lifepo4 end of life at 70% capacity

[angrygoat]

I was browsing and noticed this lifep04 cell said to stop using it once the capacity falls below 70%.

End of Life Management

Capacity/Typical Capacity ＜70% During the use of the battery, when the end of life is exceeded, should stop use the battery.

China Stock CATL 280Ah LiFePO4 3.2V Prismatic Cell with Laser-Welded M6 Studs

Note:This link is for sending inquiries only. We'll provide a complete quotation, including DDP sea shipping costs, within 24 hours.For any online orders paid via DDP sea shipping, we'll double-check and inform you if any extra DDP shipping costs are required. We apologize for any inconvenience caus

www.docanpower.com

Why? Does it become dangerous? Or is it an efficiency thing? I assume this is typical for lifepo4 cells?

Seems a shame to discard a battery that can still store a whopping 70%.

 

[sunshine_eggo]

I was browsing and noticed this lifep04 cell said to stop using it once the capacity falls below 70%.

End of Life Management

Capacity/Typical Capacity ＜70% During the use of the battery, when the end of life is exceeded, should stop use the battery.

China Stock CATL 280Ah LiFePO4 3.2V Prismatic Cell with Laser-Welded M6 Studs

Note:This link is for sending inquiries only. We'll provide a complete quotation, including DDP sea shipping costs, within 24 hours.For any online orders paid via DDP sea shipping, we'll double-check and inform you if any extra DDP shipping costs are required. We apologize for any inconvenience caus

www.docanpower.com

Why? Does it become dangerous? Or is it an efficiency thing? I assume this is typical for lifepo4 cells?

Seems a shame to discard a battery that can still store a whopping 70%.

Risk of cell failure increases.

Capacity loss accelerates as state of health decreases.

Capacity loss accelerates with age.

It's a judgment call. IMHO, I would continue using them provided they still met my needs.

 

[OffGridInTheCity]

Risk of cell failure increases.

Capacity loss accelerates as state of health decreases.

Capacity loss accelerates with age.

It's a judgment call. IMHO, I would continue using them provided they still met my needs.

Agree with @Sunshing_eggo. As long as they stay balanced and the capacity is acceptable for you, then I wouldn't put a hard 70% on it.

I'm guessing that similar to a discharge curve knee, there's a capacity loss knee - where capacity degrades gradually/linearly but a some point goes over a cliff. However, I haven't seen info, so maybe it's 60% or 50% or linear most of the way down. Balance issues should appear as one goes over the knee if there is one.

 

[RCinFLA]

There is an increase in cell impedance that comes with capacity loss due to aging. There will be greater cell voltage slump with given discharge current. 80% of original capacity comes with 2x to 3x increase in cell impedance. Where a newer cell may have 150 mV terminal voltage drop at moderate load current, the used cell with '80%' capacity remaining has 300-450 mV terminal voltage drop per cell. A fully charged used cell can drop below 3.0v in a minute or two with moderate load.

Depending on what your peak current load current requirements are, the cells may become unusable to you due to excessive voltage drop when you draw peak loads.

Inverters regulate AC output voltage meaning for a given AC load, lower DC input voltage means the inverter must draw more DC current from battery to meet the given AC output demand. This also means more cabling/connectors voltage drop and less inverter efficiency. Higher cell current and greater cell impedance has greater cell internal heating that accelerates cell degradation rate. It all stacks up to a downward spiral of performance.

As cell impedance increases with age, doing a load capacity test with less than 0.2 C(A) load current gives an artificially higher capacity result to test.

 

[chrisski]

I am hoping because I limit my draw on these cells to between .1C and .2C that a 70% capacity is way in the future.

I do not intend to capacity test my 2 x 280 ah battery banks ever.

There will be other indications that these cells are no good anymore aside from a capacity test.

As mentioned, it will either be the cell voltage drops too low after under load for a while, basically showing reduced capacity, or the voltage sags at higher state of charge as compared to when the cells were new.

In most cases, I am not a fan of capacity testing. You have a perfectly good cell from a reputable vendor, so why put it through the test of draining to too nothing? Will this test change how you assemble your battery pack? When you go that low you lose something although with lithium this is much less of a cut into the life than lead acid.

Now that you suspect the battery is dead because of voltage sag under load or reduced voltage over time, doing a cpacity test to drain that battery is even less helpful. Also, the amount of time needed to do a capacity test with a 16 cell 280 ah per cell battery pack with a .2C tester and a 10 amp power supply is at least a week.

I am a fan of capacity testing of unknown or Grade B cells when assembling them into a battery pack, especially if the cells are put into parallel before series for matcing. (2P8S).

 

[RCinFLA]

I am a fan of capacity testing of unknown or Grade B cells when assembling them into a battery pack, especially if the cells are put into parallel before series for matcing. (2P8S).

For just received cells, best, quickest test is to do a open circuit rested to load current test with 0.2 to 0.4 C(A) load current. Within about 2-3 minutes of load current the cell will reach equilibrium and you can measure cell voltage slump from rested OCV.

Matched cells will have the same terminal voltage slump vs load current. Amount of terminal voltage slump correlates well to cell capacity.

For a new, pristine condition 280 AH EVA cell this is the typical cell terminal voltage slump vs load current at 25 degC. Over the useful lifetime of cell this can increase 2x to 3x.

 

[angrygoat]

Thanks for the informative answers.

 

[Danke]

Has anyone yet reached the end of life/usefulness with their LiFePO4 batteries?

 

[Ampster]

Has anyone yet reached the end of life/usefulness with their LiFePO4 batteries?

Not yet. Before I went with LFP I was using some Nissan Leaf Modules that were at 70% when I acquired them and they worked fine with the limited usage which I put them through. Been /usinging LFP for 2&1/2 years now with no issues.

 

[scubadoo]

Has anyone yet reached the end of life/usefulness with their LiFePO4 batteries?

I may have to wait a while.
Our 4 cell 300Ah Sinopoly LiFePO4 battery has survived 9 years of fulltime travel in the motorhome with a capacity loss in Ah from the initial 315 to just 299 at the last annual C/10 capacity test last month.

I suspect that mysterious death from "old age" will likely occur either to the battery or me before that 70% level is ever reached.

 

[RCinFLA]

Usually, capacity loss comes with cell impedance rise, specifically greater overpotential voltage slump under moderate load current draw.

It is not the 70-80% capacity that stops its use, it is the inverter shutting down under heavy load due to battery voltage slumping below inverter cutoff voltage. Add in cool ambient temps on battery and the bottom falls out quickly.

'Second life', 'Grade B', a.k.a. used cells, are cells that have too much voltage slump under heavy load to support EV metro-bus high peak current demands. They will work okay for lower current demand applications drawing less than 0.3 C(A) current from cells, along with the 80% useable capacity.

This is why doing a capacity test with a 10 amp load current on a 280 AH cell is pretty much a waste of time. Much quicker and more effective to do a rested open circuit to loaded voltage slump test for 5 minutes of 0.2 to 0.4 C(A) current draw.

 

[Sojourner1]

It will be 8 years this April for my LFP batteries fulltime living use in the 5th wheel. 2,652 days out of 2,806 have been boondocking living off of the batteries and solar. Using all appliances in the 5th wheel including the air conditioner. Even 25-30% SOC can use the microwave or coffee maker without issue of the inverter kicking off which is set at 12.0v.

Daily consumption from sundown to sunup is 175 - 250ah depending on time of year.

I'm sure there is some capacity lose but it's not showing in the daily use of living off the system. I figure there is over 1,060 full cycles so far.