This is my first Diy battery rack. Any advice on how to improve it or important things i may have missed?

[Dave911]

Oh not at all. I greatly appreciate feedback from people that knows more than me. Without you all i couldnt have built this.
The lithium fire risk is kind of scary. Since i see commercial lithium battery racks all over i didnt gave it a lot of thought, but chinesium is a dangerous chemical element as they say
Thank you for bringing awareness to this issue and for your detailed explanation.

I thought that lithium ion batteries were the big fire hazard, and Lifepo4, not so much. There are youtube videos of people trying to get lifepo4 cells to burn and runaway and it doesnt seem to work.

 

[robbob2112]

I thought that lithium ion batteries were the big fire hazard, and Lifepo4, not so much. There are youtube videos of people trying to get lifepo4 cells to burn and runaway and it doesnt seem to work.

LFP batteries can have single cell damage from punctures or shorting, but once they vent if the hydrogen sticks around it can cause damage. They only produce a marginal amount of oxygen when the electrolyte breaks down unlike other chemistries so they won't spontaniously break out into a smal scale nuclear bomb type event.

 

[upnorthandpersonal]

I thought that lithium ion batteries were the big fire hazard, and Lifepo4, not so much.

The main difference is thermal runaway. The Cobalt Oxide bond in NMC cells and the like is weaker than the Phosphate oxygen bond in LFP. So much so that, thermal runaway can occur with Cobalt based chemistries when there is a failure (e.g. overcharging), while this is not the case with LFP. It is important to understand the concept of thermal runaway: it means that the cell is able to supply its own oxygen in case of a catastrophic failure (due to breaking down of the oxygen bond) which in turn allows increases in temperature and this leads to a chain reaction with nearby cells.

This is entirely different from a fire due to breaking down of the electrolyte. However, it doesn't automatically ignite: you need an ignition source. Once you have a fire however, you can get LFP in thermal runaway because the fire has the potential to add the energy needed to break down the Phosphate bond. This is much harder to induce however.

 

[Ampster]

it means that the cell is able to supply its own oxygen in case of a catastrophic failure (due to breaking down of the oxygen bond) which in turn allows increases in temperature and this leads to a chain reaction with nearby cells.

Thanks for that explanation. I had always thought that LFP does not give off oxygen during thermal runaway and I stand corrected. That sheds some light on some recently reported fires. It also explains why the increased scrutiny about battery storage by Fire Marshalls when applying for a building permit.

 

[robbob2112]

Thanks for that explanation. I had always thought that LFP does not give off oxygen during thermal runaway and I stand corrected. That sheds some light on some recently reported fires. It also explains why the increased scrutiny about battery storage by Fire Marshalls when applying for a building permit.

The batteries that free oxygen when they break down are not the LFP they are another type. As I mentioned the LFP only produce a small amount of O2 when the electrolyte breaks down. Something on the order of 3% of the vent gases verse 30% for some other chemistry. So the LFP oxygen release is very small. Oh yeah, to breakdown the electrolyte and produce the oxygen it takes a very hot fire in the 600c range, think internal cell short and dendrite punctures inside. Otherwise only H2 and some other trace chemicals are released from a vent.

I have it from my brother-in-law who is a city fireman they just use a class D extinguishing agent. The one he mentioned was a slurry of sodium chloride and potassium that was kind of foamy. A real mess to clean up and the salt mixture is both conductive and corrosive so it will also short out anything that isn't already burning.

 

[upnorthandpersonal]

I had always thought that LFP does not give off oxygen during thermal runaway and I stand corrected.

By definition a thermal runaway even has to release oxygen. The difference is at what temperature this starts to occurs: 125C for NCA, 210C for NMC, 270C for LFP if I remember all that correctly.

 

[upnorthandpersonal]

The batteries that free oxygen when they break down are not the LFP they are another type. As I mentioned the LFP only produce a small amount of O2 when the electrolyte breaks down.

Don't confuse vent gasses and thermal runaway releases. The oxygen comes from the phosphate/cobalt bonds, not the electrolyte, in the case of thermal runaway.

The electrolytes of both types release hydrogen gas as one of their major components, but more so with LFP electrolytes.

 

[Aridom82]

Don't confuse vent gasses and thermal runaway releases. The oxygen comes from the phosphate/cobalt bonds, not the electrolyte, in the case of thermal runaway.

The electrolytes of both types release hydrogen gas as one of their major components, but more so with LFP electrolytes.

So if you have Lifepo4 cells in an enclosure situation what safety systems or gadgets do you use to prevent worst case scenario or at least limit the damage?

 

[upnorthandpersonal]

So if you have Lifepo4 cells in an enclosure situation what safety systems or gadgets do you use to prevent worst case scenario or at least limit the damage?

First and foremost: make sure your cells always stay within voltage range (never below 2.5, never above 3.65) and below 40C temperature wise (and don't charge below, say, 5C). If any of those parameters are exceeded, the BMS has to cut off the pack or prevent charging/discharging depending on the parameter.

Secondly: the battery goes in a different building from the one where I sleep, and if that building were to burn down, it is far enough from the main residence for safety.

 

[Aridom82]

First and foremost: make sure your cells always stay within voltage range (never below 2.5, never above 3.65) and below 40C temperature wise (and don't charge below, say, 5C). If any of those parameters are exceeded, the BMS has to cut off the pack or prevent charging/discharging depending on the parameter.

Secondly: the battery goes in a different building from the one where I sleep, and if that building were to burn down, it is far enough from the main residence for safety.

But i thought that 0º celsius was the limit point for charging. In the winter it gets 3º or 4º celsius where my system is. Is that dangerous?
Is there any system to control or put down the fire if it starts when you are not there?

 

[upnorthandpersonal]

But i thought that 0º celsius was the limit point for charging.

Like most things in physics and chemistry (i.e., the real world) it's not a hard cut-off: it's a more gradual slope/process. You can still charge below 0, but very slowly. You can charge at 1C, but you shouldn't at the same rate you would at 25C.



So why not play it safe, and disable charging at around 5C or whatever. If you're charging in winter (it gets -30C and below here), add a small heater for example.

 

[upnorthandpersonal]

Is there any system to control or put down the fire if it starts when you are not there?

No, but the room is vented so that potential gasses released can not build up to the point of explosion.

 

[Brucey]

Like most things in physics and chemistry (i.e., the real world) it's not a hard cut-off: it's a more gradual slope/process. You can still charge below 0, but very slowly. You can charge at 1C, but you shouldn't at the same rate you would at 25C.

View attachment 214147

So why not play it safe, and disable charging at around 5C or whatever. If you're charging in winter (it gets -30C and below here), add a small heater for example.

Ideally I think keep them around 25C, right in their sweet spot? Assuming you have the available energy to maintain battery enclosure heaters.

 

[upnorthandpersonal]

Ideally I think keep them around 25C, right in their sweet spot?

Yes, absolutely. 25C is the sweet spot for LFP.

 

[Dave911]

There are some videos on Youtube that shows what happens when you spear a Lifepo4 cell with a steel conductive spear. Things get hot, but its not a runaway situation. Pretty impressive actually.

 

[Supervstech]

There are some videos on Youtube that shows what happens when you spear a Lifepo4 cell with a steel conductive spear. Things get hot, but its not a runaway situation. Pretty impressive actually.

Yup, hightechlabs channel

 

[Aridom82]

Yup, hightechlabs channel

Jesus Christ! And i have 48 of those...
Lifepo4 is scarier that i thought. One puncture and bye bye! I have to get back to work and put those cells in a protected enclosure.

 

[timselectric]

It should go without saying.
Don't spear or shoot batteries. lol

 

[Supervstech]

Jesus Christ! And i have 48 of those...
Lifepo4 is scarier that i thought. One puncture and bye bye! I have to get back to work and put those cells in a protected enclosure.

Actually, it took two punctures, and the venting cought fire... it did not explode or burn out of control.
But, yeah, in a catastrophic situation, the battery is flammable.

 

[upnorthandpersonal]

Lifepo4 is scarier that i thought. One puncture and bye bye!

You're talking about a jelly roll consisting of anode/cathode material holding a charge, with a potential energy release of up to 1kWh per cell. You have to respect those things.