LiFePO4 prismatic cell short circuit current and main circuit protection

[HRTKD]

If you short between two terminals of the same cell and neither terminal is fused, it won't help to have other terminals on other cells fused, will it? It seems to me that if you're really guarding against short circuiting within the battery, you must put a fuse on a terminal of every cell.

 

[ArthurEld]

You will not be protected from a single cell short but it will be impossible to short all of the cells.
I am fairly sure that the more cells involved the worse it is
So the more you break it up the less # of cells can be shorted without blowing a fuse

 

[Whinny]

Electrically a fuse can go anywhere in a circuit and still give protection, practically it should be a close to the positive as reasonable. The only way an additional fuse in a series circuit would give more protection is if it was lower value. The fuse won't blow in order of perceived current flow.
BTW-is the "current" thinking still neg to pos or pos to neg?.....there were 2 thoughts on it when I was in school....
More fuses post to post would protect the cell to cell circuit internal to your battery as you say though.

 

[cinergi]

Electrically a fuse can go anywhere in a circuit and still give protection, practically it should be a close to the positive as reasonable. The only way an additional fuse in a series circuit would give more protection is if it was lower value. The fuse won't blow in order of perceived current flow.
BTW-is the "current" thinking still neg to pos or pos to neg?.....there were 2 thoughts on it when I was in school....
More fuses post to post would protect the cell to cell circuit internal to your battery as you say though.

In a negative-grounded system the fuse should be on the positive.

To a prior point I made, a fuse in the middle of the pack helps reduce the chance of an unfused short between cells *and* if the fuse blows, the maximum remaining unfused voltage is now 24 volts (it electrically breaks the pack into two pieces).

 

[ArthurEld]

I am very confused about the path of current.
I dropped a wrench across 2 busbars and the fuse didn't blow. It kept sparking and melting my wrench until I removed it.
The fuse was a MRBF fuse in a terminal mount. Why didn't it blow? Why did only some of my terminal screws seize?
And why did only some of my terminals get pitted from the heat?

cinergi answered before I posted

I think it is called a short because you make a shortcut in the path

 

[Whinny]

trying to figure out how to explain my thoughts.....
If you have an external short with 1 fuse your 48v battery will always have 48v potential across the blown fuse, doesn't matter where it is.
This is getting too confusing...
Your fuse didn't blow because your short had enough resistance in it to keep current flow low enough to not blow it. A welder can keep a nice 3/8ths arc at 20ish volts and 60 amps...

 

[ArthurEld]

trying to figure out how to explain my thoughts.....
If you have an external short with 1 fuse your 48v battery will always have 48v potential across the blown fuse, doesn't matter where it is.
This is getting too confusing...
Your fuse didn't blow because your short had enough resistance in it to keep current flow low enough to not blow it. A welder can keep a nice 3/8ths arc at 20ish volts and 60 amps...

Well, the reason I am asking is because I don't understand and I don't want to add a bunch of fuses that aren't going to help anything.
What you are saying makes sense.

I am being careful and I will do what I can to make sure I never short again.

What you are saying is that a fuse will not help with the problem I had no matter where I put it. That is disturbing.

 

[John Frum]

current flows from its source to the polar opposite.
Your wrench made a circuit that bypassed the fuse.

 

[Whinny]

No, a fuse will help. I may have misunderstood your initial short. If the fuse was not in the "short"circuit (post to post battery) it would not blow as smoothJoey said.

 

[ArthurEld]

I could cut my wrench in half and put a fuse in it.

 

[John Frum]

I could cut my wrench in half and put a fuse in it.

Your wrench was the fuse.
Just a very high amp rating.
You could alternatively insulate the wrench.

 

[HRTKD]

I could cut my wrench in half and put a fuse in it.

What size/type of fuse?

 

[Whinny]

I must apologize for any confusion from my posts, I forgot the original cause of the short.
When I worked in a telephone central office 45 years ago all the tools were taped up really well except the working part. We had rubber blankets for working on power circuits. 48 volts unlimited current, more than a few tools still melted.

 

[ArthurEld]

I will continue researching this idea. I already covered my tools in electrical tape and I am being very careful.

 

[pablo fiasco]

(I've also read the replies below).

Use a fuse. What if the BMS short-circuits? The point of a fuse is to protect against downstream failures and a BMS is a great example of something that can fail. A BMS with short-circuit protection is not a fuse. A fuse is simple. A BMS is not.

Do you have to? Of course not - it's your money and your house or whatever that you're risking and only you can determine the level of risk you're willing to take. We're just here to inform you so you can make an informed decision.

According to my information the gap necessary to insulate against 12V is .004mm. My feeling is that a fuse is a good idea but a Class-T is way overkill.

 

[cinergi]

According to my information the gap necessary to insulate against 12V is .004mm. My feeling is that a fuse is a good idea but a Class-T is way overkill.

That information misses the point. There's a difference between creating a spark and extinguishing one. There's also a difference in how the fuse fails under extreme load. ANL (for example) may fail in such a way that there are small enough gaps to keep the current flowing via arc'ing. A Class T is better designed to properly interrupt the circuit under the currents we see in short circuits on LiFePO4 batteries.

 

[pablo fiasco]

That information misses the point. There's a difference between creating a spark and extinguishing one. There's also a difference in how the fuse fails under extreme load. ANL (for example) may fail in such a way that there are small enough gaps to keep the current flowing via arc'ing. A Class T is better designed to properly interrupt the circuit under the currents we see in short circuits on LiFePO4 batteries.

I have sorted out the difference between the amps to blow a fuse and the volts to arc the blown fuse. There's lots of overkill advice around on any subject you care to mention.

 

[cinergi]

I have sorted out the difference between the amps to blow a fuse and the volts to arc the blown fuse. There's lots of overkill advice around on any subject you care to mention. Just look around at how the world is in lockdown.

I prefer to operate based on facts for which I can provide evidence. There is more than sufficient evidence that AIC ratings are important: https://electrical-engineering-port...ive-device-ratings-you-must-properly-select#3

If you can provide evidence that you can short-circuit a 12v 280Ah LFP pack (4/0 wire equivalent everywhere) and that an ANL fuse will safely blow 100% of the time in a statistically relevant sample, then we will all have learned something.

 

[HRTKD]

I prefer to operate based on facts for which I can provide evidence. There is more than sufficient evidence that AIC ratings are important: https://electrical-engineering-port...ive-device-ratings-you-must-properly-select#3

If you can provide evidence that you can short-circuit a 12v 280Ah LFP pack (4/0 wire equivalent everywhere) and that an ANL fuse will safely blow 100% of the time in a statistically relevant sample, then we will all have learned something.

From the article, this is fairly cut and dried:

As a general rule, the ampere rating of a fuse or a circuit breaker is selected at 125% of the continuous load current. Since the conductors are generally selected at 125% of the continuous load current, the ampacity of the conductors is typically not exceeded.

Where I think a lot of confusion exists is in what type of over current protection is to be used. I went with a Class T fuse immediately downstream of my LiFePO4 battery because that's what was recommended. I have DC circuit breakers elsewhere in the system, because that's what was recommended.

I couldn't provide an explanation for why a Class T, ANL, MRBF, circuit breaker or whatever, should be used in each case. I suspect it comes down to the interrupt rating (section 3 of the article), but even after reading the article I don't understand it. It sounds like a high interrupt rating is good, but I don't know how to right-size my OCP.

 

[Gazoo]

I have a 100 amp Samlex MRBF fuse connected to my 24 volt pack. The specs follow so I think I am ok. If I had a 48 volt system then I would use a class T fuse.

• Marine Rated Battery Fuse (MRBF Series) made by Cooper Bussmann
- Voltage rating of max 58 VDC
- Current ratings of 100 A (MRBF-100), 200 A (MRBF-200) and
300 A (MRBF-300)
- AIC of 10000 A at 14VDC, 5000 A at 32 VDC and 2000 A at 58 VDC
- Ignition protected as per SAE J1171
- Weather Proof (IP66)
Caution! The fuse should be placed as close to the battery Positive terminal as possible, preferably
within 7” of the battery terminal.

 

[sduser]

There are two ratings for an over-current protection device (OCPD): current rating and interrupt rating. The device will safely open and clear the circuit between these two points. There is a time and current component associated with the current rating defined by the trip curve. Below and to the left of the trip curve, the device will not open (normal operation). Above and to the right of the trip curve the device will open.

If the current exceeds the interrupt rating (such as in a short circuit), the device can fail in such a way to still allow current to flow. This is the topic of this discussion, not the ordinary current rating or what type of device to use.

 

[Gazoo]

I don't know who you are referring to but since you replied after I posted I hope you are not referring to me. My post is on topic.

or what type of device to use.

Not sure what you are getting at here because the type of device is related to the topic. Can you explain please? Thanks.

 

[sduser]

I don't know who you are referring to but since you replied after I posted I hope you are not referring to me. My post is on topic.


Not sure what you are getting at here because the type of device is related to the topic. Can you explain please? Thanks.

I was replying to @HRTKD but we replied at the same time. This topic is on short circuit protection not device type preference or meeting other requirements (such as local building codes). Both fuses and circuit breakers can be capable of meeting a given current and interrupt rating, so in my mind device type is not really important for the purposes of this discussion. I may be missing something though..

EDIT: I was only considering fuses, so my original question was tailored towards fuse type, but the same general principles apply to all OCPDs I think. Sorry about the confusion.

 

[cinergi]

Yes I'm roughly equating type with AIC. That's why I used ANL in my example as a bad type of fuse (because its AIC is too low for this application). Strictly speaking, it's more accurate to only discuss the ratings (AC/DC, voltage, current, and AIC). Circuit breakers have the same specs.

 

[pablo fiasco]

I have sorted out the difference between the amps to blow a fuse and the volts to arc the blown fuse. There's lots of overkill advice around on any subject you care to mention. Just look around at how the world is in lockdown.

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. I understand that an MRBF blows internally unlike the Class-T that blows externally. I am confident that the gaps resultant inside a blown MRBF are enough to stop a 12V arc. I respect your opinions. You're all much smarter and educated than I. However these are the conclusions I've come to. If I burn my unit down I can't say I wasn't warned.