Capacity of LiFePo4 beyond 10k cycles?

[A.Justice]

I see that LiFePo4 can last up to 5-10k (depends on how much you charge / discharge, of course) cycles before it suffers around a 20% loss. Does anybody have data on a battery that has more than 10k cycles? I would be interested to know at what point they loose about 50% of their useable capacity, which is about all she wrote, in my book.

A battery that has 30% unusable capacity, in my mind, still has 70% of usable capacity, which could be quite a bit of juice, depending on how much you need, and how big / cheap the battery's are.

I would also be interested in other chemistry lithium battery, long term specs if anyone has them.

 

[Bob B]

I'd be curious to see charts about cycle life down to 50% capacity. The curious thing to me is that at 50% capacity, they will still have as much usable capacity as most brand new lead batteries.

 

[A.Justice]

I'd be curious to see charts about cycle life down to 50% capacity. The curious thing to me is that at 50% capacity, they will still have as much usable capacity as most brand new lead batteries.

My thoughts exactly. I would imagine batteries that have been used for 10k or more cycles are pretty cheap too.

 

[zorlig]

No one really knows what will happen in off grid setups long term. Even what they do have is lab testing with quick charge/discharge. And I'm not sure we have hard data on calendar aging either - got to wait 20 years for that! They might easily last that 20 years too if they are kept properly compressed with good temperature control and low charge/discharge rate and no BMS failure type events.

I suspect self discharge is what will eventually cause a battery to be decommissioned. Brand new these things maintain 3.4v with almost zero power though maybe a watt per 16 cells.

 

[sshibly]

I bought used cells off BH from 2011,
That is like 10 year old, say 200 cycles a year, that is around 2000 cycles,
LIFEPO4 cells are at 80% or lower.

10000 cycles, at daily cycle rate, will take around 30 years at one day cycle
3x a day, will take 10 years.

If the capacity is linear (I am guessing it is not), one would probably lose 50% 60% capacity

 

[Luthj]

Once capacity loss reaches a certain level, the cells will no longer be balanced with internal resistance and sudden failure can become an issue. for commercial applications this is typically the 80% level. Though some folks have been successful down to 70 or 65%. There are two main causes of capacity loss. One is loss of active lithium due to oxidation or other chemical sequestration. And the other is breakdown of the anode or cathode material. If the graphite matrix can no longer effectively except lithium ions that typically indicates mechanical or micro structure breakdown. The type of physical damage that results in capacity loss overtime also makes the cell prone to internal failure. Less commonly this results in internal short downs or excessive self discharge. But it can also result in the pack being unable to sustain higher currents.

It's been discussed a fair bit and other threads, but calendar aging can be on the order of 1 to 5% per year.

With the exception of very specific cases which have low stress life and usage profiles , I think it's fairly ridiculous to expect more than 5,000 full/rated cycles.

Now some folks will muddy the waters by redefining what a "cycle "means. If you redefine a cycle to be a 10% discharge and then recharge, you could get five or six cycles per day.

The best practice is to use the manufactures rated cycle definition. Which is typically around 85-90% depth of discharge.

 

[sshibly]

Once capacity loss reaches a certain level, the cells will no longer be balanced with internal resistance and sudden failure can become an issue. for commercial applications this is typically the 80% level. Though some folks have been successful down to 70 or 65%. There are two main causes of capacity loss. One is loss of active lithium due to oxidation or other chemical sequestration. And the other is breakdown of the anode or cathode material. If the graphite matrix can no longer effectively except lithium ions that typically indicates mechanical or micro structure breakdown. The type of physical damage that results in capacity loss overtime also makes the cell prone to internal failure. Less commonly this results in internal short downs or excessive self discharge. But it can also result in the pack being unable to sustain higher currents.

It's been discussed a fair bit and other threads, but calendar aging can be on the order of 1 to 5% per year.

With the exception of very specific cases which have low stress life and usage profiles , I think it's fairly ridiculous to expect more than 5,000 full/rated cycles.

Now some folks will muddy the waters by redefining what a "cycle "means. If you redefine a cycle to be a 10% discharge and then recharge, you could get five or six cycles per day.

The best practice is to use the manufactures rated cycle definition. Which is typically around 85-90% depth of discharge.

wow, good info,
Another BIG reason not to buy used cells.

 

[zorlig]

wow, good info,
Another BIG reason not to buy used cells.

Yeah I'm amazed so many people did. Especially the big recycled modem cell power walls, every time another cell fails you have to rebuild the pack!

 

[Luthj]

Years ago I worked on a couple budget installs that utilized used telecom lead acid backup batteries. Lets say there is a reason those 9 year old batteries were cheap!

 

[A.Justice]

Once capacity loss reaches a certain level, the cells will no longer be balanced with internal resistance and sudden failure can become an issue. for commercial applications this is typically the 80% level. Though some folks have been successful down to 70 or 65%. There are two main causes of capacity loss. One is loss of active lithium due to oxidation or other chemical sequestration....

That's good to know, I really appreciate the well thought out response! So they really do have a safe useable life, and don't just continue to hold less energy. My batteries are in my home and in my trailer, so failure is a concern that I most definitely have.

 

[Luthj]

The good news is that LFP failures don't cause fires or other damage, they just suddenly or rapidly stop delivering usable power.

 

[A.Justice]

Yeah I'm amazed so many people did. Especially the big recycled modem cell power walls, every time another cell fails you have to rebuild the pack!

I never understood why people use small cells for large applications, other than e-bikes and vehicles that require a very high voltage to run.

I use 18650's in my vape, and I've seen them fail. When you have a pack made out of hundreds of batteries, that's hundreds of connections, cells, BMS boards, and mechanical things that could go wrong. I would rather use the smallest number of batteries possible, as it leaves the smallest amount of things to go wrong.