Which is worse, discharging to empty or charging to full?

toms

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They are large format prismatic (pouch).

They look like very early black Sinopoly cells - but they are Australian (made in Taiwan) LiFeTech cells. I believe the earliest of these cells was 2008.

This is after about 5 years of neglect - fitted with Batrium. I blew the dust off and fitted a REC after that once REC started making 16cell BMS’s.

I’m not saying there won’t be better cells available in a decade, fact is these are a decade old and the same thing was said back then.
 

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NwCali

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They are large format prismatic (pouch).

They look like very early black Sinopoly cells - but they are Australian (made in Taiwan) LiFeTech cells. I believe the earliest of these cells was 2008.

This is after about 5 years of neglect - fitted with Batrium. I blew the dust off and fitted a REC after that once REC started making 16cell BMS’s.

I’m not saying there won’t be better cells available in a decade, fact is these are a decade old and the same thing was said back then.
Just know that dust of organic nature can burn...grain dust, wood dust, even shaving from plastic. If enough dry dust of those types is suspended in the air it can go boom...if a ignition source is present.
 

toms

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Good thing my blower is brushless! Seriously, i have revised my climate control system and the electronics room is now dust free 😀
(the dust in that pic is of the Australian bulldust variety - straight off the desert plains)
 
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toms

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Better sometimes is just lower cost.

I agree, if my system costs twice as much it better throughput more than twice the kwh, or its a fail.

I suspect the OP is looking to maximise pack life without going overboard. The difference between mainly living in the bottom 25% of the SOC or mainly living in the top 25% of the SOC is likely one of the lower factors in the life of his pack.

Based on my experience, i would rate it like this:
1- high temperature (above 30°C)
2- low current charging in the upper knee
3- high current charging in the upper knee
4- time spent in the upper knee
5- high current (>1C) charging
6- high current charging at low SOC
7- time spent in the bottom knee

My cells are OK with it, but many cells are damaged by any charging below 0°C.

The first two combined are by far the greatest killers i have experienced.
 

fafrd

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I agree, if my system costs twice as much it better throughput more than twice the kwh, or its a fail.

I suspect the OP is looking to maximise pack life without going overboard.
Precisely.
The difference between mainly living in the bottom 25% of the SOC or mainly living in the top 25% of the SOC is likely one of the lower factors in the life of his pack.
Meaning less likely to make as much difference compared to the other factors you’ve listed below, correct?
Based on my experience, i would rate it like this:
1- high temperature (above 30°C)
2- low current charging in the upper knee
3- high current charging in the upper knee
4- time spent in the upper knee
5- high current (>1C) charging
6- high current charging at low SOC
7- time spent in the bottom knee

My cells are OK with it, but many cells are damaged by any charging below 0°C.

The first two combined are by far the greatest killers i have experienced.
So just to make sure I understand correctly, allowing your cells to reach temps of over 30C / 86F is the single most damaging thing you can do followed by ‘floating’ or allowing low-current charging such as what you get in Constant Voltage Charging mode above the upper knee (in the handle of the hockey stick), am I getting that right?

If I’ve understood your list correctly that’s the first clear suggestion that time spent near low levels of charge in the bottom knee is less critical to reduced lifetime than time spent up in the handle of the hockey stick (especially if continuing to charge at low current levels up there).

Thanks for sharing the benefit of your experience (and I had no idea the ‘pouch’ design and manufacturing method had been around that long).
 

Ampster

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There is a lot to digest in this thread, There is no way that I can address all the issues but for analytical simplicity it might be useful to break it down into whether the pack has been top balanced or bottom balance, I used to frequent DIY EV forums and I suspect in some circles the top balance versus bottom balance debate rages on, I an not proposing to revive that debate here, but the answer to the question in the title of this thread may turn on whether the pack is top balanced or bottom balanced.
Anytime a user anticipates taking a pack to anywhere near the top or the bottom there are risks. My own experience with using a pack for load shifting and occasional backup means that my pack sees various levels of discharge but typically gets fully charged each day. For that reason I have top balanced my pack and rely on my BMS/balancer to adjust the balance during the typical charge cycles that take place each day. I have set my inverter to give me a long absorb time (CV phase} to optimize that process. My use case is that after the pack finishes charging I have only a few hours until my system moves from using solar production to drawing down the battery to provide power during peak TOU period. Therefore my pack does not stay at a high state of charge for very long, My point is the answer to the question in the title depends on each users circumstances, In other words it all depends on where you are standing whether discharging to empty or charging to full is worse.
 
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fafrd

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What c rate constitutes low current?
Good question. Since EVE’s spec is for charging at 0.1C to 3.65V, I guess I assumed it meant charge levels well below that (such as charging in the low single-digit amp level) as the cells charge up above 3.5V towards 3,65V.
What c rate constitutes high current?
Another good question which I assumed means charging to 3.65V at ‘safer’ specified charge levels of 0.1C or higher.
 

toms

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What c rate constitutes low current?

What c rate constitutes high current?

Depends on individual cells, generally the lithiaton reaction is different above 0.5C charge rate. (it is also different in the upper voltage knee).

That’s where i separate high and low current charging, my packs never see above 0.5C.
 
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toms

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You’ll need to find a chemist that is experienced in LiFePO4 design to explain it further, as my understanding of chemistry limits my ability to explain.

My understanding is that there are two distinctly different types of lithiation reactions, at high SOC and high current the intercalation is more localised and is a higher chance of seeding a dendrite.

This risk isn’t present at low current or low SOC.

The advice i was given is after SEI formation, don’t take the cell into either knee, don’t charge above 0.5C, and keep below 30°C.

Plenty of people have other ideas, i’ve seen a lot of short-lifespan packs that have followed a different path.

Nothing i’ve seen has shown me i should change the way i use this chemistry.
 

DLTooley

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That absorb time issue at lower charge voltage cut offs led to me use the default Victron profile, charge to 14.2, 3.55 plus two hours of absorb at that same voltage.

My use is such that I will only occasionally use anything near full capacity. As with lead acid that infrequent deep discharge is acceptable.

In ten years there may not be a better tech, but batteries should be cheaper for expansion, including expansion with a used pack.

The biggest question for me is due my widely varying use pattern where for months my usage is very limited. As such I’m wondering how to set my solar charge profile to avoid extended time at 100%.
 
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smoothJoey

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Depends on individual cells, generally the lithiaton reaction is different above 0.5C charge rate. (it is also different in the upper voltage knee).

That’s where i separate high and low current charging, my packs never see above 0.5C.
Low current is < .5c, high current is > .5c and .5c is the Goldilocks value?
 

fafrd

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There is a lot to digest in this thread, There is no way that I can address all the issues but for analytical simplicity it might be useful to break it down into whether the pack has been top balanced or bottom balance, I used to frequent DIY EV forums and I suspect in some circles the top balance versus bottom balance debate rages on, I an not proposing to revive that debate here, but the answer to the question in the title of this thread may turn on whether the pack is top balanced or bottom balanced.
Actually, I’d guess the answer to the question will likely be an important consideration in whether you elect to top-balance or bottom-balance…
Anytime a user anticipates taking a pack to anywhere near the top or the bottom there are risks. My own experience with using a pack for load shifting and occasional backup means that my pack sees various levels of discharge but typically gets fully charged each day.
Our application is identical, but we made opposite choices. In my case, my battery is larger than my highest daily production level and letting my SCC charge the battery up to the point that it exited CC mode just translated to lost solar energy I otherwise could have used/captured.

Hence my decision to switch from top-balance to bottom-balance. The ‘fixed point’ in my system now is cutting off discharge once the battery has been depleted to whatever voltage threshold I have set (currently at 25.2V).

The next day, charging will start and all of that day’s production will be used to power loads or be captured in the battery by the time the sun sets. My SCC never gets out of boost and my battery never gets close to fully-charged.

Then after the sun has gone down, battery energy co to use to power loads through any Peak Rate period and into the night until the battery is once again depleted.


For that reason I have top balanced my pack and rely on my BMS/balancer to adjust the balance during the typical charge cycles that take place each day. I have set my inverter to give me a long absorb time (CV phase} to optimize that process.
Yes, that’s the way to do it if you need to maintain a top-balance, but that long CV time translates to wasted solar energy that you otherwise could have captured…

My use case is that after the pack finishes charging I have only a few hours until my system moves from using solar production to drawing down the battery to provide power during peak TOU period.
If your daily production is barely-enough to cover your peak usage, that’s the best way to do it (and the way I originally had my system set-up).

In my case, I realized I was getting daily production of ~50% more than I needed to cover Peak Period Consumption so I could start self-consumption ‘early’ (before start of Peak TOU rates) on a high-production day. By starting up self-consumption once the battery has reached ~75% SOC, I’m able to reserve what little production I get on poor-production days to offset Peak Consumption while being able to start self-consumption early on high-production days (which has the added benefit of less stress on the battery).

Therefore my pack does not stay at a high state of charge for very long, My point is the answer to the question in the title depends on each users circumstances, In other words it all depends on where you are standing whether discharging to empty or charging to full is worse.
My system was originally configured just like yours. I was ‘holding’ at a high SOC for a short amount of time in order to maintain balance.

Primarily to capture the added solar energy represented by that time my SCC was in CV mode, I switched to a bottom-balanced configuration where I now spend 0 time high and hours low (as in 8-10 hours).

So my question actually translates to whether balancing low is more damaging than balancing high? Is it more harmful to hold a LiFePO4 battery low for hours and never change it up past ~75% than to hold it high in the knee for only 1-2 hours and never deplete below ~25% (where it will remain for the same 8-10 hours)?

The one thing I think we can all agree on is that balancing is only effective well into the steep part of one of the knees, so which is worse (degrades cycle lifetime more rapidly), holding for a shorter amount of time (~1-2 hours) high in the upper knee or resting for a longer amount of time (~8-10 hours) down in the lower knee?
 

fafrd

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You’ll need to find a chemist that is experienced in LiFePO4 design to explain it further, as my understanding of chemistry limits my ability to explain.

My understanding is that there are two distinctly different types of lithiation reactions, at high SOC and high current the intercalation is more localised and is a higher chance of seeding a dendrite.

This risk isn’t present at low current or low SOC.

The advice i was given is after SEI formation, don’t take the cell into either knee, don’t charge above 0.5C, and keep below 30°C.
I never even reach 0.1C or 25C, so I’m fine on those last two.

I need to learn more about SEI formation (since I have absolutely no idea if I have any or not), but is there some generally-accepted conventional wisdom about what voltage levels represent the beginning of the upper and lower knee?

Is below 3.15V ‘in’ the lower knee? Below 3.2V?

And is ‘into’ the upper knee above 3.375V? Above 3.363V? Above 3.35V?

Plenty of people have other ideas, i’ve seen a lot of short-lifespan packs that have followed a different path.

Nothing i’ve seen has shown me i should change the way i use this chemistry.
 

thegoogler

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It's a lot harder in my experience (practically speaking) to prevent charging to 100% than it is to prevent discharging to 0%. For the latter you can always just set your BMS cut-off higher. For the former though using voltage is a poor way to judge SoC, it's difficult to find a charging system which actually counts columbs and will stop at the right time, and even quality chargers like Victron which support tail voltages will overcharge your battery if you get lots of sun every day but don't discharge your battery once it's near the top (like those of us who have their setup in an RV or boat and only use it on the weekends).

I suspect there's more damage in keeping your cells topped up or holding them at 0% SoC than briefly moving into that range. But if you have to choose running them at 0-90% is probably better for the cells than running them at 10-100%. Unfortunately your use case will probably dictate your charging profile, i.e.:
  • Top balance and fully charge if you're relying on solar or another power source and your discharge depth varies or is often shallow. RV or boat with solar is a good example here where if you sit idle for a few days you'll always end up topping off the cells (unless you disconnect your solar panels). Again I've personally found it's hard to stop the charge before the knee unless you manually watch and disconnect so if you don't top balance then you'll likely end up with an HVD anyway.
  • Bottom balance if you expect a full discharge regularly, since a mostly full charge will get you to 99% anyway so the top is only perhaps a 1% loss but the lower knee is ~10% capacity. A e-bike is a good example here where you may or may not fully charge it depending on your scenario, but if you bike a lot you probably will fully *discharge* it (and then pedal the rest of the way). An RC car or plane is another example.
 

thegoogler

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I never even reach 0.1C or 25C, so I’m fine on those last two.

I need to learn more about SEI formation (since I have absolutely no idea if I have any or not), but is there some generally-accepted conventional wisdom about what voltage levels represent the beginning of the upper and lower knee?

Is below 3.15V ‘in’ the lower knee? Below 3.2V?

And is ‘into’ the upper knee above 3.375V? Above 3.363V? Above 3.35V?

CATL recommends keeping my battery in the 10-90% SoC range. I think most of the LiFePO4 batteries are similar. However below is my actual battery voltage during a discharge with a <0.01C load applied (which is super low, I know). I started the discharge 24 hours after charging, so the battery pack had settled at 13.99-14.00V.

From the chart I would infer the upper knee starts above 99.5% (which although not graphed here was 13.31V, while 99.9% was 13.91V) and the lower knee starts around 8-9%, at least with my cells.

1637615921964.png
1637616464686.png
 

toms

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Low current is < .5c, high current is > .5c and .5c is the Goldilocks value?

That’s what i use, picture 2 “phases” of lithiation, one resulting in an even spread of intercalation, and one resulting in more localised intercalation. You want to avoid the second phase. Depending on your exact cell chemistry (even within LiFePO4) the parameters will be slightly different.

The point of this thread is that the second phase lithiation reaction also occurs at low current, high SOC. This can be eliminated by staying in the low SOC range, which is what the OP was asking.
 

smoothJoey

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That’s what i use, picture 2 “phases” of lithiation, one resulting in an even spread of intercalation, and one resulting in more localised intercalation. You want to avoid the second phase. Depending on your exact cell chemistry (even within LiFePO4) the parameters will be slightly different.

The point of this thread is that the second phase lithiation reaction also occurs at low current, high SOC. This can be eliminated by staying in the low SOC range, which is what the OP was asking.

I have eve 280ah cells in 8s.
.5c is 140 amps.
My bms is only rated for 100 amps continuous.
My ac charger can only deliver ~27 amps of which ~7 is consumed by my base load so ~20 amps into the battery.
Even if I upgraded the charger then next bottle neck would be the ac outlet which is limited to ~1440 watts continuous.
I'm well and truly OCD but I'm going to have to live with it.
Oh well.
 
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