diy solar

diy solar

LiFePO4 prismatic cell short circuit current and main circuit protection

How about a 48V pack made from three cells in parallel and 16 of those groups in series 3P16S? that is 840 Amphours and at 20C short circuit current would be 16,800 Amps. Is my calculation correct.
Yup. 17ka at 48V could certainly cause some fireworks. Not something to be taken lightly!!! I think I'm glad I stayed with 12V!!! And 400Ah.
 
Last edited:
Perhaps in AC power?

My professional work hasn't been very deep into serious power, but I have had some exposure to issues regarding arcs, fuses and breakers, inrush, etc.


Double check the let-through current curves for class T fuses.
The way I read them, if you have 200,000A available fault current, a 400A fuse will interrupt fast enough that equipment doesn't have to be able to withstand more than about 28,000A. But if hit with 20,000A, let-through is still about 15,000A.

So I think the current limiting is mostly a benefit at the much higher AC current these fuses are rated for.


Although, you mention slow ramp up of current from lithium batteries (meaning they are doing the current limiting, not the fuse.)
Do you have links on the current/time response of lithium batteries to short circuits? All I have to go on is the measured internal resistance on vendor data and BMS measurements that people have posted.

BMS is MOSFETs, so I'm not inclined to trust them. I've seen enough problems with inrush limiter circuits.

I have found LFP short-circuit current graphs. Enter "lithium battery short-circuit current" into your search window and click on "images" and look for something like this link.


This particular one bothers me a bit in that the peak current is indicated to be 1075 amps for a 160 Ah battery. That seems low. Also the current rise time is maybe 5 seconds. That's slower than I recall seeing in the past. Though I can't find that one.

Here's another one. An 8 Ah battery this time. And a faster rise time. Looks to be 2 or 3 seconds. Again, max is about 10C.


If you combine the above with this time-current curve, it seems that the Class T fuse can open in about 10 milliseconds at a current that is about 550% of the fuse rating. If the above examples are correct, a 150 amp Class T on a LFP that can deliver about 10C short-circuit current will interrupt before the current hits its peak (e.g., not long after the current hits about 5C).


If I'm right about the above, the "current limiting" function of a Class T on an LFP battery is a different mechanism than that where it's applied at AC. While the short-circuit current rise time in a worst-case AC short-cicuit (with 1.6x DC component) is about 3 ms, and there is current limiting and a let-thru, with the battery, it seems the fuse simply times out while the current is on its way up.

A year or two ago I thought I was looking at an LFP short-circuit curve that rose faster than the few seconds I'm seeing above. That leaves me a bit unsettled.

The first (left) let-through set of curves in the link below are for an AC & DC rated Class T fuse. It's an AC set of curves, but does indicate current limiting action at currents that we might see from LFP (unless the above recordings are correct). If an LFP DC current can rise more quickly than an AC current, that should only hasten current limiting action I think. So I'm not sure there wouldn't be some current limiting with DC. In these curves a 200A fuse limits a 2000 amps symmetrical available to a peak of 3000 amps. This kind of says a 10kA DC current would be limited by the same fuse .... I think.


Tomorrow I'm going to look further for LFP short-circuit current recordings.
 
Perhaps in AC power?

My professional work hasn't been very deep into serious power, but I have had some exposure to issues regarding arcs, fuses and breakers, inrush, etc.


Double check the let-through current curves for class T fuses.
The way I read them, if you have 200,000A available fault current, a 400A fuse will interrupt fast enough that equipment doesn't have to be able to withstand more than about 28,000A. But if hit with 20,000A, let-through is still about 15,000A.

So I think the current limiting is mostly a benefit at the much higher AC current these fuses are rated for.


Although, you mention slow ramp up of current from lithium batteries (meaning they are doing the current limiting, not the fuse.)
Do you have links on the current/time response of lithium batteries to short circuits? All I have to go on is the measured internal resistance on vendor data and BMS measurements that people have posted.

BMS is MOSFETs, so I'm not inclined to trust them. I've seen enough problems with inrush limiter circuits.
AFAIK batteries wont do much to limit the current rise time. Wiring inductance limits the current risetime but things get smoky pretty fast especially on 48v systems.
Wiring inductance is roughly 1uH/meter, assuming short occurs 1 meter from bms the loop inductance is ballpark 2uH. Lets say bms and battery wiring increases this to 5uH: 50v short results in 10 amps per microsecond current risetime. 200uS breakin time would result 2000A (doable to break with proper design)
On some other bms I have seen claimed 600us response time, this would be 6000A

On 12 volt system the current risetime is only 1/4 of the 48v system and things are lot easier
 
Several LiFePO4 cells (forget if it was 100 Ah or 280 Ah) had ratings IR = 0.25 milliohm, and test results about 0.17 milliohm.
Using that and 3.4V per cell, I came up with 20,000A (regardless of pack voltage) for a single series string.

I found one report of 100 Ah AGM having 4000A short circuit current.
Automotive battery CCA current and voltage ratings indicated about 3000A into short circuit, zero volts.
I figure these don't scale with capacity due to resistance of plates and busbars/terminals.

The LiFePO4 batteries probably have something involving ion transport that drops current below my simple ohms law calculation. A forum member did mention something on the topic.

There was a video of a battery bank being shorted through a lower rated fuse, which interrupted the current just fine. But I calculated the resistance of wire shown was sufficient to reduce current to its AIC rating.

In AC applications, line voltage comes from a transformer rated for a few percent regulation, could be 2.5% above nominal at no-load dropping to 2.5% below at full load. So 5% voltage drop for a 200A transformer means it could deliver 200A / 5% = 40,000A. Wire resistance can further reduce that. Typical residential service I think is about 20kA fault current, so breakers rated 22kA AIC are sufficient. Industrial, can be 100kA and higher.

The class T fuse curves show 0.01 second interrupting up to a few thousand amps, which is 1/2 line cycle at 50 Hz. They don't show min/max time as breakers often do. They don't show what the faster trip times are for 10x higher currents; only the let-through curves suggest that.

For AC applications, one of the concerns is an arc among three phases continuing beyond 1/2 line cycle, when one line crosses zero (so would want to extinguish the arc), because another phase puts current through the same plasma fireball. With a fuse on each phase, the high current is supposed to open the fuses, so you don't get a sustained 0.5 second or so arc continuing to deliver intense heat and UV. (That could be hundreds of times more power than my arc welder, and safety calculations determine protection required based on several calories per square centimeter on the surface of your body.)

With DC from battery, no zero crossings to interrupt arc or help fuse open. AIC rating is 20kA.
With AC, maybe current isn't interrupted in less than 0.01 seconds, but voltage is dropped across the plasma and sand inside fuse. AIC rating is 200kA.
The 600V fuses are several inches long.
 
Several LiFePO4 cells (forget if it was 100 Ah or 280 Ah) had ratings IR = 0.25 milliohm, and test results about 0.17 milliohm.
Using that and 3.4V per cell, I came up with 20,000A (regardless of pack voltage) for a single series string.

I found one report of 100 Ah AGM having 4000A short circuit current.
Automotive battery CCA current and voltage ratings indicated about 3000A into short circuit, zero volts.
I figure these don't scale with capacity due to resistance of plates and busbars/terminals.

The LiFePO4 batteries probably have something involving ion transport that drops current below my simple ohms law calculation. A forum member did mention something on the topic.
There is suprisingly little info "out there" about the shot-circuit capability of prismatic LiFePO4.
Best I have seen is http://pe.org.pl/articles/2017/5/13.pdf but I just find it hard to believe that large prismatic cells are limited to apprx 6C

I could measure one myself but only LiFePO4 cells I have are 18650 cylindrical A123 high current cells that happily deliver 30C at 2.5 volts and probably roughly 100C on short-circuit :cautious:
 
There is suprisingly little info "out there" about the shot-circuit capability of prismatic LiFePO4.
Best I have seen is http://pe.org.pl/articles/2017/5/13.pdf but I just find it hard to believe that large prismatic cells are limited to apprx 6C

I could measure one myself but only LiFePO4 cells I have are 18650 cylindrical A123 high current cells that happily deliver 30C at 2.5 volts and probably roughly 100C on short-circuit :cautious:
The paper you referenced is clearly the source of the images I linked to above (that I randomly found on the web). It would be nice to have some other reference with similar results. I found a half dozen papers that might contain that but most were not accessible without some login or didn't sound promising enough to download.

The #1 battery shown in Figure 1 of the paper looks to be a Calb battery very similar to the SC-200 Calb batteries in my system. Kind of reassuring I guess. Presumably the author didn't use 12 feet of #6 wire to his short-circuit switch. I didn't see anything on his test setup. I'll look closer when time permits.

 
I will do my best to be sure it doesn't happen again. You are welcome to learn from my mistake or make it yourself.

I think most of the aluminum melted and exploded off of the terminal immediately. But that didn't break the connection.
The stud burnt off and disintegrated. The busbar melted. All that still didn't break the connection.
The arch didn't stop until I picked up the ratchet. I don't know what would have happened it I didn't remove the ratchet.
I suspect it would have burned everything possible until there was nothing left to arch to.

I shorted on half of a 48V battery. The stud nuts seized on the cells that were involved in the short.
Since the terminal is destroyed on one cell, I can't use it. The other cells seem to be fine.

So, this lightning bolt from hell only cost me about $100. But it reminded my of the ticking time bomb I have in my garage.
I have to do what I can to reduce the likelihood of this happening again. And it seems like I should try to warn others.
But so far no one wants to hear my warning. They think I'm a party pooper. I'll give up soon.
right on buddy, I think too many people are carrying over their lifelong experience with AGM and autos, who has done a short messing with cars, mostly its small wiring/screwdriver etc bit of a flash but never a sustained arc AFAIK. But a bank of Lions well , whole different scene, and they look so innocent dont they, you dont get a shock or anything, so get complacent till the worst happens. Anyone know how to extinguish an arc? My guess bucket of sand on hand, water? Perhaps a friendly stick welder could help if he has a dc stick welder (most modern ones are). Someone need to try it out Please and let us all know
 
I've more or less given up trying to get the actual SCCR for my battery. It's been suggested to me that the battery BMS might be built to limit the SCCR regardless of what one might calculate - but who really nose? My inverter mfg wants me to use a Class-T but when I quiz them closely they concede that the MRBF will be okay. Sounds like we're chasing the same bunny around the same bush. I'm going with the MRBF.
The Nose Knows! To my knowledge, a BMS can not be made to restrict the SCC of a battery. They are electrical devices (Usually MOSFETS) and WILL burn up when their specs are exceeded. SCC is a CATASTOPHIC failure of the battery internals.

I find it funny that people are spending thousands on batteries, chargers, buses, inverters, cables, etc. but baulk at spending less than $100.00 USD for a fuse that may save most of their investment in the case of a failure.
 
The Nose Knows! To my knowledge, a BMS can not be made to restrict the SCC of a battery. They are electrical devices (Usually MOSFETS) and WILL burn up when their specs are exceeded. SCC is a CATASTOPHIC failure of the battery internals.

I find it funny that people are spending thousands on batteries, chargers, buses, inverters, cables, etc. but baulk at spending less than $100.00 USD for a fuse that may save most of their investment in the case of a failure.
Indeed I have been banging on about this very concern. Such trivial attention is given to the design of the dc distribution system . Most have little idea of whats needed to break a full load current from an array of panels on a full sunny day. I got the collective yawn when I proposed old school large knife switches (used by millions throughout far east). Distance quenches ELV arc flashes but the switchgear and fuse mafia dont want you to know this.Screenshot 2024-03-02 072646.png
 
Thats w
AFAIK class T fuse is expensive because of the fast acting and "current limiting" feature.

NH blade fuses are like 10 usd per piece and often rated 40kA 250 VDC or 80 kA 440 VDC
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
 
@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)
 
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
Screenshot 2024-03-02 153631.png.
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.
 
Last edited:
@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).
 
Back
Top