LiFePO4 prismatic cell short circuit current and main circuit protection

[Boron]

@MattiFin sadly you have shown that you are under-informed on the matter. DC systems have used blade switches since 1920+. You can see evidence of this in railway electric traction switchboards (600Vdc 1000A), Electroplating, early telephone exchanges, UPS etc.
However, and this is where your ignorance shows, Electrical power distribution and switch rooms requires operators to be specially trained, equipped full body flash suits, gloves, head googles when operating manual switch gear. There are YT vids to educate you.
On a more domestic note, the vid you showed, deliberately abused circuit breaking for clicks and drama. In reality you wear gloves to protect your hands from flash burns, wear googles to protect your eyes from UV and sparks. Its a moments operation in msecs to pull down a knife switch and the distance of separation immediately quenches the arc flash. For 12V there is no arc flash very minimal risk in auto systems. For 24V you can stick weld but the flash with quench easily within <5mm. 48 volt systems need much larger distances (I estimate 8x ) so proposition is that 50mm is a safe guideline - which I hope to demonstrate in a vid when I get all apparatus together.

Note - dc stick welding is generally ca 20V at work piece (once the arc is struck and in the CC region but starts at ca 60V to strike the arc) and 100-200 A dc. So gloves and face protection is essential but its no great drama.

Ill informed amateurs dont realise a 48V panel system pushes out full current on a sunny day and improper specified breakers may fail to cut the load in a roof fire for example (see vids of mcbs failing including a very popular one in OZ until recently banned

Here is a Vid showing positive support for hypothesis that a large knife switch will work well.

Notice the system is 48v (4 panels) in full sunlight. The arc flash is produced and sustained BUT when the distance is ca 2" it quenches. BTW you can blow the flame out or remove its heat (needed to maintain ionisation temp of ca 6000c by a simple plant mister filled with water.)

I propose also that an immediate arc fire remedy in ELV systems can use a simple plant mist bottle (as well as cable cutters)

 

[Boron]

Thats w

That is what I use. A fraction of the cost of class T but they do the job perfectly with the same AIC capability.
It just shows what a rip off Class T fuses are

FWIW I investigated this Cabal of fuse making. It is after all a piece of wire and its there to protect the installation wiring. Over current surges of 5x can be tolerated in slo-blo cases (motor starters etc). Just consult the graphs showing I^2t values.. Much of the cost of a fuse is in setting up test facilities and gaining accreditation - hence huge cost of Class T. Same applies to Siemens type NH blade fuses in EU - not nearly so expensive.

For the DID guy this is all prohibitive but a fuse link is necessary.
I found that the word "fuselink" was coined during the 30s for automobiles. You can still buy a simple wire with crimped spade terminals. Its ca 4" long with fireproof braid and breaks at ca 80A. It can be attached between the battery terminal and the distribution board, or the alternator or even the starter motor
.
Its the last resort to protect entire car wiring. Courtesy Ford Motor Co.

So the resourceful DIY hacker might think, why not go back to basics.
I did some further digging and came up with a selection of silicon sheathed wire (called flexible wire). This can be sized for 5A upwards choose your AWG. The advantage here is that the Si sheath is fireproof and decomposes at 600C. Copper melts at 1000C but rapidly looses strength before then. So a piece of Si wire say 2" long stretched with a modest spring should surely constitute a credible fusible link. The other concern is to contain the flash particles and potential ignition source, ( glass tube? fibre sleeving used in heaters). Its what I would do and you cant beat the physics. Just test the link and record results and you have taken proper duty of care. Thats enough heresy for today.

 

[Johan]

Another data point: I just dead-shorted a 150A MRBF fuse using 330Ah 48V LFP and 4/0AWG cable. Results are here.

 

[meetyg]

@MattiFin sadly you have shown that you are under-informed on the matter. DC systems have used blade switches since 1920+. You can see evidence of this in railway electric traction switchboards (600Vdc 1000A), Electroplating, early telephone exchanges, UPS etc.
However, and this is where your ignorance shows, Electrical power distribution and switch rooms requires operators to be specially trained, equipped full body flash suits, gloves, head googles when operating manual switch gear. There are YT vids to educate you.
On a more domestic note, the vid you showed, deliberately abused circuit breaking for clicks and drama. In reality you wear gloves to protect your hands from flash burns, wear googles to protect your eyes from UV and sparks. Its a moments operation in msecs to pull down a knife switch and the distance of separation immediately quenches the arc flash. For 12V there is no arc flash very minimal risk in auto systems. For 24V you can stick weld but the flash with quench easily within <5mm. 48 volt systems need much larger distances (I estimate 8x ) so proposition is that 50mm is a safe guideline - which I hope to demonstrate in a vid when I get all apparatus together.

Note - dc stick welding is generally ca 20V at work piece (once the arc is struck and in the CC region but starts at ca 60V to strike the arc) and 100-200 A dc. So gloves and face protection is essential but its no great drama.

Ill informed amateurs dont realise a 48V panel system pushes out full current on a sunny day and improper specified breakers may fail to cut the load in a roof fire for example (see vids of mcbs failing including a very popular one in OZ until recently banned

Here is a Vid showing positive support for hypothesis that a large knife switch will work well.

Notice the system is 48v (4 panels) in full sunlight. The arc flash is produced and sustained BUT when the distance is ca 2" it quenches. BTW you can blow the flame out or remove its heat (needed to maintain ionisation temp of ca 6000c by a simple plant mister filled with water.)

I propose also that an immediate arc fire remedy in ELV systems can use a simple plant mist bottle (as well as cable cutters)

I think the problem with the first video is incorrect wiring (he mentioned it also).

DC breakers should have arc chutes in them, but in polarized (polarity sensitive) these are only on one side, including magnets to pull the arc into the chute.
When you run them the wrong way, the arc isn't pulled into the chute and combusts the breaker into flames (the arc has nowhere to go).
Take a look at this DC breaker teardown:

That's also why you shouldn't use a polarized DC breaker between an inverter/charger (AIO) and your batteries, because current can flow in both directions.

But for PV panels, I don't think it should be a problem, if the breakers are properly rated and are from a good brand (unfortunately lots of fakes out there).

 

[skarp0ye]

So I am planning on installing 4x Victron LiFePO412,8V/200Ah Smart in my boat. Would a MRBF on each battery be sufficient?

Other alternatives I am concidering, 4x Class-T/NH one per battery. Or 4x MRBF + 1x Class-T main fuse after busbar.

The reason I am concidering MRBF and not just Class-T is more to do with space constraints and ease of installation (tight space) than cost. But on a boat safety is primary concern, and I need to know it would be safe.

What would you recommend?

 

[HRTKD]

The American Boat and Yacht Council was supposedly going to approve the use of MRBF on batteries. That's a nice compact solution. However, the height of where the cable lug attaches adds leverage against the cell terminal. You would need to provide support to the cable so it doesn't put weight on the MRBF. This is all to avoid straining the cell terminal.

On the issue of 4 MRBF + 1 Class-T, that's probably how I would go. I'm a bit attached to the Class-T fuses and would feel more comfortable with it in the circuit.

 

[skarp0ye]

On the issue of 4 MRBF + 1 Class-T, that's probably how I would go. I'm a bit attached to the Class-T fuses and would feel more comfortable with it in the circuit.

Thanks for your input. How would you size the fuses in that setup?

They are all going into the Victron Lynx BMS 500A rated.

Was thinking 125A for each MRBF and 500A for the Class-T?

Or should maybe the Class-T be slightly lower at 450?

Expected max load is a 3000W inverter + 12V stuff. So probably a lot lower than 500A.

 

[Hedges]

What are the batteries rated for, 200A each?
Maybe current won't split evenly. The goal is to never blow a fuse.

3000W / 10V / 90% efficiency x 125% margin x 112% ripple factor = 467A

500A seems good for single fuse.
Maybe fuse per battery should be 200A each, even 250A.

You would like to be able to operate if a battery goes offline, and not blow fuses one after another if that happens.

 

[skarp0ye]

Yes batteries are 200A continous, 400A bursts.

Good points. 200 or 250 is probably good. But that would also require a bit thicker wiring i guess.

 

[osman.me]

http://www.dcmax.com.tw/LF105(3.2V105Ah).pdf

Looking from manufacturer specified graphs it looks like one cell would have:
3.30v @0.33C discharge (0.33*105=35A
3.07v @ 3C discharge (3*105=315A)

extrapolating to dead short and 0 volts (dI/dU) we get 280A/0,23V = 1217A short circuit current per cell.
2 cells in parallel would be ~2400A.
Note: connecting more cells in series doesn't increase the maximum short circuit current as you add also more resistance every time.
Real-world "short-circuit" current often increases with series connection as the cabling might be the actual limiting factor. But it is always below the maximum short circuit current in any case.

LiFepo4 seem to have rather small short circuit current compared to something like power-optimized Li-Po.
But that is what makes it safer...
(5Ah Li-Po is capable of 2000A short circuit current.)

extrapolating to larger systems 6000A breaking capacity ANL fuse would be ok up to 600Ah LiFePo4.
And lot less with some other lithium chemistries.

I don't think, LiPo batteries have higher short circuit current than that of an equivalent LFP one (when optimized for power only). In fact a single LFP cell can generate enough amps to make a wrench red hot

 

[Hedges]

This was the "cell-short-circuit-current-and-main-circuit-protection" thread,
not the "Will's racing drone" thread

What we're really interested in here is peak current that a pack can deliver, to determine what fuse is required.
Is 20kA interrupting class T necessary? Sufficient? for our kind of ESS.
That's the current I predict for 100 Ah LiFePO4 based on 0.17 milli-ohm IR, but maybe it is lower.

It may well be your Lipo packs can deliver even higher short circuit current than our LiFePO4.

Maybe the LG-RESU 48V packs need higher AIC fuses than the LiFePO4 server racks and PowerPro.
Or maybe those cells don't have the peak performance of ones you used for racing.

 

[osman.me]

This was the "cell-short-circuit-current-and-main-circuit-protection" thread,
not the "Will's racing drone" thread

What we're really interested in here is peak current that a pack can deliver, to determine what fuse is required.
Is 20kA interrupting class T necessary? Sufficient? for our kind of ESS.
That's the current I predict for 100 Ah LiFePO4 based on 0.17 milli-ohm IR, but maybe it is lower.

It may well be your Lipo packs can deliver even higher short circuit current than our LiFePO4.

Maybe the LG-RESU 48V packs need higher AIC fuses than the LiFePO4 server racks and PowerPro.
Or maybe those cells don't have the peak performance of ones you used for racing.

I don't know very well about what classes fuses do you need, but I'm pretty sure that most LFP can indeed deliver higher short circuit current than a similar LiPo one. Not the vice versa...!

 

[MattiFin]

Back to topic, something to muster:

https://www.enatel.net/wp-content/uploads/2024/08/Short-Circuit-Testing-of-Li-ion-in-standby-applications-Murray-Wyma.pdf

https://poweronaustralia.com.au/wp-content/uploads/03.-Power-On-Australia-POATLFeLi-48100F-E-Fault-Current-Test-Report.pdf

short-circuit currents seem to range 30C to 40C in these. Battery construction unknown, could be cylindrical high current or thick electrode high capacity prismatic cells.

 

[Will Prowse]

This was the "cell-short-circuit-current-and-main-circuit-protection" thread,
not the "Will's racing drone" thread

What we're really interested in here is peak current that a pack can deliver, to determine what fuse is required.
Is 20kA interrupting class T necessary? Sufficient? for our kind of ESS.
That's the current I predict for 100 Ah LiFePO4 based on 0.17 milli-ohm IR, but maybe it is lower.

It may well be your Lipo packs can deliver even higher short circuit current than our LiFePO4.

Maybe the LG-RESU 48V packs need higher AIC fuses than the LiFePO4 server racks and PowerPro.
Or maybe those cells don't have the peak performance of ones you used for racing.

A huge bottleneck I have found after testing piles of batteries with my surge station is that even if the cells can pull a ton of current, the small conductors that supply the bms and terminal heat up quickly And limit the current. For example the 100 amp hour seikon. Sure it's programmed to output 850 amps, but you have a couple small 8 gauge conductors supplying the bms. Best I can usually do is 550 amps from each pack. Regardless of the capacity and cells capability.

 

[Will Prowse]

Going from a 4/0 cable down to a BMS 6 gauge, when you're trying to pull over 500 amps, it's going to be a huge bottleneck.

When designing the system with that in mind, the big issue is having lots of battery packs in parallel.

2/0 and 4/0 is still very hard to push over 500 amps. So typically the resistance created will lower the current so that almost any DC fuse can stop it.

Also depends on the BMS over current protection design. The server racks BMS ovp trips incredibly fast. But for a millisecond, there will be a large DC pulse.

 

[Will Prowse]

If you're using a contactor or relay like a golf cart battery bms, you need a t class fuse. Those things can output a ton of current. Need some serious disconnect power on those.

And that's why they use t class fuses.

 

[Will Prowse]

All of this discussion about the cells true output is disregarding the entire circuit. If you're attaching 5X 4/0 to a single cell without a bms, you might have an issue.

To be on the safe side, I calculated some years back that every 30kWh of server rack batteries could create a good amount of current when connected to bus bars with equal length conductor. So I separate each 30kWh tower with its own t class fuse. And I actually had a dead short on camera and the fuse worked great. Not sure how it would hold up if I used a cheap ANL or marine grade, but for batteries this large, it's still preferred to throw a t class fuse on there anyways. Why not.

 

[Will Prowse]

Another data point: I just dead-shorted a 150A MRBF fuse using 330Ah 48V LFP and 4/0AWG cable. Results are here.

Exactly. And that's without an internal BMS. If you add a BMS to that test, using a typical fuse will produce the same results. Very easy to achieve a disconnect and extinguish the arc.

 

[robbob2112]

I calculated 8000a for a 12.8v280ah battery based on the internal resistance reading... if connected without a bms that may be possible, but it would require a hard short with a large conductor on purpose.

 

[Will Prowse]

I calculated 8000a for a 12.8v280ah battery based on the internal resistance reading... if connected without a bms that may be possible, but it would require a hard short with a large conductor on purpose.

Yes exactly

I'm putting lots of batteries on my resistor bank and I cannot get above 560A with the packs that have an internal BMS. Would be cool to see someone post a higher results.

 

[robbob2112]

Yes exactly

I'm putting lots of batteries on my resistor bank and I cannot get above 560A with the packs that have an internal BMS. Would be cool to see someone post a higher results.

Ha, take the bms off and bolt it together with a sacrificial switch.... make sure you are in full arc flash gear...

 

[Will Prowse]

I think if you want to pull more than 600A from a single 100Ah battery w/ bms, you need 3x 4/0 cables. My test rig with 2x 4/0 cables isn't cutting it. And even then, I just don't think it will manage to pull it off. My rig should be pulling 630A and the most I can pull with an 850A output battery is around 550A.

Server rack and vertical mount batteries OCP is extremely fast and trips before most fuses. Unless you pile them up and parallel a large number. That's the only time I see issues.


Great way to test all of this is to use a carbon pile load tester. Try it out. Let me know what number you get.

Look at the limit of cranking amps on the new lifepo4 cranking batteries. Those BMS are made for that and still are not pushing crazy high numbers. They actually don't have to because the voltage drop with lifepo4 is so much less than lead acid. When voltage stays high, the required current drops. Furthermore, motors like a stable voltage. Having it drop will heat up the coils and less total power output will occur.

 

[MattiFin]

Look at the limit of cranking amps on the new lifepo4 cranking batteries. Those BMS are made for that and still are not pushing crazy high numbers. They actually don't have to because the voltage drop with lifepo4 is so much less than lead acid. When voltage stays high, the required current drops. Furthermore, motors like a stable voltage. Having it drop will heat up the coils and less total power output will occur.

Somewhat correct for most AC motors but not true for DC starter motors. Starter motors spin faster and draw same or even higher current with higher voltage.

 

[MattiFin]

I think if you want to pull more than 600A from a single 100Ah battery w/ bms, you need 3x 4/0 cables. My test rig with 2x 4/0 cables isn't cutting it. And even then, I just don't think it will manage to pull it off. My rig should be pulling 630A and the most I can pull with an 850A output battery is around 550A.

Your ”rig” is probably the limiting factor.
2 meters of 4/0 cable shorting out ”infinite” 3.2v cell gives 10kA current and 40kA for 12v battery. Cable size is obviously not the first limit.

 

[Will Prowse]

Somewhat correct for most AC motors but not true for DC starter motors. Starter motors spin faster and draw same or even higher current with higher voltage.

That's very strange I'm going to have to research that. Typically when you have a voltage drop of any kind even with inductive load, the current goes up. That's why you can't run motors with lower voltage because they heat up excessively. Until the laquer on the coils starts to smoke. Then it creates a massive short circuit. I'm going to have to put a meter or scope on a DC motor when starting with lithium iron phosphate. They startup so much faster with that higher voltage. No struggles.