Fusing questions (Breakout from- what i would do diff)

The other thing to consider is that fuse wire is short. The longer the wire, the more heat dissipation can occur and the more current it can handle. So the same wire at 3cm will blow faster than one at 30cm. So this is probably why this same wire, even though rated higher amp, blows at a lower one because the runs are so short. The bus bar can't pull enough heat out of it faster than it's building up and it blows.

Korishan, I'm interested to know how you arrived at your conclusion. A short fuse blows quicker than a long fuse, all other things being equal?
Up until a few weeks ago, I would have agreed with you. But after watching one of Joe's fuse test video's, I found myself having to agree that a longer fuse blows quicker than a short fuse. To be more precise, the short fuse requires more Amps to blow than the long fuse.
Reason being that the connections on the short fuse (< 10 mm) act as heat sinks, whereas on a long fuse, the heat can't dissipate as quickly...
Your thoughts? cheers.

It depends. There is a sweet spot in middle often. ie less than 8mm slower. above 12mm slower... No idea where it goes but i have seen this behaviour myself.

But the relation of the length compare to current should be 1 to 1. But it have several outer factors that can cause this above to happen.

There are plenty of threads of the subject.
https://www.physicsforums.com/threads/length-in-relation-to-fusing-current.110357/

So it's the "clamping" effect. After following the link, I came away with: It should take the same amount of current to blow a fuse irrespective of length. Except when the fuse becomes so short that clamping effect comes into play.

As an armchair physicist, this is challenging some of my beliefs. :idea:

A "fuse" is a "fuse." It's designed to blow/ burn up, at a predetermined current rating. As such, the length of the "fuse", if changed, will no longer be the same "fuse." You can not use the same gauge wire at different lengths and call it the same fuse. You can test various fuses, all different in length until you find one that burns, consistently, at the same current, but if you change the length at all, you must test it again. Since 90% of the builds I e seen here are all over fused, I'd be very careful when replicating any of these designs.

At first Joes video was just good. He tested some wires and showed the results. Then he made it great by telling everyone to DO THEIR OWN TESTS! That's the point you should take from his experiments.

If you are using a wire that burns at 6A, with a 2000 mAh cell, a laptop cell at that, one rated for 1-1.5C max, per the data sheet, you are letting it pull 3C with a 6A fuse! They do make high drain 18650 which can be used a 3C, hell some up to 6C, but these laptop cells are not designed that way. The good thing is, and the reason no one has had a serious problem yet, is because they haven't actually pushed the cells that hard in their system...the packs have been build so that no single cell has to cure t that much current, which is good, but then why fuse them for it?

Before you are quick to reply, take out a paper and pencil and do the math yourself...then think about it...and you'll have no choice but to say, damn that assholes right...

Ok. Aside the possibility of standing on someones toes. Fusing at a cell level is essential. If chance in a million a cell goes short circuit there is going to be a fire. Fusing on a pack level is common sense, who wants to replace every fuse one every cell. Ok the resistance caused by fuses may be a factor, but I don't think that the energy lost would even be wort calculating. Fusing at a system level is essential. Google Jacobs Ladder.

No one is saying don't fuse them brotha...that's insanity. We all agree it's a must. But if your going to go through the work, then do the math and select the correct rating.

Edit to above:

The 6A fuse will protect the down stream bits, cables, inverters, etc..but not the cells. If you choose a correct fuse, you can protect your cells as well. Not that 6A would destroy the cell, but it would certainly deteriorate much more quickly.

Inverted18650 said:

A "fuse" is a "fuse." It's designed to blow/ burn up, at a predetermined current rating. As such, the length of the "fuse", if changed, will no longer be the same "fuse." You can not use the same gauge wire at different lengths and call it the same fuse. You can test various fuses, all different in length until you find one that burns, consistently, at the same current, but if you change the length at all, you must test it again. Since 90% of the builds I e seen here are all over fused, I'd be very careful when replicating any of these designs.

At first Joes video was just good. He tested some wires and showed the results. Then he made it great by telling everyone to DO THEIR OWN TESTS! That's the point you should take from his experiments.

If you are using a wire that burns at 6A, with a 2000 mAh cell, a laptop cell at that, one rated for 1-1.5C max, per the data sheet, you are letting it pull 3C with a 6A fuse! They do make high drain 18650 which can be used a 3C, hell some up to 6C, but these laptop cells are not designed that way. The good thing is, and the reason no one has had a serious problem yet, is because they haven't actually pushed the cells that hard in their system...the packs have been build so that no single cell has to cure t that much current, which is good, but then why fuse them for it?

Before you are quick to reply, take out a paper and pencil and do the math yourself...then think about it...and you'll have no choice but to say, damn that assholes right...

Click to expand...

Short a laptop cell full charge. It will do 10c or more even 50c. Even though it wasnt intended to. LiFePo4 are even more scary. Full charge, they can do 100c. Explode instantly. Lithium battery technology is scary.

So, do you fuse to protect yourself, or your batteries, or somewhere in the middle?

No shit! I know what they CAN do. (Note you cannot get 100A from a single 18650, not possible dude, the short circuit current MAX will be I= V/R where voltage is the state of charge ofthe cell, and R is the internal resistance). A "high drain" cell has a very low IR, much lower than a standard 'laptop' 18650. Please do a little homework. That's why I said before your quick to reply, do the math.

If you push a 1.5C cell to 3x it's recommended power output, you'll get three times the heat due to the cells IR. It's just the math brotha, that's all.



Edit:There are somehigh-drain 18650 with an IR under 40mOhmthat will shortcircuit over 100A. I was incorrect above, it can happen.

The fuse is not about protecting based on current from 1 cell.. Its about protecting current comming from xx of the other cells pushing there current into the single cell that is shorted... Thats when the 5-10A fuse is not a problem what so ever.

For instance my 80p packs can easily send 300A into 1 single cell if that cell werent fused and would have been shorted.

So you've designed the fuses to protect from a current inrush? There is no "outbound" protection.If, let's say, 10 of the cells in your pack, go high impedance (not shorted, just get old and stop doing there job) which causes the other xx cells to carry the load, those remaining cells now have to do all the work. I. Your build you would not know there was a problem until enough cells went out to cause the remaining to push enough current to blow your fuses.

I do see your point, I fully understand you can fuse your pack to protect the rest of the system and that's totally acceptable and will work, more times than not, without any issues. Again though, why not design the fuses to protect the cells as well? If you're going to do the work anyway, why not do the math, find exactly how much current you expect each cell to provide, and fuse it properly? Why not protect your investment? Someone please answer that question...

if you expect that each cell will share "x" amount of the load, why not fuse it that way?


Edit: and your allowing an "inrush" of 5-10A? Per cell? That's a massive tolerance if 1C = 2000/2500mAh.

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the entire point I want to make here is "Put some thought into what you are doing and why." You all know that I didn't just built one, I've built a bunch and lord willing, I will keep building them...but I want to encourage people to think about their designs, ask "why is it this" so the builds get better, designs get safer, and we all learn more...

Fuses are important so ask yourself, what am I trying to protect? What degree of tolerance do I need? What does the datasheet for the cell I am using say is safe? Am I exceeding that? Mike built a database, it's an awesome resource, use it.Etc, etc...

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A year ago I could mail these things all over the world no problem, now there's a fucking book of rules. After the article that was just released, more people are going to try this and that means more mistakes and once a few houses burn down, or someone gets really fucked up, it'll be impossible to find these cells anywhere...and forget shipping them...think about it, don't just copy what someone else does and test it yourself...if you can't do that, you should go buy a battery instead.


# RANT OVER!!! Haha

Its a fine line where you want and how you want your fuse wire. You want it protected but at same time you done want it to create a massive resistance either. (Yes i know, proper designed and all it wont be a big issue...)

I have designed my packs to max 1A per cell. I have fused mine with a 5A fuse wire. The reason is simple. The cells will easily break the fuse single handed. I do not want to raise the resistance to much and even though 1A resistance might not be high if so i need to go for glass fuses because of the booring task to solder tinned copper wire that thin

If 10 of my cells go high impedance out of 80 that wont cause much of an issue in the long term and no it wont cause the rest 70 cells to blow since that raise is so small. The likelyhood of inrush instead of outrush is bigger. The outrush is defined on the whole pack to 80A per pack. If 10 out of onepack goes bad lets say stone dead or just lack of charge/discharge my BMS will show me that in terms of that it will not balance with the other packs and fall out of that part.

Not Im NOT tollerating inrush of constant 5-10A if that would happen i would have had other fuses blown or even the inverter to blow long before that. It can potentially be 5A to single cell due to the fact that the fuse is not protecting it but then all other cells need to reject to take current. Since the pack has a main fuse to set the total input.

And as you said its important to have put a thought into the packs and the design about the safety part and more importantly you need to understand what protects against what and what parts are left out because of something else.

The reason you cannot ship them does not have much todo with Powerwallers building packs. That is general Lithium issues and i would say originate from mobile phone batteries going haywire on different places. (Its just a big guess though)I cant even ship RC batteries any more due to this and now i need to buy the more expensive ones locally or have it shipped by special delivery from abroad

Take me right now m8. We are all on same page here

I do agree, thereare nevertoo much safety systemsin a battery bank, even more important, is to usethe right kind of fuse for the job.

On your first pont, ( fuse against a cell that has a high impedance) i think there is no way to fuse against that... This has to be monitored, a bms willdetect a pack with a fewcells gone bad.

Those cell fuses we are using, are only good in most casesto "disconnect" a cell with a short... still not a bad idea,and easy to do, so whynot.
To calculatethis fuse with abreaking amperageat the same amperage you think to draw,iagree, but what to do with the occasional spikes in draw if there is ahuge loadlike a compressor starting?So someplay hereis needed.

For me, a good bmsand a "master fuse"with shunt trip controlled by the bms is the minimum you need.
There is a threadabout different kinds of breackers with shunt trips, there are like 3 different posts there ( one from me )... i am wondering why there are not more...

I think (hope) there will come more threads of breakers with shunt trips and such in future as this whole thing grows. Many people still uses normal fuses/breakers as main and perhaps due to that those with trips are expensive for some.

Another thing to consider is the placement and shape of the wires. We all know that heat rises. So if you build your pack to sit with the cells below the busbar, and the fuse wire is heavily inclined up towards the bar, then the heat will flow out of the fuse into the bar. This will increase the time it takes to blow. Now, it may only be like 2 seconds or 5 or sometime.

If you build the fuse wire to have a wave in it, then it will more consistently blow at the desired amperage for the given length. AvereRage Joe actually showed this in one of this videos. He took several pieces of wire, same length. One was curved up, another straight. The one that curved up actually blew faster. The straight one started to get red hot first before it blew. We don't want a fuse to glow hot for any period of time. We want it to blow at a particular amperage. At least, if you're custom making your fuses, I would suggest fast blow fuses. If you want slow blow fuses, buy the manufactured ones because they are pretty certain without a doubt gonna work as intended and no guessing work needed. Plus, they come in a snazzy glass shield to keep molten metal from landing on stuff.

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I agree with daromer, we need more threads on it. I'm not familiar enough with them and not at that stage in my own build to be really worried about them. But I do enjoy reading about what others have to say about these. I do plan on getting shunt trips and contactors to keep all that other stuff safe, and me, too

Good tip there.

The wire in all its glory the glass fuses are more accurate in any situation frankly. If i would start all over or if you do more packs that might be the one go to instead.

daromer said:

I think (hope) there will come more threads of breakers with shunt trips and such in future as this whole thing grows. Many people still uses normal fuses/breakers as main and perhaps due to that those with trips are expensive for some.

Click to expand...

Too expencive? nothing, i repeat, nothingis too expencive too protect those beautifull magnificent packs with many many many houres if work in them.

wim: don't forget the dogs and cats, too

Has anyone ever tried poly fuses?http://secondlifestorage.com/t-Fuse-Choices-Polyfuse

If not I shall experiment. My packs will consist of small building bloks 5a each. Polyffused to a bus bar

If the tests work

I havent. There is something in back of my head that tells me that they arent optimal due to how they work. Feel free to test. Im interested in the results!