Single vs. Multiple Class-T Fuses

[John Frum]

Update: My friend contacted EVE (as we currently have a combined order in for 120 LF280Ks) and asked what the prospective fault current is. Their immediate response was 2000-4000A.

Is this cells or batteries?
Perhaps 2000 amps is empty and 4000 amps is full?

 

[John Frum]

Nice - so the breaker gives people a false sense of security, as I feared.

IMO it is worse than useless.

 

[740GLE]

Their manuals include this disclaimer
"The included breaker is not a guarantee of battery protection. Size and install the correct over
current protection for conductors and battery if not included with the product." -- https://signaturesolar.com/content/documents/EG4/EG4-LL 48V-24V Manual-1.2.0.pdf

ah yes but they also market that same CB as and “added safety” feature. Almost seems like that needs a little astrick, or a question mark after it, lol.

 

[cs1234]

We should point Signature Solar over to the Fortress battery thread to show them what we think of misleading advertising.

 

[RV10flyer]

I have a 100 amp DC breaker on each of my 10 280aH batteries. I have a 125 amp class. T fuse at the midpoint between cell 8 & 9 of each battery. Then I have a main 250A DC breaker on each Sol Ark 12K.

 

[cs1234]

I have a 100 amp DC breaker on each of my 10 280aH batteries. I have a 125 amp class. T fuse at the midpoint between cell 8 & 9 of each battery. Then I have a main 250A DC breaker on each Sol Ark 12K.

What 100 amp DC breaker are you using?

 

[cods4]

Is this cells or batteries?
Perhaps 2000 amps is empty and 4000 amps is full?

Its a single cell.

Obviously it would only decrease slightly for any number of cells in series (someone did the calculations for that earlier in the thread).

And if you are paralleling up cells then it would increase. I.e for a 2p16s battery made of LF280K cells it would be ~4000-8000A.

It seems fairly well understood from what I've read that temperature also affects these values by quite a lot, probably a lot more so than SOC.

I'd suggest that it would be 4000A at around 40-55°C, and 2000A at < 0°C.

SOC would probably only have an effect at quite low values.

 

[John Frum]

Its a single cell.

Obviously it would only decrease slightly for any number of cells in series (someone did the calculations for that earlier in the thread).

That was me.

It seems fairly well understood from what I've read that temperature also affects these values by quite a lot, probably a lot more so than SOC.

My mistake, I was assuming standard test conditions of 25C.

SOC would probably only have an effect at quite low values.

I consider my numbers a 0 order estimate.
I suspect the voltage sag has to be significant into a dead short and I have not accounted for that.

 

[John Frum]

My ~1500 watt tea kettle boils a liter of ~25C water in ~200 seconds.
At 4000 amps a big blue prismatic cell would be something like a ~13000 watt sealed tea kettle.
Does that seem roughly accurate?

 

[cods4]

I suspect the voltage sag has to be significant into a dead short and I have not accounted for that.

Yeh I think that's the case. The chemical reaction in the cell can't keep up with the short circuit situation and the voltage drops right off.

 

[cods4]

My ~1500 watt tea kettle boils a liter of ~25C water in ~200 seconds.
At 4000 amps a big blue prismatic cell would be something like a ~13000 watt sealed tea kettle.
Does that seem roughly accurate?

Not exactly since we expect the voltage will not be staying at 3.2V.
Instead we can calculate the power from the cell using P=I²R.
If we assume the internal resistance will remain the same. Then P=4000²x0.0002 = 3200W

And the voltage of the cell can be calculated with V=IR. So the cell voltage will be as low as 0.8V.


These are all pretty rough calculations since we don't know exactly how EVE measured the short circuit current.

 

[John Frum]

In light of new info from @cods4 it seems mrbf and anl fuses have sufficient breaking capacity for simple 24 volt lfp batteries.

 

[John Frum]

On the other hand...
I wonder if the dead short ampacity is higher than 4000 amps for the first milliseconds.
Possibly long enough to form a plasma filament across the air barrier in the blown fuse.
Once the plasma filament is established it may sustain at lower current and voltage.

 

[cs1234]

On the other hand...
I wonder if the dead short ampacity is higher than 4000 amps for the first milliseconds.
Possibly long enough to form a plasma filament across the air barrier in the blown fuse.
Once the plasma filament is established it may sustain at lower current and voltage.

So it will fix itself then.

 

[John Frum]

So it will fix itself then.

You would still have 16 ~3000 watt electrolyte kettles.

 

[cs1234]

You would still have 16 ~3000 watt electrolyte kettles.

When life gives you lemons!

 

[John Frum]

When life gives you lemons!

Its got electrolytes!

 

[RV10flyer]

What 100 amp DC breaker are you using?

CHYTI 100A/600V DC/10,000A

 

[Mithril]

On the other hand...
I wonder if the dead short ampacity is higher than 4000 amps for the first milliseconds.
Possibly long enough to form a plasma filament across the air barrier in the blown fuse.
Once the plasma filament is established it may sustain at lower current and voltage.

Someone should convince Electroboom or styropyro to do an experiment and see the actual volts and amps required to sustain an arc for various fuses

 

[Mithril]

Ok, so how do people feel about the need for T-class fuses "beyond" the main system fuse(s). Assume batteries and battery banks are all fused correctly (enough AIC, correct size to protect wires). Is there any actual NEED for T-class level AIC on branches? In theory if a 50A fuse blows and there's a 5000A arc the main T-class is ALSO going to blow and it has sufficient AIC. I think the only concern is if you used a fuse with such a low AIC that it could form an arc that would NOT kick the main fuse(s).