24 volt 280ah compressed..isolated studs...fire

joeblack5

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Please comment on this proposed build.

After reading thru some of the fire / burn threads I changed my design to incorporate space between the battery that can be flooded with water in case of an emergency.

The frame is welded from bed frame angle iron. 3/8" wood in the bottom and 1/16" pvc sheets on the side to prevent the cells to contact the steel frame..strips of 1/16" rubber roofing glued with pieces of double side tape to separate the cells from each other.

To avoid mechanical stress on the terminal I propose two layers of braided tinned copper in diagonal. Thought is here larger distance/ surface allowed for better thermal separation between cells and at the same time allows more flexibility.

Used stainless studs without starter threads as to give maximum resistance against pulling out the aluminum thread.

To avoid or better limit current thru the stud I want to use an insulating washer between the top of the braided wire and the compression nut. This I hope will limit potential heat generation in the stud.

My other thought is that the flat braided strip will make multiple contacts with the aluminum battery terminal.

The stud that is supposed to carry no current will have the bms wiring on top thru the red ring terminal.

The idea with the rubber roofing strip is to allow space between the cells to insert temp sensors and in an emergency the ability to cool the cells with pouring water over them.. emergency only!

The steel frame is a tight fit, the last cell will have to be pushed in. There is no space for the cells to expand.

Please give me your thoughts about the proposed approach.

The next one I would like to build in an aluminum or even stainless box that can better contain a thermal runaway event and also that I can flood the box with water to reduce the chance of more cells getting affected.
 

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joeblack5

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Some more details..bms chargery..
Cells listen 272ah
Total voltage 24 volt
Max cel voltage 3.45 Volt

Max charge current 50 Amps
Max discharge current 150 Amps

Johan
 

Bob B

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You have really put a lot of thought into this .... thanks for posting.

I am curious how the flexible braid works out. I am considering using it when I re-do my pack configuration, but I am planning to press copper around the ends and then tin the copper. This is the first time I have seen it used without doing that.

I personally would spend more time thinking about trying to control the cell compression .... and less time thinking about spacing to make it easier to control a fire. Fire is a very rare event with the LFP cells, but compression has been shown to affect the life of the pack significantly.... If you are going to have the pack tight with no ability to expand, it needs to be sized so it is tight at full charge since the cells naturally expand and contract even under optimum compression. It the compression on the pack exceeds 17 PSI, that will shorten the pack life.

I am planning to use sand as an extinguishing method rather that water.

What are you going to be using the pack for?
 
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Rider

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After reading thru some of the fire / burn threads I changed my design to incorporate space between the battery that can be flooded with water in case of an emergency.
I'm thinking back to my chemistry days, and it seems to me that water and lithium is not a good idea at all.

Lithium reacts intensely with water, forming lithium hydroxide and highly flammable hydrogen. The colourless solution is highly alkalic. The exothermal reactions last longer than the reaction of sodium and water, which is directly below lithium in the periodic chart.
 

joeblack5

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Thank you Bob, seeing the couple of accidents, the diy approach, the low quality of the bms systems and the unpredictable quality of the cells I feel that I have to do a lot more.

Reading thru fhorst's unfortunate fire issues and everyone's opinion it seems that this technology easily can have issues.
I am curious about experience with bulging of the short sides of the listen and eve cells.. also what people think about the open space between the cells where there is no rubber separator.. if the cell casing would get distorted and bridge that gap then that would not be good.



This bank is for our DIY expedition rig.

The metal holding case is dimensioned so that new cells that still have concave sides will fit tight, so they can expand to flat.

I have used some Chevy volt batteries in my EV project and those pouches can and will swell if not contained.. it seems that is all what Chevy did.. so I figured that keep it from expanding beyond the cell case's normal dimension is good enough but I do not know if that is the whole story with the lishens and eve's.

I am also interested in people experience with higher current going thru the stud and the I2R heat developed in the stud. The heat and loctite are no friends either.

Then another part is the contact between the battery post and the connecting bar.. the smallish M6 and the aluminum thread pull out limitation does not give a lot of contact pressure to actually confirm the the tinned copper to the imperfect flat area of the battery post. ...and that is besides the aluminum oxide problems.

I wonder how people are sure that the both areas are indeed in full metallic current conducting contact.

That is why I thought it would be better for multiple smaller contact, similar as the gold collar washer that is used between a microwave and it's antenna cavity.... I know skin effect reasons do not apply here with DC.
So the easy way was using the braided sleeve directly as then there are hundreds of contact spots.

Rider, With the couple if burn pictures it seems that if you can contain the thermal runaway then at least not the whole pack will catch on fire.

The opinions about lithium ion fires are highly debated.
The aircraft industry tell the flight attendants to douse the lithium fires in water and be aware of potential toxic fumes.. I remember the highschool experiment with pure lithium or sodium and water but very little in our batteries is pure.. I also remember the 2H2 and O2 exploding balloons. Fun times.. So a bit of burning of of H2 with an already open fire would not be my main concern.. going from one runaway cell to 6 runaway cells seems to be much worse as nothing can be done at that moment anymore..

Please discuss,
Thanks Johan
 

HighTechLab

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Hasn't fhorst had 2 battery bank fires within the last year, meanwhile everyone else is doing just fine following standard build practices?

How do you plan to compress your cells with a gap between them?

I'm certainly not a fan of the metal braiding.
 

joeblack5

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He htl,. The gap is filled up with the rubber strips, see pic.. so about 1/16".

I did not study other people's setup but fhorst has/ had a very large setup..most people seem to be satisfied with much smaller systems. He seems to be also pretty forward with what he did. Maybe others are a little more shy about their pyrotechnics,...especially when insurance is getting involved and having spaghetti wiring images and non UL listed bms systems.

Can you elaborate why you are not a fan of braided wiring? I gave you my arguments why I choose it.

Thks, johan
 

wiseacre

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I'm certainly not a fan of the metal braiding.
Can you elaborate why you are not a fan of braided wiring? I gave you my arguments why I choose it.
Inquiring minds would like to know.

I know next to nothing about building a battery but somewhere on the forum was a discussion about using DYI cables and how if they were not exactly the same size would have different resistances and the BMS would read those differences (and not the true cell voltage or whatever) leading to something, something or other but the gist was not good.

I'd like to build my own but not until I can grok everything that goes into it.
 

joeblack5

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In this picture you can see the battery cage and the rubber separators. The set of 8 is in there right. So any swelling can I lay take the the concave part out of the cells.

I am surprised that not more people are using flexible connections.. it seems to be bad practice to put stress on battery terminals. I expected some more comments .

Johan
 

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HighTechLab

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Normally the braided cable like shown in the picture has a solidly crimped end so that all strands have solid connection. It's very common to see this material as ground straps in industrial control panels.

When setup like this, only the area under the washer has direct contact and is a part of the circuit, whereas with a solid busbar, everything has direct contact to either itself or the terminal. The strands that don't have solid/direct contact to the stud/flat now are an even higher resistance path for current to be carried across, which means more heat.

I see your logic behind it but I don't agree that it solves a problem without creating more additional problems.

See crimped ends: https://www.mcmaster.com/braided-cable-grounding/emi-reducing-grounding-ribbon/
 

Short_Shot

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Flooding an electrical/lithium fire with water would be just about the worst thing I could think to do.

My thought was to use a relatively large C02 cartridge and a spring loaded discharge pin with a fusible link if you must do something like this. Normally c02 extinguishers aren't recommended due to their low physical reach but inside a battery box this is irrelevant.
 

mrzed001

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I think it is good and bad in the same time :)

Compression is good.
But there is no electric separator between the cells and the metal frame. On the side and on the bottom.

If you want a reliable fire extinguisher then put a sandbag on top of the battery. In case of a fire the plastic melts and sand flows down.
 

Short_Shot

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I mean sure. But lipo and lifepo4 are different and have very different working extinguishing methods.
 

Short_Shot

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The problem with sand is it must be spread out over everything.

That "bunker" solution will not do that for a pack our size. It will just dump a small pile on top wherever the hole is. If you need to fill the whole thing, you need sufficient clearance over the top of it for the pile to form and naturally spread.

A C02 canister can flood virtually any size container and should more or less suffice for our type of build since any fire occurring will likely just be insulation on wiring. The cells themselves do not combust on their own like lipo does as long as you can prevent a nearby fire from igniting the electrolyte.

This also depends on proper fusing of course.


At least that's the theory as I understand it.

Water will also do a fun thing where it shorts out all your remaining cells together.
 

Boondock Saint

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In this picture you can see the battery cage and the rubber separators. The set of 8 is in there right. So any swelling can I lay take the the concave part out of the cells.

I am surprised that not more people are using flexible connections.. it seems to be bad practice to put stress on battery terminals. I expected some more comments .

Johan
OK then . . .

As much as the braid might have enough mass between cells to carry the load without introducing resistance, the contact of the braid isn't as much as a BB, even compressed by the bolts and washers. Secondly it leaves the metals subject to oxidation which will introduce resistances, varying. So over time you may have to redo and clean that aluminum shoulder. Then there's the matter of inconsistent readings if you use a BMS.

People DO use flexible BB, premade with flat lugs on the ends in order to have hard contact controllable mating. This gives you then best of both worlds. Personally I'm not doing it due the nature of my 2P16S setup.

And about your metal housing and insulation. Lack of insulation. No joke. Buy some HDPE matt and insulate the hells out of the sides and ends of the batts to the metal frame. If it were me, I'd be on amazon RIGHT now buying HDPE oven matt that's wide enough to lay across the entire bottom and up both sides taller than the rim of that box. Just lay it on top of the wood like a cradle, place the batts back like you have them and compress them before it's too late.

I think in case of a "thermal runaway event" where these were to catch on fire, then only running away if going to be you running away with flames in the background, not in slo-mo.


.
 

mrzed001

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The problem with sand is it must be spread out over everything.

That "bunker" solution will not do that for a pack our size. It will just dump a small pile on top wherever the hole is. If you need to fill the whole thing, you need sufficient clearance over the top of it for the pile to form and naturally spread.
If you take a big (or more small) sandbags and put it on top of the cells like in a mesh structure then it will flow where the heat is.

A C02 canister can flood virtually any size container and should more or less suffice for our type of build since any fire occurring will likely just be insulation on wiring. The cells themselves do not combust on their own like lipo does as long as you can prevent a nearby fire from igniting the electrolyte.
Do not forget that CO2 dissipates fast. Sand stays there. :)

Water will also do a fun thing where it shorts out all your remaining cells together.
Yes, avoid water
 

Diysolar123

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I'm thinking back to my chemistry days, and it seems to me that water and lithium is not a good idea at all.
lithium batteries do NOT contain lithium metals...its a lithium salt so there is no worry about active lithium.

the problem with lifepo4 is that it is a high energy object and using something like inert gases or water to "oxygen starve" the energy release does absolutely no good at all...
most fire suppression is to stop "burning" which is the oxidation of the material...burning wood/plastic/whatever...you get rid of the oxygen and the fire stops.
batteries would "burn" in a vacuum because they are dumping stored energy!

you are not going to put it out, your goal is to isolate and minimize damage to surrounding areas.

there was a large (cargo container size) tesla battery at a power plant that caught fire and all they could do was let it go until it had exhausted itself.
 

Short_Shot

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If you take a big (or more small) sandbags and put it on top of the cells like in a mesh structure then it will flow where the heat is.
By the time you have melted through your sand bags the damage has long been done because it will have been on *fire* for a while.
 

mrzed001

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lithium batteries do NOT contain lithium metals...its a lithium salt so there is no worry about active lithium.

the problem with lifepo4 is that it is a high energy object and using something like inert gases or water to "oxygen starve" the energy release does absolutely no good at all...
most fire suppression is to stop "burning" which is the oxidation of the material...burning wood/plastic/whatever...you get rid of the oxygen and the fire stops.
batteries would "burn" in a vacuum because they are dumping stored energy!

you are not going to put it out, your goal is to isolate and minimize damage to surrounding areas.

there was a large (cargo container size) tesla battery at a power plant that caught fire and all they could do was let it go until it had exhausted itself.

That is NMC. A little bit other than LFP :)
 
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