I need help with NFPA 855. I have 4 Ecoflow delta pros in my basement which failed an electrical inspection. Can I use them in a residence? Inspector said they need to be in utility closet with a fire door. Is this true? Where do I get the specifications of a battery storage utility closet? I’m in USA Michigan. Thanks
Ask Me Anything About LFP Batteries and More! – Industry insider at top EV firm
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Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
Every Country has its' assorted codes & requirements. Then every State/Province in said country can plopp something else in and therefore it varies everywhere. You MUST work within what your region wants, regardless if it is intelligent or not. Unfortunately far too many jurisdictions are stuck with very old rules & regs that were created when Lead Acid & Variants were the only thing out there. Sad to say but far too many are Battery System Stupid !
SoggyShoes
Solar Enthusiast
I have some questions about LFPs regarding low temperatures and storage.
I have an RV that is stored outside. Where I live, we can get up to 2 weeks below freezing. During this time it could get as cold as -18°C (0°F).
Part a:
We always hear never ever charge FLP below freezing, but then there're those that seem to be able to charge at low C-rates without apparent problems. (Are they doing doing long term damage?)
The debate on damage to LFP when discharging or just storing seems even less clear.
So how bad is it on cells to be charged, discharged or stored in freezing temperatures, at what temperature is the cell permanently affected, and does the duration have an effect?
Part b:
Some batteries have internal heating pads. These pads tend to be on one side or the bottom of the cells so the BMS temperature probes can be on the other side.
It seems this would cause a large temperature gradient from one side of a cell to the other.
Is this a concern?
If both part a and part b are major issues, what is the balance?
There are a lot of members that have RVs or seasonal cabins.
I have an RV that is stored outside. Where I live, we can get up to 2 weeks below freezing. During this time it could get as cold as -18°C (0°F).
Part a:
We always hear never ever charge FLP below freezing, but then there're those that seem to be able to charge at low C-rates without apparent problems. (Are they doing doing long term damage?)
The debate on damage to LFP when discharging or just storing seems even less clear.
So how bad is it on cells to be charged, discharged or stored in freezing temperatures, at what temperature is the cell permanently affected, and does the duration have an effect?
Part b:
Some batteries have internal heating pads. These pads tend to be on one side or the bottom of the cells so the BMS temperature probes can be on the other side.
It seems this would cause a large temperature gradient from one side of a cell to the other.
Is this a concern?
If both part a and part b are major issues, what is the balance?
There are a lot of members that have RVs or seasonal cabins.
Boron
New Member
I'm referring to him indicating ESS = ultracapacitors. Electrostatic Storage
DIYrich
Solar Wizard
What are "likely" situations that would result in thermal runaway of an LFP battery?
Overheating (120 f in Phoenix Arizona)?
Sustained overcharging?
Charging while frozen?
Defect in the battery not revealed during grading?
Running into the battery with a car (battery in the garage)?
Batteries in a flooded basement?
Other accidental shorting?
Overheating (120 f in Phoenix Arizona)?
Sustained overcharging?
Charging while frozen?
Defect in the battery not revealed during grading?
Running into the battery with a car (battery in the garage)?
Batteries in a flooded basement?
Other accidental shorting?
DIYrich
Solar Wizard
Any off-gassing issues with LFP?
Hedges
I See Electromagnetic Fields!
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In that case, how is balancing performed?
Without any charging, voltage drops. After a full charge, if not discharged immediately, do typical EV packs stay high enough to perform necessary balancing?
We would hold a "bulk" voltage for some period of time, giving BMS time to bleed off or transfer current to rebalance the cells.
Newer pack, probably remain in balance better. With age, I'd expect self-discharge would increase. Greater balancing current or time would extend life of the pack.
Not for us DIY types, they ain't.
As I understand, we get to buy the rejects which didn't make the EV cut.
hwy17
Anti-Solar Enthusiast
Not that I disagree, but now that I think about it, have we ever discovered which models of vehicles anywhere in the world are actually using the EVE 280-304Ah cells?
Would be funny if we've been confused this whole time and these were never used in EVs.
jpwhit
Solar Enthusiast
Terms such as Bulk, Float, Absorption used with lead acid batteries are specific to the chemistry of a lead acid battery. They really have no place in describing the charge cycle of any Lithium battery chemistry.
You don't need a constant voltage part of the charge cycle to check for balance. Just because the charge cycle is not regulated by a specific voltage does not mean that your BMS system can't look at relative voltages between cells as one metric to determine the state of balance between cells. Algorithm for the charge cycle and algorithms for balance logic do not necessarily depend on each other.
Sverige
A Brit in Sweden
Welcome to the forum
It’s nice to get a different perspective on LFP batteries.Just about every LFP ESS cell datasheet specifies charge termination at 3.65V and 0.05C tail current and just about every DIY battery user and 100% of solar power YouTubers interpret this (wrongly) as “charge up to 3.65V all the way down to zero current”.
In what I regard as the danger zone, 3.65V and under 0.05C, how much damage is done? What I mean is when people continue to trickle charge a large capacity bank of paralleled LFPs at currents which represent a tiny fraction of C, it is (according to cell manufacturer specs) an overcharge, but how damaging to cycle life is this? There are some people who will say that lithium plating of the electrode is occurring in this region of overcharge, but this seems unlikely. What do you say about this?
Edited to add: in EVs with small capacity cells stacked in series to high voltages, perhaps charging into the region below 0.05C is not likely to occur as the current involved would be quite small. In DIY ESS setups where people parallel connect large capacity cells, 0.05C can be a large current. In my case with a 4S4P setup, 0.05C is 58A and charging CV to 3.65V below that current is pushing the cells to a higher SOC than the cell manufacturer says is the charge termination point. Charging to 3.45V at 5A could even be a higher SOC than 3.65V at 58A!
Last edited:
battery_professional
New Member
Part a:
LFP can be charged below freezing. EVs do have thermal management to heat its pack but for slow AC charging they will do around 0.1C-0.2C charge daily at -10 degree Celsius for LFP batteries. I would say most first-tier EV batteries manufacturers' LFP can do 0.2-0.3C charge at -10 to 0 degree Celcius and 0.1C charge at -20 degree Celcius without any problem. For EV batteries that produced within last 3 years, just stop your charge at 3.65 Volts and do not do the constant voltage part and its fairly safe. To give further safty margin, you can cut the charging rate by 1/2.
Its not bad for cells to be charged at low temperature as long as there are no lithium plating. Battery manufactuers will have a datasheet that details the charging strategy (temperature - SOC - crate limit map). As long as the BMS follows this map, you have no problems. I think the problem with DIYers are they cannot get this map and they cannot implent it into the BMS algroithms. But I think you be conservative and just use a constant 0.1C to 3.65 Volts and I don't think you will have problems for temperatures above -10 degree Celcius.
Discharge at freezing temperatures won't hurt the batteries, its the BMS. LFP batteries have large hysteresis and large DCIR for current pulses and your BMS will start to think LFP's SOC jumped to single digits and the BMS will stop the discharge.
Storage at low degree Celcius is actually GOOD for cycle life. Simple fact: The lower the temperature, the slower the side reactions that consumes lithium.
Part b:
This is a very good question. For modern EVs and energy storage plant, temperature gradient is very bad and they do almost all their design work to minimize thermal gradients. This is definitely a concern. For DIY project, I think you should your BMS probes at the point where you think has the lowest temperature. Thats where the actual electrochemical reactions limits your performance or the weakest point.
Its best to put the thermal pads inbetween your cells where have the largest surface area for heat exchange and to minimize thermal gradients.
You questions are very good and on point.
battery_professional
New Member
Most EV burn you saw on social media is either because metal debris defects in batteries or the pack was hit by rocks/metals on the road which make the battery interal short.
120F is bad for cycle lifes but LFP are expect to not have thermal runaways before almost 200 degree Celcius. It's much safer compare with NCM.
Sustained overcharging is not that dangerous either. We do 5 Volts floating charging for safety test and it has no problem with thermal runaways. But its bad for cycle life. LFP are very different from NCM. In NCM when you do overcharging, the electrons start to oxidize oxygen in the NCM lattice. Oxygen atoms is the backbone of NCM lattice and that will make the cathode extremly oxidative and unstable. SO its very dangerous for NCMs. But for LFP when all the irons lose one electron, you are just oxidizing electrolyte during overcharge so your cycle life will decrease significantly but will not normally leads to thermal runaways. Very different mechanisms.
Charging while forzen: As long as you don't have lithium plating, its safe. But if you do have, then its a bit danagerous.
Defect in the battery: This is why top tier manufactuers spend millions to buy online CCD in a production line and use computer vision to find those spots. Its dangerous, but as a consumer you cannot check it by yourself.
Crash into a battery is defnietly very dangerous as long as you have obserable deformation on you battery case.
Battery short with water or salt water is not dangerous for battery itself because they both are not good conductors. So the short current will be limited. Some EV packs even use salt water to discharge the pack before disintergration.
Last, it really depends on the peak current and temperature for shorting, if its large than its dangerous because it might melt the separators and leads to thermal runaways.
battery_professional
New Member
I don't think EVs will use cells with 280-304Ah. You can tell this battery very likely is used for either energy storage or electric buses or trucks if you are working in this field.
Modern EV's normally got less than 200 Ah capacity for safety purposes. When you make an EV, you want the voltage of the pack to be 600 - 900 Volts for thinner copper bus bars, better powertrain and electricmotor efficiency. Especially when you use SiC and not IGBT, you want 800 Volts. Considering most cars' total kWh is less than 100 kWh, the cell would be around 125 Ah.
Large batteries are very hard for thermal mangement and safety control because its have almost 1kWh energy you need to control during thermal runways. Also, during fast charging consumers will need at least 3C peak charging power, if you have a 300 Ah cell, you needs almost 900A current and the design for all the connectors\busbars will be very expensive.
So just like solar panels that have an upper limit of voltage string limit (600-1000v)
These Eve prismatics, do they have similar upper limits?
Such that if one wanted to build a EV school bus out of these, would there be an issue with an 800v 280ah cells? Or could you mitigate this with adding more insulation around the thin blue plastic?
These Eve prismatics, do they have similar upper limits?
Such that if one wanted to build a EV school bus out of these, would there be an issue with an 800v 280ah cells? Or could you mitigate this with adding more insulation around the thin blue plastic?
Brucey
Solar Wizard
I think some EV designs use two ~400V packs to get to their 800V target to enable faster charging etc.
Also these prismatics being used in buses and other larger vehicles makes sense because there's lots of stories about EV buses being total financial disasters with many sitting idle due to lack of support or insufficient range in cold weather etc. So there may well be a glut of these not going into EV buses and therefore available to us at ever lower prices.
Hedges
I See Electromagnetic Fields!
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Most (or is it all?) BMS only perform balancing when voltage of cells is high enough that voltage is an indicator of state of charge.
Below some voltage, no way to tell by voltage if the cells are imbalanced, or in which direction.
Many people might charge to 3.65V/cell, hold a while, then drop to 3.4V
What do you suggest, to allow balancing to occur?
Hedges
I See Electromagnetic Fields!
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Too big for cars, but I thought they were for busses, etc.
Were these cheap cells really made for us DIY consumers?
battery_professional
New Member
In theory they don't have a upper limits as long as your powertrain can take that voltage. And the insulation at every spot of your pack can tolerate that voltage.
800V 280Ah has no problems for buses where the got a lots of room for power distribution systems, heat management and wires. It can be mitigated by adding more insulation and specifically design for this senarior. But it will all comes to the COST. In buses, they usually got more than one pack and charging port to make life easier and they don't really need fast charge which will make the pack way more easier to design.
hwy17
Anti-Solar Enthusiast
Nuvation does claim to have middle balancing magic. I do not know how or whether it's legitimate.
wheelman55
New Member
Battery Professional. Thanks for your input here.
Regarding storing batteries in hot climates.
We have a North America winter only solar powered off-grid home in far west Texas.
We leave in April and return in November. Ambient temps are 100+ Fahrenheit for five of those months. We use Discover LiFePO4 batteries.
We can either turn the system off during our absence OR we can leave it on, untended, in conditioned space, with a mini split set to 77 F. We keep a fridge on as well, so the battery bank will be cycled to some extent daily.
Which way is better for the batteries?
Thanks.
Regarding storing batteries in hot climates.
We have a North America winter only solar powered off-grid home in far west Texas.
We leave in April and return in November. Ambient temps are 100+ Fahrenheit for five of those months. We use Discover LiFePO4 batteries.
We can either turn the system off during our absence OR we can leave it on, untended, in conditioned space, with a mini split set to 77 F. We keep a fridge on as well, so the battery bank will be cycled to some extent daily.
Which way is better for the batteries?
Thanks.
battery_professional
New Member
I think you can set your system to around 40-60% SOC depends on the self-discharge rate and turn if off.
Storage at 100+F with a relative low SOC won't hurt the battery by much. Just be sure its SOC won't drop to below 10% SOC when you got back.
Storage at very low SOC will hurt the SEI(solid-electrolyte interface) of the battery and have a problems like gasing.
wheelman55
New Member
Thanks Battery_Professional!
Cabin Rising
New Member
I would like to know what determines the battery size? Why are they all so small and then tied together?
Sverige
A Brit in Sweden
Just quoting my question above in case you missed it @battery_professional as you’ve answered questions posted after it.Welcome to the forum
Just about every LFP ESS cell datasheet specifies charge termination at 3.65V and 0.05C tail current and just about every DIY battery user and 100% of solar power YouTubers interpret this (wrongly) as “charge up to 3.65V all the way down to zero current”.
In what I regard as the danger zone, 3.65V and under 0.05C, how much damage is done? What I mean is when people continue to trickle charge a large capacity bank of paralleled LFPs at currents which represent a tiny fraction of C, it is (according to cell manufacturer specs) an overcharge, but how damaging to cycle life is this? There are some people who will say that lithium plating of the electrode is occurring in this region of overcharge, but this seems unlikely. What do you say about this?
Edited to add: in EVs with small capacity cells stacked in series to high voltages, perhaps charging into the region below 0.05C is not likely to occur as the current involved would be quite small. In DIY ESS setups where people parallel connect large capacity cells, 0.05C can be a large current. In my case with a 4S4P setup, 0.05C is 58A and charging CV to 3.65V below that current is pushing the cells to a higher SOC than the cell manufacturer says is the charge termination point. Charging to 3.45V at 5A could even be a higher SOC than 3.65V at 58A!
Also, to tack on another Q, can you expand on how oxidation of the electrolyte due to overcharge limits cycle life in LFP? I would have guessed it would reduced capacity rather than cycle life, but will be interesting to learn more about that. On a similar subject can you explain how overdischarge damages LFP and where the danger zone begins?
And I’d better stop there as I’m asking altogether too many questions!
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