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Fire resistant battery bank
As practice of my assembly technique for the power wall I am making a medium? sized 4.2kWh power bank.
3s208p. 624 cells total.
Built into a rolling tollbox as it will weigh 40+kg by the end of it.
One of my concerns was what happens in case of fire?
Can it be contained or at least directed up without setting stuff nearby alight.

1. Build the power bank from scratch with fire resistant materials. eg steel
2. Modify something off the shelf to suit.

Option 1. Cost and time.
I do not have the tools / work space to complete the build.
I do have access to a workshop nearby but not as convenient as working on something when you have 30mins spare here and there.
It can be done.

Option 2 it is.
To help with fire proofing,tile underlay (concrete board) will be used to sandwich the cells.
Make it as 3 separate layers. 2 separators per layer. A total of 9 fire cells.
The sides of the container will be coated with fire proofing silicone.
Strategic internal partitions to direct fumes/flames up and away from other electronics.

Fans to draw in air.
Through a foam filter as it will be used in dusty environments.
The cells are generously spaced to help with heat dissipation.
Max peak power will be 0.5A per cell. Limited by the BMS.
With individual cell fusing and the BMS monitoring temperature/voltage, there should be a very low chance of fire.
Separate environmental monitoring board with own battery to monitor temperature in case BMS fails.

I still wanted to test the fire silicone for my own piece of mind.

A small 2x3 pack was constructed of rejected cells.
Some plastic was covered with 4mm of silicone.
Thermal camera at the ready.
Let the science being Smile
As always be extra careful around these cells. The potential for damage/injury is HIGH when abused.

Test 1. No fire
My test power supply only went up to 5A.
It warmed up the cell but no fire.
The cells warmed, 70°c,  up then went open circuit. I tested 3x.

Test 2. No fire
Heat up the cells directly with a blow torch. Propane > 1100°c.
After a minute I though it was not a good idea to be that close to one if it actually vented.
Still it got at least to 240°c (Max of thermal camera) and still no venting.
Those cell holders sure let off a bit of black smoke.

Test 3. Fire.
I gave up trying to get one into thermal runaway and attacked the plastic directly.
The fire proof silicon did it's job well Smile

I had a lithium fire with one of the a123 26650 lifepo4 cells so I was able to observed up close. The cell caught on fire while I was taking it apart from a battery pack, I accidently shorted out the cell which caused a punctured on the side of the cell.
When the cell catches on fire it will burn for about a minute until all the internal pressure is released. Whatever fire resistant container you build has to be able to contain the fire for at least 1 minute. The fire will be similar to the blow torch you were using. I was surprise the cells in your blowtorch test didn't vent. If they had vented it would have ignited.
One cell catching on fire might not do too much damage if it is contained but once its surrounding cells catch on fire, that will be a fire that can't be put out. 
I think something like a water sprinkler system might work to contain the fire and prevent the surrounding cells from overheating. It will be a mess to clean up, but better then having a massive fire.
The fire I had, I was able to contain with water. The water would knock the flames down, but it would reflash as soon as I removed the water. But what the water did was keep the surrounding cells from overheating.
But from your tests it seems the cells can handle alot of abuse without catching on fire. But its good to have a plan for worst case scenario.
nz_lifer likes this post
Thanks for sharing your experience. Cool
Where it will be deployed there is also a lot of sand nearby if required.
I guess either way, if a fire starts I should consider the whole pack compromised and concentrate on minimising the size of the fire.
Rather than trying to salvage the remaining cells.
Have a 10L bottle of water nearby to dump into the container and wait it out.

The silicone has a 4hr rating. So should last a while even at the elevate burn temperature.
With water flooding the burn area I am now very confident I will be able to manage a fire.
Here's an actual 18650 lithium-ion fire / cascading fire that started in a tesla pack that they overcharged......    it really highlights what happens as 1 cell causes the neighboring cells to ignite.   See 3:06 onward in this youtube -

I've come to believe that one of the very best things one can do for fire safety beside physical protection and short protection is to *avoid overcharging*!
nz_lifer likes this post
My main concern would be that that is a closed box, IF one will ignite, it wont take long before the others will burn, ignite or even explode.
My best advice in this case will be: make sure the pressure can be direct iit to one direction away from the other cells.Up
So every "chamber" has its own "barrel" sort of speak.
Fill all the chambers with vermiculite pellets, or a other heat and fire proof material, like calcium cilica sheets(sorry dont know the propper english translation).
So the sparks and flames wont reach or heat up the other cells.

If you use a proper bms, well testes cells and cell level fusing, it wont go this far.
In the video, i thought those guys did not use any kind of bms nor protection.

In my own endeavors i throw some (25 pcs packed)in a camp fire, i was standing 5 meter away when the fun started, one hit me in the chest....left a nice blue/purple spot....
I often wonder if that was a healthy full charged cell or a specific chemistry, there where more flyers btw

Don't worry so much and use common senses.
With best regards Igor
nz_lifer likes this post
That video is a worst case scenario for sure.
No BMS and just plugged in their Lead acid charger.

The box will only be sealed during storage.
Although I think I may now add vents just in case.
In use the lid will be open.

I will study the tesla power bank design to see how they separate their packs.
I have a rough idea of what to do after being around the fire guys as they test and commission commercial buildings I have been working at.

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