Pack Fusing / Fuse Wire / Failure

OffGridInTheCity

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I'm sure you have been following @hbpowerwall's series on powerwall/pack failure. The recent youtube today (THANK YOU!)
shows that 4'ish cell level fuses out of 40 blew during a short but 4 did not. This seems consistent with the fuse wire tests - e.g. the majority of fuse wires burned thru but a few didn't.

I wanted to discuss expectations about packs and cell level fusing. For me - I've always thought of cell level fuse / fuse wire as a way to protect the overall pack from a failed individual cell that shorted. And indeed, its pretty clear that if 1 cell out of 40 shorts, the other 39cells will produce A LOT of amps and likely cause that cell to disconnect from the busbar electrically.

I've never thought that fuses / fuse wire would *all* (100%) burn thru if a pack shorted. This is why I have battery level circuit breakers / fuses - so that if there's a battery level short it doesn't depend on cell level fusing to all 'blow'. As we see from @hbpowerwall's work - after a catastrophic event you could very well have a few cells (say 4 of 40) that are still electrically active. So I have some brain-fuzz and would like to discuss....

Its not clear to me that 4 (of 40) cells that are continuing to be electrically active after a catastrophic event is a failure. Because....
1) With battery level circuit breaker/fusing - we should have adequate protection from 'system shorts'.

2) If a cell or 2 (a minority of cells) in a pack shorts - the fuse wire will electrically disconnect it from the pack as the pack will have enough amps to blow a lot of the fuse / fuse wires.
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3) This case... an overheating cell started a cascading fire?. Not sure how cell level fusing is supposed to protect against this - what am I missing?

4) Side question - if the remaining 4 of 40 cells are so week they can't muster 2a or 20a to burn a 2a axial fuse or 30awg fuse wire... do we think they will catch fire? or will they just short out till they drain power? They may get hot but will they burst into flames - I'm not so sure from various you tubes I've seen.

Not sure I was all that clear - but I'm confused about the takeaways so far and would appreciate comments :)
 

floydR

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Good question. I too was confused. One of the things I took from this was to fuse only one side of the battery. But I could be wrong, Another was 30 ga tinned copper wire burned at around 8 amps. Would it pop in a case of one cell shorting out internally? Which is why I thought was the purpose of the fuses. I think in the video before this one HBpowerwall tested tried to test this with mixed results. Another is I want a thermal camera.

Later floyd
 

daromer

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1. yes main fuse is for main short. Cell level should never take up that unless you screw it up.
3. Its not protecting towards that. Cell level fuse is ONLY and i say this for the xxxx time ONLY for pure shorts on cell level or if you skip the main fuse.
4. They can but they are not there to protect from main short. That is the main fuse purpose. HB bypassed that.

Im not confused over why a few were left. They just didnt short enough current to blow the last fuses. Period. They are there for high current dead shorts when you have large packs and not a few cells only ;) Can be calculated if you want.

None of this things we have done protects and makes it 100% safe. There is no such thing in terms of Lithium cells like the ones here. Its safer. And keep that min mind
 

Redpacket

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Could the CID's on some have opened?
 

daromer

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Redpacket yes they for sure could have! That is a good thing to check.
 

hbpowerwall

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Just finished a video after I saw this and another comment about the fuses I'll post it here soon as it uploads. All the fuses were attached well, and 4/5 fuses that didn't blow in the test were still ok. Still not sure whats going on but I hope my tinkering helps others.


1.jpg2.jpg3.jpg
 

hbpowerwall

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daromer

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Checked the CID on the cells?
 

Dr. Dickie

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So, am I not understand what is going on, or shouldn't it be (theoretically speaking of course), that the voltage of those cells were not high enough that given the resistance in the circuit they could not produce the necessary 2 A to blow the fuse--given the fuses were operational and properly functioning. So, would it not be that either the voltage of those cells were too low, or the resistance in the circuit (including internal) was too high. That resistance would include the fuse and the wire on the negative end of the cell.
Of course being in the pack, the cells would all have the same voltage--at least at the start. Could the cells have dropped in voltage fast enough to not blow the fuses given a low enough resistance? If not, then the resistance must have been different (at least at the start)--is the expected resistance of the fuses known?.
I am not really adding anything, just trying to make sure I understand what is going on--as I will be building my packs in a month or two, so this is every much in my interest.
 
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daromer

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The voltage cant be different since they are in parallel and during the intitial pop they cant drop fast enough unless
1. They werent properly connected from start
2. They are broken. ie either high ir and cant deliver the current or they have popped cid

The fuses was ok as i understood it.
 

Dr. Dickie

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The voltage cant be different since they are in parallel and during the intitial pop they cant drop fast enough unless
1. They werent properly connected from start
2. They are broken. ie either high ir and cant deliver the current or they have popped cid

The fuses was ok as i understood it.
Is there a positive temperature coefficient (PTC) device in these cells as well?
I am just learning about all this stuff, so I don't know if these cells come with one. Couldn't that also make the resistance shoot up?
I don't know if they react fast enough to have caused the non-event. Or whether it would have had to get waaaayyy to hot for it to have an effect.
 
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daromer

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Doubt it. its not that common but i shouldnt say no without knowing exactly what cell type it is. But with that said to build temperature it takes some time and not half a second. The CID on other hand could or can have been triggered by the pressure before or after.
 

ajw22

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As several already said, I too think there are 2 possibilities as to why the cell fuses did not break:
1. Dead cell (not shorted), as in CID popped or something.
2. Cell is degraded with very high IR, so much so that it can neither absorb nor supply 2Amps (or whatever fuse rating) to break it. Testing the cells for capacity and IR should be very telling.

My theory re the cell fire is that it was a case of #2, in addition to it having started to heavily self-discharge at perhaps a rate of 200mA~1000mA. Generate enough heat inside the cell to slow cook itself to catastrophic death, but not enough to activate the CID or break the fuse.

No evidence whatsoever, but I have a feeling that heavy self-discharging rarely starts suddenly under proper use, but that it starts with a barely detectable trickle and get progressively worse over time. So I think it is critically important to detect even slightly self discharging cells and remove them ASAP. That in turn leads to my firm belief that unmonitored high balancing current is potentially dangerous, as it masks self discharging cells.
 

Dr. Dickie

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Doubt it. its not that common but i shouldnt say no without knowing exactly what cell type it is. But with that said to build temperature it takes some time and not half a second. The CID on other hand could or can have been triggered by the pressure before or after.
Yeah, I kinda figured that out after posting that. That is only for a cell building up a lot of heat.
Thanks, still learning.
 

anton_voltx

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Edit: deleted post looked closer and see the bus bar system you are using. I would suggest potentially separating it a bit more from the cell level with some insulation. Like making the fuses poke up through some barley paper.
 
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