SoC degradation of LiFePO4

svetz

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We've probably all heard to store LiFePO4 at around 45% SoC.

Anyone seen a chart that indicates bad thing vs. storage SoC? I'm guessing the bad thing is a softening of the electrodes measured in reduced capacity independent of cycle-life.

The reason I ask is I have LiFePO4s that are essentially for emergency use (e.g., perpetually at 100% SoC) and wondering what the trade-offs are. Obviously 100% charged gives the most power when needed, but when it's not hurricane season I won't need 100% power.
 
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Maast

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From my LFP research folder: Give this a read as it goes into heat and high SOC LFP degradation modes.
 

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svetz

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From this ref they call it capacity fade. Looks like a linear loss:

1602168734336.png

Looks like a battery would be down 10% capacity in 3 years. Given the battery has a warranty guarantee of 70% capacity for 10 years, that lines up.
Charge discharge curve of cells stored at 100%

1602168225603.png
Top is irreversible loss, bottom is reversible loss
1602169075463.png
I hadn't quite expected anywhere near that amount of degradation. That paper only covered 100% and 0%, 0% was worse. It would still be interesting to see a plot at varying states of charge.

From my LFP research folder: Give this a read as it goes into heat and high SOC LFP degradation modes.
Skimmed through it, but didn't spot anything relevant.
 

svetz

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This reference did tests on Lithium ion at 30%, 65%, and 100% at 24 months:
1602172954891.png
So, it indicates little aging difference at 65%.
This reference indicates for LFP being below 70% seems to be the
best spot, but it also shows 0 as the best...which conflicts with the
prior paper and from what I've heard in general... so probably not understanding something.


1602172331585.png

Still, all in all seems pretty clear to minimize aging of an emergency storage system it's best not to keep it at 100%.
 
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Ampster

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Still, all in all seems pretty clear to minimize aging of an emergency storage system it's best not to keep it at 100%
If you stored at 80% then you could always go to 100% when the big event gives you some warning. If no warnng, at least you would have 80%. In your case, you paid the bucks to let Ephase manage that risk. Correct?
 

svetz

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If you decide to set your max voltage down, keep in mind that most BMSes top-balance and don't start until they hit a high voltage. So you'll probably want to run them up to 100% every so often just to balance the cells.

If you stored at 80% then you could always go to 100% when the big event gives you some warning. If no warnng, at least you would have 80%. In your case, you paid the bucks to let Ephase manage that risk. Correct?
That's what I'm thinking. Enphase also has Storm Guard, so if the NWS puts out an alert it should automatically go up to 100% in anticipation of an event and revert afterwards.
 
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If you decide to set your max voltage down, keep in mind that most BMSes top-balance and don't start until they hit a high voltage. So you'll probably want to run them up to 100% every so often just to balance the cells.
Alternatively the top balance point could just be lowered to the new top.
 

founder

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Alternatively the top balance point could just be lowered to the new top.
I am for all phase balancing.
I am for registering each cell by the BMS software using the QR-code, so a history of each cell can be stored.
I am for monitoring to identify problem cells.
I am for helping identified problem cells to keep them as healthy as the average of the pack.

Needs special BMS and software.
 
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