arcing even with the fuse has broken. I have an older boat but elected to add a 400 amp class T fuse which is about 6 inches from the bank - not because I had to, but because it make sense from a safety perspective - but I have 3 batteries each rated at 200 amps continuous, going into a bus bar, and a 4/0 cable from the bank to the selector switch.

If I were you I'd call the battery manufacturer and see what they recommend.

[AKA-None]

I wish I’d had a breaker on my starter battery as that would have saved me a lot of repair work

[CarlF]

If you have an insurance claim for something caused by the battery, your insurance company would likely deny it if you don’t have a Class T fuse installed.

But a bigger problem is that your new lithium battery may not be able to support the loads. Check the battery specs for the maximum continuous discharge (which is usually less than when the BMS cuts out). Routinely exceeding that will void your battery warranty and damage them.

[goboatingnow]

Originally Posted by CharlieJ

As I tell my clients when they disagree with the Standards: "IYB...It's your boat."

And, by the way, this voluntary requirement in the USA is a legal requirement in those countries that abide by the ISO standards.

Not post construction per se

This because if the work was termed a major craft modification the whole boat should be subjected to a professionally supervised post construction CE certification. It’s not enough to merely follow the specs ( that helps of course )

It’s a huge grey area in Europe with so called professionals doing modifications and effectively ignoring the CE issue. The whole thing survived because the owner holds on to the original certification and happily hands it to the next owner ( conveniently ignoring any “ major craft modifications “)

The issue is what’s an add-on and what’s a major craft modification

The general view is there is no “ legal “ obligation post construction to abide by iso standards. This was clarified after the sinking of the yacht megawatt , of Northern Ireland , where a incorrectly machined rudder shaft failed ( auto pilot install ) , the shaft failed and the yacht sank

[goboatingnow]

Originally Posted by gsalgado

Hi folks,



I've recently learned that I need over current protection on the wire going from my house bank to my switch, and since that bank powers my windlass, watermaker and other house loads, I'd need at least a 200A fuse to avoid nuisance tripping. However, I've just switched my bank to a single 300ah LifePO4 with a BMS that limits continuous discharge to 200A, so I'm wondering if it'd be ok to rely on that to protect the wire from the battery to the switch?

It’s a call quite frankly. But good practice would be to fuse it at the battery post bms disconnect

The bms disconnect is there to protect the battery.

The fuse is there to protect the wire


Every connection wire to a battery should be fused at the battery. This includes things like shunt sense wires. I use a plethora of inline sealed car style fuses for smaller gauges like shunt and battery meter wires.

[CharlieJ]

However, I've just switched my bank to a single 300ah LifePO4 with a BMS that limits continuous discharge to 200A, so I'm wondering if it'd be ok to rely on that to protect the wire from the battery to the switch?

The OP asked if it was ok to depend on the current limiting feature of an unknown BMS in or integrated with a LFP battery of unknown manufacture in lieu of an OCPD to protect the conductors to some large electrical loads. It is not ok from a Standards perspective, from an engineering standpoint, from an insurance standpoint, from a "best practices" standpoint and from a common sense standpoint. This was not a question that needed a nuanced answer. My original answer (Post #2) was therefore not nuanced.

[CaptainRivet]

Originally Posted by gsalgado

Hi folks,

I've recently learned that I need over current protection on the wire going from my house bank to my switch, and since that bank powers my windlass, watermaker and other house loads, I'd need at least a 200A fuse to avoid nuisance tripping. However, I've just switched my bank to a single 300ah LifePO4 with a BMS that limits continuous discharge to 200A, so I'm wondering if it'd be ok to rely on that to protect the wire from the battery to the switch?

congrats your 200A BMS won't survive long.
Your 200A BMS is facing an anchor windlass that easily pull 400A startup surge or when the anchor is stuck....won't survive that long...

no its not ok. and your lifepo4 300AH house needs also an over current fuse class T or NH2, 300AH with the typical inner resistance of 12mOhm will create more A when shorted then most know. an ANL or Megafuse will be melting close or bypassed by a light bow if that bank gets a short or similar. the root cause of most lifepo4 fires.

[LifesBetterWhenYoureBeating]

Originally Posted by CaptainRivet

congrats your 200A BMS won't survive long.
Your 200A BMS is facing an anchor windlass that easily pull 400A startup surge or when the anchor is stuck....won't survive that long...

no its not ok. and your lifepo4 300AH house needs also an over current fuse class T or NH2, 300AH with the typical inner resistance of 12mOhm will create more A when shorted then most know. an ANL or Megafuse will be melting close or bypassed by a light bow if that bank gets a short or similar. the root cause of most lifepo4 fires.

I'm not at all familiar with the inner workings of a BMS so I'd appreciate if you could elaborate on what will happen over time if I use a windlass that pulls 400A on startup? I guess you wouldn't recommend using my LifePO4 for such a windlass, then?

As for the fuse, would an MRBF work or does it have to be class T/Nh2?

[CaptainRivet]

Originally Posted by gsalgado

I'm not at all familiar with the inner workings of a BMS so I'd appreciate if you could elaborate on what will happen over time if I use a windlass that pulls 400A on startup? I guess you wouldn't recommend using my LifePO4 for such a windlass, then?

As for the fuse, would an MRBF work or does it have to be class T/Nh2?

what's exactly the nominal wattage of your windlass 700, 1000, 1200 or even bigger?
What's the BMS you are using manufacturer/model or if it's a drop in which one?
what's the short circuit amperage of an MRBF? Ait should be minimum 20kA. most likely not seeing the distance and isolation.
if you operate windlass actually only the NH2 is appropriate because class T is fast acting and will blow during startup. you need a slow acting in cable protection and fast in short curcuit perfect for NH2

[LifesBetterWhenYoureBeating]

Originally Posted by CaptainRivet

what's exactly the nominal wattage of your windlass 700, 1000, 1200 or even bigger?
What's the BMS you are using manufacturer/model or if it's a drop in which one?
what's the short circuit amperage of an MRBF? Ait should be minimum 20kA. most likely not seeing the distance and isolation.
if you operate windlass actually only the NH2 is appropriate because class T is fast acting and will blow during startup. you need a slow acting in cable protection and quick in short curcuit perfect for NH2

My windlass is a Maxwell 1200W. Battery is a drop in replacement, Ampere Time 12v 300AH

MRBF Interrupt Capacity: 10000A @ 14V DC