LifePO4 degradation. Time vs cycles

[Despotes]

Which contributes to degradation more? Charge cycles or time? If one were to cycle a >4000 cycle @ 80% rated battery once a month, could the battery last at >80% for more than 20 years? If not, specifically what chemical changes are occurring?
Since we know higher quality cells have a higher cycle life, does that mean they also have a longer life apart from the number of cycles? What exactly is chemically different in higher quality cells to make them last longer?

 

[sunshine_eggo]

Opinion:

Cycle life degradation + 1-2%/year degradation = total degradation.

In your scenario, there would also be degradation associated with storing it at a high state of charge. This would become worse at elevated temperature.

I think most LFP options are going to hit 80% state of health 10-15 years independent of actual usage profile provided battery is kept well inside data sheet limits and neutral ambient temps (25°C).

Your scenario presents an extremely expensive option per cycle/kWh delivered. I would choose AGMs for that.

 

[Hendrik]

I would like to know how you calculate battery cycles. My dyness batterys spec's says 6000 cycles.

When will one cycle be completed? Will it count as a cycle when it dischages from 100% to 85% and then charges backup to 100% again,

or will one cycle be from 100% to 20% discharge and then charges up to 100% again.

Can someone explain to me please.

Thanks.

 

[sunshine_eggo]

I would like to know how you calculate battery cycles. My dyness batterys spec's says 6000 cycles.

Those are typically some specific definition like 80% depth of discharge yields 6000 cycles before capacity degrades to 80%

When will one cycle be completed? Will it count as a cycle when it dischages from 100% to 85% and then charges backup to 100% again,

Yes, that's a cycle, but it's not a FULL cycle as defined by the "6000 cycles" rating.

or will one cycle be from 100% to 20% discharge and then charges up to 100% again.

That would be a "full" cycle according to the rating.

1. If your discharge depth is more shallows than spec, you will get more cycles.
2. If your discharge current is less than speci, you will get more cycles.
3. If your charge current is less than spec, you will get more cycles.
4. If you charge to lower voltage, you will get more cycles.

There's no way to calculate how less strenuous use will impact cycle life, but you can be confident that it will be more favorable than rated. EXAMPLE:

12.8V, 100Ah battery, 6000 cycles to 80% DoD.

That's 100Ah * 12.8V * .8 * 6000 = 6.144MWh of total energy delivery.

If you cycle per the specs, that's what you should get.

If you underutilize the battery according to the specs, and apply one of more of items 1-4 above, you will get MORE than the 6.144MWh of total energy delivered.

 

[Hendrik]

Thank you.

 

[scubadoo]

A recent post of mine on this forum:
"My 4 cell Sinopoly 300Ah LFP battery has delivered about 5000kWh over the last nine years and counting with a 5% or so capacity loss. 315 down to 301Ah last November.
Number of cycles? Absolutely no idea!
I anticipate a few more years of usable life."

It will die either of old age or cycle death one day. I will leave that calculation to the theoretical experts among us to estimate when.