Lithium and fuses

[lustyd]

You seemed to suggest that thinner wires increase resistance, which drops voltage so produces lower current.
I was confused by this assertion too but thought Paul would be better placed to discuss.

 

[noelex]

You seemed to suggest that thinner wires increase resistance.

Which, of course, it does. A thinner conductor or, perhaps more accurately, a conductor with a lower cross-sectional area will have higher resistance per unit of length (assuming we are considering conductors of the same material).

In post #50, you attempted to predict the short-circuit current of a battery by using the internal resistance of the cells. To be accurate, we need to add in the resistance of the bus bars and the contact resistance between the bus bars and the cells. If the battery has a BMS, we need to also include the resistance of this as all these elements contribute to the internal resistance of the battery as a whole.

This will give some idea of the potential short-circuit current at the battery terminals (although, as I indicated, this is not necessarily accurate).

If we want to calculate the potential short-circuit current at the fuse, to see if the IC rating is adequate, if the fuse is not mounted on the battery terminals, we need to take into account the resistance of the wire between the battery terminals and the fuse together with the contact resistance of the various connections.

It is the total resistance, within the circuit that will influence the current.

 

[PaulRainbow]

The current capacity of any-sized wire is easy to look up.

Only if you know the wire size "thin" is not a quantifying size.

Regardless of your definition of a "thin" and "thick" conductor (assuming identical materials), what I have written in the quoted post (post #77) is accurate. Quantifying the values, any way you wish, does not alter the soundness of the principle.

Can you think of an exception?

 

[PaulRainbow]

The rubbish posted on here gets deeper every day

 

[RogerJolly]

Your particular installation will help reduce the required IC rating.

The thin wire adds to the total resistance a short is likely to experience, and hence it reduces the peak short-term current that is possible. Naturally, a thinner wire demands an appropriate and lower fuse rating, but it also, at least in theory, reduces the IC rating needed for the fuse.

Pretty sure the wire is 6mm2. Maybe this is all ‘Dad logic’, but here goes…

To exceed the 10,000 AIC of the MRBF we’d need a resistance of 1.2 milliohms or less across the battery - meaning a dead short that forms a circuit including less than about 0.5m of above wire, that’s assuming zero BMS resistance.

Maybe the short happens when a supply and earth wire rub against each other near the battery and vibration wears through the insulation.

In a good outcome there’s a small contact area between the wires, representing a certain resistance rather than dead short. This is enough to draw more than 40A so something happens to the battery fuse (40A MRBF) - the resistance means less than 10,000 A is drawn, so the fuse blows safely and breaks the circuit. In another good outcome the BMS is fried and stops conducting. In another good outcome both previous good outcomes happen.

In a bad outcome the shorting wires manage to make enough contact to draw more than 10,000 and the fuse carries on conducting (strikes some sort of arc?) – caries on conducting enough current to overheat wires and cause fires. Also the BMS fails in a bad (conducting) way.

I’m thinking the good outcome is more likely (hopefully not wishfully).

 

[noelex]

Pretty sure the wire is 6mm2. Maybe this is all ‘Dad logic’, but here goes…

Your "Dad" logic is reasonable . Shorting 6mm² main battery cable will produce a lower potential peak short circuit current than shorting a thicker main battery cable (mine is 120mm² ), all things being equal.

However, we want protection systems that work in the less likely scenarios, especially when the cost and trouble of such protection systems is not high (in boat dollars) as it only requires installing the correct fuse.

The current standards, such as the American ABYC standard, only consider the battery size and technology when specifying the IC rating of the main battery fuse. Obviously, the actual value of the fuse will be different depending on the main battery cable size, but not the minimum IC rating.

You can use the battery manufacturer’s specifications to determine the IC rating, but as few manufacturers supply this, you are required to fit a fuse with a 20kA or higher IC rating when installing lithium batteries to conform to the standards. This is irrespective of the size of your main battery cable. I think the logic is that a short could occur at or very close to battery terminals, so the main battery cable size is irrelevant or at least less important. They also demand that the main battery fuse is within seven inches or less of the battery terminals (unless you employ extra conduit to eliminate the risk of a short circuit). The European ISO/RCD is similar and requires the fuse be within 200mm.

The ABYC standard is an American publication and is not even a legal requirement in the USA, but the guidelines should at least be considered.

 

[PaulRainbow]

Your "Dad" logic is reasonable . Shorting 6mm² main battery cable will produce a lower potential peak short circuit current than shorting a thicker main battery cable (mine is 120mm² ), all things being equal.

However, we want protection systems that work in the less likely scenarios, especially when the cost and trouble of such protection systems is not high (in boat dollars) as it only requires installing the correct fuse.

The current standards, such as the American ABYC standard, only consider the battery size and technology when specifying the IC rating of the main battery fuse. Obviously, the actual value of the fuse will be different depending on the main battery cable size, but not the minimum IC rating.

You can use the battery manufacturer’s specifications to determine the IC rating, but as few manufacturers supply this, you are required to fit a fuse with a 20kA or higher IC rating when installing lithium batteries to conform to the standards. This is irrespective of the size of your main battery cable. I think the logic is that a short could occur at or very close to battery terminals, so the main battery cable size is irrelevant or at least less important. They also demand that the main battery fuse is within seven inches or less of the battery terminals (unless you employ extra conduit to eliminate the risk of a short circuit). The European ISO/RCD is similar and requires the fuse be within 200mm.

Do you have a link to the source of this information please ?

 

[noelex]

Do you have a link to the source of this information please ?

I presume you are referring to the part of my post you have bolded:
"The European ISO/RCD is similar and requires the fuse be within 200mm."

I cannot provide a direct link to the European ISO/RCD standards because this behind a paywall.

However, I can share a link to an article by Rod Collins that highlights this requirement of the European standard. As I indicated, the main fuse must be within 200mm of the battery terminals (with some exceptions, similar to the ABYC standard).

Battery Banks & Over Current Protection - Marine How To

Rod Collins is a member of the electrical sub-committee responsible for developing all ABYC electrical standards. If you click on the above link, you’ll find many other excellent articles. They are all worth reading.

 

[PaulRainbow]

I presume you are referring to the part of my post you have bolded:
"The European ISO/RCD is similar and requires the fuse be within 200mm."

I cannot provide a direct link to the European ISO/RCD standards because this behind a paywall.

However, I can share a link to an article by Rod Collins that highlights this requirement of the European standard. As I indicated, the main fuse must be within 200mm of the battery terminals (with some exceptions, similar to the ABYC standard).

Battery Banks & Over Current Protection - Marine How To

Rod Collins is a member of the electrical sub-committee responsible for developing all ABYC electrical standards. If you click on the above link, you’ll find many other excellent articles. They are all worth reading.

The EU/UK ISO isn't behind a paywall, as such, you just have to buy a copy of it.

I assume you mean this:

"The ABYC requirement is for a battery bank fuse is to be within 7 wire inches of the battery bank. The European ISO/RCD (which is law) requires the fuse be within 200mm. 200mm equates to about 7.9”. In this regard, the European ISO/RCD is essentially the same as the ABYC’s 7” requirement. The difference here is the ABYC standards are still voluntary. I only point this out because we hear time and again that the ABYC is not a law, and it is not, but the ISO/RCD (recreational Craft Directive) is a law."

That's incorrect, battery fuses do not need to be within 200mm. The European ISO/RCD is not a law, it's a requirement for EU RCD and only applies to new build boats.

There are also no particular requirement in the ISO regarding Lithium installations (yet).

 

[pandos]

Just to update the class T fuses and holders , with covers arrived today, via FedEx from Texas made by Bourns in Costa Rica, cost including vat carriage customs etc 92 euros for two holders and two 150amp fuses

 

[RogerJolly]

Struggling to finding an exact definition of a class T fuse, with hard parameter values etc - is there one? Anyone know what organisation defines them (guessing it's not British Standards, but maybe something similar)?

Was thinking the standard would include a physical format, so they all fit in a standard holder. Don't think this is true though, so you wonder exactly what is defined.

Littlelfuse are listing class T fuses with all shapes in sizes in the picture, down to 1 A - really confused now!

 

[noelex]

Struggling to finding an exact definition of a class T fuse, with hard parameter values etc - is there one? now!

Unlike some other fuse types T class fuses are different sizes depending on the amperage rating. See post #66. The most common sizes used in marine applications are:

Length: 61.9mm (110-200A), 69.9mm (225-400A), 77.8mm (450-600A)
Bolt hole size: 8.7mm (110-200A), 10.3mm (225-400A), 12.3mm (450-600A)

 

[RogerJolly]

Ah yes. Sorry missed that.