LiFePO4 prismatic cell short circuit current and main circuit protection

[cinergi]

Voltage is a huge factor here. The higher the voltage, the higher the chance of unintentional arc'ing. So while the fuse might be fine to interrupt 10kA at 12v, it might not be at 48v (not to mention you'll have more amperage on a short circuit of an LFP bank at 48 than at 12). And while this guy's test proved OK, it might not be OK 100% of the time. You don't want to be the 1% guy whose fuse fails to function properly and burns your house/RV/boat down. Personally I would never use MRBF as a main fuse for LFP, especially in a 48v configuration.

And as others have noted, the rig itself can be a factor -- it depends on the size of the wire exiting the fuse holder (and the wire involved in the rest of the short circuit). Personally, I'm going to assume a very large crescent wrench is providing the short circuit

I moved to a DC 250 amp circuit breaker because I blew two 300 amp class t's (80 bux) with inverter inrush current. The breaker was ~$90 and is rated 50kA. And now I also have a nice battery switch.

 

[HRTKD]

Voltage is a huge factor here. The higher the voltage, the higher the chance of unintentional arc'ing. So while the fuse might be fine to interrupt 10kA at 12v, it might not be at 48v (not to mention you'll have more amperage on a short circuit of an LFP bank at 48 than at 12). And while this guy's test proved OK, it might not be OK 100% of the time. You don't want to be the 1% guy whose fuse fails to function properly and burns your house/RV/boat down. Personally I would never use MRBF as a main fuse for LFP, especially in a 48v configuration.

And as others have noted, the rig itself can be a factor -- it depends on the size of the wire exiting the fuse holder (and the wire involved in the rest of the short circuit). Personally, I'm going to assume a very large crescent wrench is providing the short circuit

I moved to a DC 250 amp circuit breaker because I blew two 300 amp class t's (80 bux) with inverter inrush current. The breaker was ~$90 and is rated 50kA. And now I also have a nice battery switch.

I could use more information on this, because I don't understand the inadequacy of an MRBF.

Regarding your last paragraph, how did a 250 amp circuit breaker work when a 300 amp fuse didn't? I'm using a Class T fuse, so I'm interested.

 

[cinergi]

I could use more information on this, because I don't understand the inadequacy of an MRBF.

Regarding your last paragraph, how did a 250 amp circuit breaker work when a 300 amp fuse didn't? I'm using a Class T fuse, so I'm interested.

Oh, sorry yeah I left the explanation out of the fuse vs breaker. I could have stuck with the fuse because I now manually precharge the inverter capacitors. But I grew tired of the cost of replacing the fuses and went with a breaker which also gave me the ability to turn off the battery (including the BMS). Incidentally that breaker has a longer trip time so it doesn't trip even if I don't precharge. The Class T fuse was fast-acting (vs. slow-blow).

As for the MRBF problem -- the reason it has a lower interrupt rating than class T is because of the way the fuse blows. The metal that makes up the interior of the fuse melts and deposits in various locations inside the fuse. In a high-current situation, it's possible these molten pieces don't end up breaking the circuit because they fall too close to each other and the current doesn't get interrupted -- it arcs across the various bits of melted fuse. And the higher the voltage, the more likely that is to happen. A class T fuse "explodes" internally so this sort of failure can't really happen.

 

[MattiFin]

Voltage is a huge factor here. The higher the voltage, the higher the chance of unintentional arc'ing. So while the fuse might be fine to interrupt 10kA at 12v, it might not be at 48v (not to mention you'll have more amperage on a short circuit of an LFP bank at 48 than at 12). And while this guy's test proved OK, it might not be OK 100% of the time. You don't want to be the 1% guy whose fuse fails to function properly and burns your house/RV/boat down. Personally I would never use MRBF as a main fuse for LFP, especially in a 48v configuration.

And as others have noted, the rig itself can be a factor -- it depends on the size of the wire exiting the fuse holder (and the wire involved in the rest of the short circuit). Personally, I'm going to assume a very large crescent wrench is providing the short circuit

I moved to a DC 250 amp circuit breaker because I blew two 300 amp class t's (80 bux) with inverter inrush current. The breaker was ~$90 and is rated 50kA. And now I also have a nice battery switch.

Almost anything works at 12 volts, 48 volts gets lot harder to extinguish the arc.

Ever try to weld with large 12v battery? Really hard or impossible to keep the arc going no matter what amps you have.

Maybe also worth mentioning that not all ANL fuses are same. Renogy for example specifies only 180A ! breaking capacity for 20A ANL fuse

https://www.renogy.com/content/files/Manuals/ANL-Fuse.pdf

 

[cinergi]

Almost anything works at 12 volts, 48 volts gets lot harder to extinguish the arc.

Ever try to weld with large 12v battery? Really hard or impossible to keep the arc going no matter what amps you have.

Maybe also worth mentioning that not all ANL fuses are same. Renogy for example specifies only 180A ! breaking capacity for 20A ANL fuse

https://www.renogy.com/content/files/Manuals/ANL-Fuse.pdf

Indeed. But who knows how close these metal fragments can be inside the fuse.

Note that we're talking about MRBF not ANL. ANL is even worse for AIC ratings.

 

[HRTKD]

Is there a definitive guide for determining when to use a Class T, MRBF, ANL or circuit breaker? Something that says at 12v you need this, but at 48v you need to step up your game and use that.

I think there is a tremendous amount of confusion on what size of over current protection to use as well as what type.

 

[pablo fiasco]

Almost anything works at 12 volts, 48 volts gets lot harder to extinguish the arc.

Ever try to weld with large 12v battery? Really hard or impossible to keep the arc going no matter what amps you have.

Maybe also worth mentioning that not all ANL fuses are same. Renogy for example specifies only 180A ! breaking capacity for 20A ANL fuse

https://www.renogy.com/content/files/Manuals/ANL-Fuse.pdf

Indeed. But who knows how close these metal fragments can be inside the fuse.

Note that we're talking about MRBF not ANL. ANL is even worse for AIC ratings.

You didn’t mention how you came to blowing 2 300 amp fuses. Dead short or over load at the inverter? I don't know a terrific lot about electricity but I think you're right about the importance of voltage to the thinking. I suspect that a lot of the comments on this thread about fuses come from people concerned with higher voltages or pure theory. But really, how did you blow those fuses?

 

[Sabre36]

I think the question is boils down to...

q: How much dead short amperage can a lifepo4 battery deliver?

This video gave me confidence in mrbf fuses

Then one of the more knowledgeable forum members cast some doubt on the test.
From memory its was the relatively long loop of relavtively low awg wire causing a bottleneck.
I wonder how different the results would be with a short bit of 4/0 awg.

In the marine market the ABYC is calling for Class T fuses for any Li-Ion bank. Class T has 20,000 AIC at 125V and much more at 12 or 24V (MRBF is 10,000 AIC at 14V. A single 100Ah G-31 AGM is capable of 5000A into a short.. While no MRBF fuses failed unsafely in that test, some ANL fuses did. Unsafe fuse failures are a concern on boats where LPG etc can accumulate. This is why fuses on boas should also be ignition protection rated and have a minimum AIC threshold.

 

[cinergi]

You didn’t mention how you came to blowing 2 300 amp fuses. Dead short or over load at the inverter? I don't know a terrific lot about electricity but I think you're right about the importance of voltage to the thinking. I suspect that a lot of the comments on this thread about fuses come from people concerned with higher voltages or pure theory. But really, how did you blow those fuses?

It was from the inrush current of charging the inverter capacitors.

 

[pablo fiasco]

It was from the inrush current of charging the inverter capacitors.

Okay, hadn't heard of that bend in the trail. Be curious to know some details of your batteries and inverter. Also wondering how a smaller breaker is going to solve the problem if the larger fuse couldn't handle the inrush.

 

[Ampster]

Also wondering how a smaller breaker is going to solve the problem if the larger fuse couldn't handle the inrush.

Earlier he mentioned two things, reducing the inrush with a precharge resistor and the difference between slow blow and fast acting.

 

[pablo fiasco]

Like I said I hadn't heard of the capacitor inrush problem. But did occur to me to suggest a resistor to slow it down. I will be doing that to avoid sparking even though my inverter mfg assures me mine is built to standards wherein damage to the capacitors is no a problem. They didn't bother to mention the potential for blown fuses. Mine is a 2000W peak 3500W inverter. All my research leads me to conclude that I need a 300A slow blow fuse to allow the inverter the headroom it needs. Practically speaking a 250A breaker with a high AIC would probably be a good choice for me as I'm not likely to reach peak watts.

 

[cinergi]

I have 10kVA of inverter (2 5kVa units) as well as DC-DC and solar charger (yes, the DC-DC and SCC spark, too). If the bank was low SoC (like 30% or less) it wouldn't blow the fuse. Higher SoC's blew the fuse (higher voltage = higher current).
The resistor / precharge circuit also prevent unnecessary stress on the BMS relay.

 

[pablo fiasco]

Wow, big numbers. I'm just paddling around in shallow water with 12V in an RV situation. So I think maybe I just learned something new. Question: should I be pre-chargeing SCC and B2B as well?

 

[Fallingaway]

Maybe this is just my limited knowledge but I’m having a hard time understanding why you would want to fuse it based on open short current rather than something reasonable for the application. The OP is using this for a vehicle so why not just go with something like 150a since he likely wouldn’t be pulling near that from a 12v battery? It’s intersting for sure but, a lot of times I feel like people on here over complicate things.

 

[pablo fiasco]

I'm used to the idea that most conversations on this subject will involve me being right away over my head. It's okay. It's better than being bored with people asking me if I know how to bounce a ball.

 

[Fallingaway]

I get ya for sure. This one in particular though, I feel like it started with a simple question and then I walked into an electrical engineering PhD class

 

[pablo fiasco]

Still, I did learn a thing or two. It's why I come here.

 

[sduser]

Maybe this is just my limited knowledge but I’m having a hard time understanding why you would want to fuse it based on open short current rather than something reasonable for the application. The OP is using this for a vehicle so why not just go with something like 150a since he likely wouldn’t be pulling near that from a 12v battery? It’s intersting for sure but, a lot of times I feel like people on here over complicate things.

I am using a 150A fuse. We are talking about clearing the circuit in the event of a short circuit.

 

[pablo fiasco]

If I were to use a 150A fuse it would limit my inverter to less than it's continuous output. Took me a while to sort out why 300A slow blow was being recommended. The lugs that came with the inverter are enormous. I have some welding ground cable, wire diameter near to 1/2" that fits them nicely. What's that, 4/0? And my setup will be simple and compact. I can't think there would be any advantage in an under amp fuse.