LiFePO4 prismatic cell short circuit current and main circuit protection

[Dzl]

Of course I can't "provide evidence". BTW I never mentioned ANL fuses. Not that I have anything against them. I am using an MRBF. Feedback from both my inverter mfg and my battery mfg + my own reading tell me I'm on safe ground.

Apologies if you already have and I missed it, but could you share who the battery manufacturer is (and what specifically they said)?

 

[pablo fiasco]

Apologies if you already have and I missed it, but could you share who the battery manufacturer is (and what specifically they said)?

I thank you all for putting up with me. I can sense exasperation. It's not that I don't believe your advice is good. I have learned a lot from you despite choosing not to follow your advice in every detail. I'm not a solar enthusiast. I have a particular project - a Skoolie. The electrical system is just one of many problems I need to solve. From paint to insulation to plumbing and so on, each piece of the puzzle will be something to learn about deal with and move on from. I thank you for the help you've given me but I made my decisions a month ago and must move on. I have not come to solar decisions yet. Perhaps I'll be back with more silly questions when I come to that bridge. Thanks again. Pablo.

 

[barbixy]

Okay, so that's pretty far over my head. Is that another way to state SCCR - in multiples of C?

Yea, and what causes a short circuit?

 

[John Frum]

Yea, and what causes a short circuit?

In my experience the leading cause of short circuits by a furlong is @smoothJoey

 

[John Frum]

Yea, and what causes a short circuit?

A short circuit doesn't have or bypasses a load.
Since there is no load there is very little resistance to current flow.
Usually this results in a tripped breaker or blown fuse.
Unfortunately sometimes the breaker or fuse is bypassed or malfunctions and then the result is likely to be a fire.
Even worse, depending on the battery chemistry a short circuit can result in an explosion.

 

[barbixy]

A short circuit doesn't have or bypasses a load.
Since there is no load there is very little resistance to current flow.
Usually this results in a tripped breaker or blown fuse.
Unfortunately sometimes the breaker or fuse is bypassed or malfunctions and then the result is likely to be a fire.
Even worse, depending on the battery chemistry a short circuit can result in an explosion.

Holy crap. What's the likelihood of a breaker failure?

 

[ArthurEld]

It doesn't take a breaker failure to cause a lot of damage.

The way it was explained to me is that you can run an arc welder at less than 100A.

So if your breaker is 100A you can get one hell of a welding arc going before the breaker trips.
If your flashing sparking melting experience never reaches 100A it never stops welding because the breaker doesn't trip.
That probably makes a lot more sense after you have experienced a short.

An arc welder is basically a short that is lower amps than the breaker

Hey, it's not as bad as Lipo

 

[Maast]

The trouble with some fuses like the MEGAs I'm using to connect two BYD modules is you can't tell if they're blown, and if you have multiple parallel strings you'd never know unless you noticed a capacity drop.

I soldered tiny wires parallel to the fuses to act as a indicator. I did tests and that wire pops at 22 amps.

 

[John Frum]

Holy crap. What's the likelihood of a breaker failure?

I can't give you numbers but with quality breakers of the appropriate specification its rare.
I do prefer fuses though.
I suggest to only use reputable brands for both.

 

[DerpsyDoodler]

I played it safe and went with the recommendation to use a Class T fuse. My batteries cost more than the fuse, as did the rest of the system. I can't explain the fiddly bits. I won't say cost was not important, but safety of my system was high on the priority list.

I don’t like how his video implies it’s ok to go to a higher fuse ampacity with a lower interrupt rating without upgrading the wire/cable even though he says, “i’m not recommending you do this.”

 

[HRTKD]

I don’t like how his video implies it’s ok to go to a higher fuse ampacity with a lower interrupt rating without upgrading the wire/cable even though he says, “i’m not recommending you do this.”

I'm not sure why you quoted me in your reply. My 225 amp fuse is rated higher than anything my current loads will pull, but it's slightly below the max amp throughput of my BMS/battery. I'll be upgrading my inverter eventually, and when I do, the 225 amp fuse will be just right.

 

[DerpsyDoodler]

I'm not sure why you quoted me in your reply. My 225 amp fuse is rated higher than anything my current loads will pull, but it's slightly below the max amp throughput of my BMS/battery. I'll be upgrading my inverter eventually, and when I do, the 225 amp fuse will be just right.

I quoted you because I agreed with your reasoning and logic, and thought my statement might invoke discussion of my opinion.

 

[HRTKD]

I quoted you because I agreed with your reasoning and logic, and thought my statement might invoke discussion of my opinion.

Oh, OK.

 

[pablo fiasco]

It doesn't take a breaker failure to cause a lot of damage.

The way it was explained to me is that you can run an arc welder at less than 100A.

So if your breaker is 100A you can get one hell of a welding arc going before the breaker trips.
If your flashing sparking melting experience never reaches 100A it never stops welding because the breaker doesn't trip.
That probably makes a lot more sense after you have experienced a short.

An arc welder is basically a short that is lower amps than the breaker

Hey, it's not as bad as Lipo

You can run an arc welder at less than 100A but I really doubt you'll get far at 12VDC. I'm not at all qualified to sit at this table but it seems to me everyone is ignoring voltage. Sure the amps do the damage but it's the volts that cause the arc. My reading tells me that 12VDC can't jump more than .004 mm in a normal atmospheric condition. I think the odds of any fuse blowing w/o leaving at least that big a gap are approaching zero. I noticed for curiosity sake that in the wiring schematic that Will published on this site he posits an ANL fuse - not a Class-T. Just sayin'.

 

[ArthurEld]

You can run an arc welder at less than 100A but I really doubt you'll get far at 12VDC. I'm not at all qualified to sit at this table but it seems to me everyone is ignoring voltage. Sure the amps do the damage but it's the volts that cause the arc. My reading tells me that 12VDC can't jump more than .004 mm in a normal atmospheric condition. I think the odds of any fuse blowing w/o leaving at least that big a gap are approaching zero. I noticed for curiosity sake that in the wiring schematic that Will published on this site he posits an ANL fuse - not a Class-T. Just sayin'.

Sorry, I'm 48V and I assumed you were too. You are less likely to have a short that burns your terminals off.

edit: I'm not really sure about 12V. You may not need a class T fuse but I think you still will burn your terminals off if you short.

 

[pablo fiasco]

I imagine you can weld at 48V. My tig operates at 60V and 300A.

 

[HRTKD]

I imagine you can weld at 48V. My tig operates at 60V and 300A.

I haven't dug into the DC side of my TIG machine. I used it for the first time this weekend, after I installed a new AC 240v 50amp circuit into my garage. According to the label, it pulls, at most, 33 amps from the AC side.

 

[pablo fiasco]

I don't know off hand what mine pulls AC. It's running from a 100A breaker. I've never actually cranked it up to 300A. Does what I need around 140.

 

[cinergi]

You can run an arc welder at less than 100A but I really doubt you'll get far at 12VDC. I'm not at all qualified to sit at this table but it seems to me everyone is ignoring voltage. Sure the amps do the damage but it's the volts that cause the arc. My reading tells me that 12VDC can't jump more than .004 mm in a normal atmospheric condition. I think the odds of any fuse blowing w/o leaving at least that big a gap are approaching zero. I noticed for curiosity sake that in the wiring schematic that Will published on this site he posits an ANL fuse - not a Class-T. Just sayin'.

Except that this isn't normal atmospheric condition:

An arc fault is the flow of electrical energy through an air gap by way of ionized gas molecules. Whilst air is normally regarded as a non-conducting medium, a high potential difference (voltage) between two conductors in close proximity can cause the air molecules to break down into their ionized constituents (called a ‘plasma’), which can then carry a charge from one electrode to the other.

Look at what happens in the video below -- the connecting arc is small and requires close proximity. The disconnecting arc is shockingly enormous (no pun intended).

 

[curiouscarbon]

Very impressive and respect inspiring demonstration.

Zero crossings can be a blessing! ️〰

 

[HRTKD]

That video reinforces for me why an AC (only) rated breaker isn't acceptable in a DC circuit. If I understand the test, it was 240 volts DC and very few of us see that many DC volts except for possibly in the the PV wires. You non-mobile folks probably see a lot more volts than us mobile DIY'ers.

 

[pablo fiasco]

Except that this isn't normal atmospheric condition:



Look at what happens in the video below -- the connecting arc is small and requires close proximity. The disconnecting arc is shockingly enormous (no pun intended).

Interesting video. Surprising. I wonder what that means for what goes on inside a domestic 220. I can't see them opening a 1/2" gap. Can't see them arcing like that either. I don't have the parts to chase down the science but that is in a 220VAC setup. Do again with 12VDC. That's really all I'm currently interested in. I read somewhere that the potential for arcing in DC is greater than in AC because the current flows in one direction.

 

[HRTKD]

Interesting video. Surprising. I wonder what that means for what goes on inside a domestic 220. I can't see them opening a 1/2" gap. Can't see them arcing like that either. I don't have the parts to chase down the science but that is in a 220VAC setup. Do again with 12VDC. That's really all I'm currently interested in. I read somewhere that the potential for arcing in DC is greater than in AC because the current flows in one direction.

If you watch the whole video, both AC and DC are used. AC is a big nothing burger. DC is where the action is.

 

[cinergi]

Personally I don't need to perform the science experiment to decide what's safe to buy; I know my pack resistance is a couple of milliohms so I'm planning for ~20kA AIC fuses and breakers. This industry has been around a long time and these ratings exist for a reason. Who am I to say they're wrong? For LFP packs, your safest configuration for the main battery fuse is a Class T.

 

[pablo fiasco]

If you watch the whole video, both AC and DC are used. AC is a big nothing burger. DC is where the action is.

There was just that 2 minutes with the 220V test. I wouldn't be surprised if the 'action' was on the DC side of the clip.