Why is bulk/absorption voltage used?

[hwy17]

Obviously you have not.

I did watch the portion you mentioned, I didn't find a lot of point in it. I can re watch this morning in more detail.

You can stay out of this thread if you're going to be dismissive of more discussion and not explain what you're saying. Obviously I have not why?

 

[hwy17]

Go watch those videos.
This betrays your ignorance of the chemistry.
Cell imbalance only exposes itself in the upper part of the knee of the charge curve.
Charge curve for this chemistry at the margins is logarithmic....
In a logarithmic area is where you do cell balancing, not in the fat flat center of the curve.

Yeah get out of here dude. There are other people willing to have a respectful discussion.

 

[TomC4306]

I did watch the portion you mentioned, I didn't find a lot of point in it. I can re watch this morning in more detail.

You can stay out of this thread if you're going to be dismissive of more discussion and not explain what you're saying. Obviously I have not why?

Watch and learn.
Whatever dude...

 

[TomC4306]

Yeah get out of here dude. There are other people willing to have a respectful discussion.

It's all been done before. It's all been asked before. The answer is exist. I tried to steer you to a guy that has over 300 videos exploring all aspects and charging voltages and current flows and mismatched paralleling in all sorts of stuff in 300 videos on lfp.
But you're too busy to accept the answer.
Good day. I said good day...

 

[hwy17]

I think the discussion here should be best taken to the optimal charging protocol for a single cell, not a series combination with a bms. If the 3.45 (or whatever) bulk limit, the 3.45 (or whatever) absorb, and 3.35 (or whatever) float is chosen because of balancing issues, so be it, but if it is because of electrochemical considerations of the electrodes, then that is more fundamental, and should be considered first.

I don't have a good answer, as I'm not knowledgeable with lifepos in particular, but I do understand electrodes, their thermodynamics, and kinetics pretty well, but more from a corrosion stance. I can imagine that modelling lifepo cathodes could be pretty complicated, as it involves solid state diffusion in the cathode. What I can say is that considering non-equilibrium kinetics (charging/discharging) is where a good deal of the complications arise. The most meaningful potential is that at open circuit, when there are no loads or charging, and after everything has relaxed, which can take some time. When you charge, there is an overpotential associated with ohmic loss in the electrolyte, an activation overpotential associated with driving the fundamental reaction a certain direction at a certain rate, and a diffusion overpotential, that results from the fact that some ions in solution have different diffusivities than others, and are essentially separated from one another (like a little capacitor) when they are pulled one way in an electric field, but at the same time are trying to even out concentrations due to diffusion. These overpotentials disappear when you stop charging and have no loads, after some time anyways. They are a strong function of current density, in varying and often complicated ways. What's determining the energy density of the system (roughly the soc) at this point is the electrochemical potential of the fundamental reactions, which is pretty straightforward, and the amount of 'stuff' at one electrode or the other.

All of the overpotentials result in efficiency losses, so you can see that the most efficient way to draw power from a battery, or charge it, is to draw infinitesimal currents, which is clearly not practical. It makes sense that you would try to hold a cell at its open circuit potential at full charge, which I understand is around 3.35V. It's entirely possible that some time is desirable at higher potentials for some reason related to the state of the electrodes - I just don't know.

I think what hwy17 is trying to get at is is there a point of ever being above 3.35 during a charge. The way I see it is yes, because for any meaningful charging current, there is an associated overpotential, so to snap back at the right open circuit potential of full-ish soc, you charge above it, but still below 3.65, and wait for current to taper down a bit. Outside of unknown (to me) electrode effects, you'd still get to the same soc by holding at 3.35 for infinite time, sort of like walking halfway to a wall each time you walk to it. The overpotentials diminish with decreasing current. So, in short, it is a 'time' reason. For a 16s battery, I can see there being reasons involved with the balancing dance, but these are separate.

Wonderful. Thank you for this.

So we can establish that we are at some level definitely raising voltage for the purpose of charging speed. I don't have a conceptual disagreement with that, but I do have this practical question of how high.

So electrochemically we would charge a cell to 3.35 and be done.

Practically, we need to raise voltage for 2 reasons:

1. To increase the rate of charging

2. To allow for voltage based balancing

Now what I'm wondering is if we can actually achieve both of these goals with 1 raised voltage. For fun or in the pursuit of simplicity or whatever would satisfy people. I'm obviously satisfied to keep asking it myself.

The part I'm a bit blindsided about, and maybe I missed this part of the community method, is that maybe a lot of people are not trying to balance in float? And I misunderstood that they're actually trying to achieve both goals 1 and 2 in the absorption phase and then back off to an inactive float.

That would explain a lot to me, like about the active vs passive balancing debate, or about how Orion only reticently added float modes at the pestering of the DIY ESS users while the EV community balances at their final charge voltage.

 

[hwy17]

1) What Voltage do you consider fully charged? Rested and while charging

2) What single voltage do you propose?

1. I think that once you cross 3.4v while charging the head room for unrealized SOC is small. I'm not an expert to give out precise numbers for us to argue here. I wonder if in this 3.4v while charging overhead question, we don't need to chase the 1-10% so hard. Most people will probably have a few hours of float to catch up on whatever is left in there.

2. I'm thinking of like 55v/3.437. Arguments in favor would be SOK endorses a float at 55.2v. Arguments against would include the community saying "yeah right, I'd never hold my babies at 3.437 all day you burn out your batteries if you want to". But I'm talking about something the 3.4-3.45 range for sure.

 

[Checkthisout]

1. I think that once you cross 3.4v while charging the head room for unrealized SOC is small. I'm not an expert to give out precise numbers for us to argue here. I wonder if in this 3.4v while charging overhead question, we don't need to chase the 1-10% so hard. Most people will probably have a few hours of float to catch up on whatever is left in there.

2. I'm thinking of like 55v/3.437. Arguments in favor would be SOK endorses a float at 55.2v. Arguments against would include the community saying "yeah right, I'd never hold my babies at 3.437 all day you burn out your batteries if you want to". But I'm talking about something the 3.4-3.45 range for sure.

And your ultimate point is that you want to increase battery life, correct?

And you believe that a C rate that is as low as possible to a charge level that's 10% below what's considered fully-charged will accomplish this goal, correct?


What voltage would you cycle down to?

 

[1201]

I'm entertaining the idea that what if we did, would the difference in charge performance be as great as this community's consensus proposes.

The counter arguments are:

1. You wouldn't be in good balancing voltage.

2. The charge would be too slow.

And I'm saying would it really? I think you can balance at float and that is now more or less the consensus. And would it be that slow? If we're talking about 5% SOC left on the table, is that 5% worth packing these higher voltages in every day?

Haven't read the rest of the thread but counter argument- forget float voltage entirely and just charge to full- sun goes down and battery starts discharging and the voltage will automatically drop down to where it would have been at float anyway so why even bother with float?

To me float is where you want the battery voltage at if you are going to be out of town for a few days. You don't want to keep it at the full voltage for days on end but for everyday use i don't see why two voltage are needed when the battery voltage is going to drop as soon as the sun goes down

 

[RCinFLA]

Overpotential voltage is the overhead voltage above zero cell current necessary to push lithium ions through cell. It is key to understanding charging or discharging. Overpotential has a exponential time decay.

With cell current, there is also internal conductive resistance voltage drop. This is the approx. 0.2 milliohms for a 280 AH prismatic cell. Compared to overpotential voltage bump, conductive resistance voltage drop is usually a small part of the total voltage bump during charging.

At 0.2C bulk charge rate for a 280AH cell the conductive resistance voltage drop is about 11 mV of charging voltage bump at cell terminal voltage compared to about 50-60 mV of overpotential voltage bump after 1-3 minutes of 0.2 CA constant charging current.

Overpotential voltage goes up as cell current goes up. Greater charging current requires greater overpotential overhead.

As a cell approaches full SoC the required amount of overpotential voltage increases for same amount of cell charging current. Most of the 'parking spaces' within the negative graphite electrode have been taken and it takes greater overpotential voltage to drive the lithium ions deeper into the negative graphite electrode in search of an available parking space within the graphite lattice.

As cell approaches a fixed charging voltage, the demanded cell current drops off and the overpotential overhead drops off.

An LFP is fully charged at a rested open circuit voltage of 3.43v.

Overpotential voltage vs cell current has a logarithmic relationship meaning there will be 5-15 mV of overpotential required for small amount of cell current. This is why just passively paralleling cells will not fully balance cells. Their rested open circuit voltage will not get any closer than 10-20 mV which can be up to 5-10% SoC difference.

The reason to have a higher absorb voltage is to make up for overpotential voltage loss and speed up charging. You can fully charge a battery at an absorb voltage of 3.45v but it will take a long time of a day or two. Remember when you get down to 0.01C charging current or less you will still have 10-20 mV of overpotential overhead above an open circuit rested cell voltage.

The primary longevity normal wear degradation to an LFP cell is degradation to the negative graphite anode and electrolyte of cell. The negative electrode graphite expands and contracts about 11% over full SoC range. This fractures the protective solid electrolyte interface shell around graphite making electrolyte more vulnerable to degradation and can fracture graphite granules possibly causing them to become electrically isolated from cell operation.

The SEI protective layer is regrown during subsequent recharging, but it consumes a small amount of free lithium and electrolyte to rebuild the SEI protective shell reducing cell capacity over lifetime of cell and thickens the SEI layer which increases cell internal resistance.

The SEI protective shell around graphite helps to keep electrons from escaping graphite into electrolyte during charging where they degrade electrolyte.

Higher charging current rate accelerates this degradation process rate. Exercising cells over wider SoC range accelerates the degradation process rate from greater graphite expansion/contraction.

 

[sunshine_eggo]

I'm not quite understand your assumptive charting based on the crossing. In one example, the 3.65 chart, the original chart does go into CV right at the voltage crossing. I don't understand the mathematical mechanism, but it does. In the other chart though, the original chart does not go into CV at a crossing.

"the crossing"?

I'm not following you on this. Both charts transition to CV when the absorption voltage is reached. If you are referring to where the brownish line crosses something else, that line isn't relevant to anything but reporting the momentary capacity.

Also, these two tests are at different currents, which is going to affect the CC duration as well as the voltage. I'm not saying you said they were a 1 for 1, just thinking it through here.

Not only different currents, but difference cells and cell counts. Changing the current will shorten the BULK phase only. The rest of the chart will be essentially identical. On the single cell, it would make notable difference because of the higher absorption voltage. On the 4S battery, it would make less difference because the bulk phase is only 2.5 hours out of 7.5 hours due to the lower absorption voltage.

 

[hwy17]

"the crossing"?

I'm not following you on this. Both charts transition to CV when the absorption voltage is reached. If you are referring to where the brownish line crosses something else, that line isn't relevant to anything but reporting the momentary capacity.

It looked to me like you drew red lines over a presumptive point at which a higher voltage bulk charge would have gone into absorption and I don't yet understand the presumption, or I misunderstand the straight red lines drawn on the chart.

 

[hwy17]

Changing the current will shorten the BULK phase only.

Understood and agreed.

 

[sunshine_eggo]

It looked to me like you drew red lines over a presumptive point at which a higher voltage bulk charge would have gone into absorption and I don't yet understand the presumption, or I misunderstand the straight red lines drawn on the chart.

ah... I probably didn't explain it. I was trying to demonstrate how the bulk phase would be shortened. If the bulk phase is shortened, the charge will require a longer absorption period.



In this plot the actual charge needed about 2 minutes of absorption. If this charge had been to 3.40V, it would have needed a much longer absorption period, which would have added significantly to the charge time.

You mentioned something earlier - simply charging at 3.35V to full. I don't think this is possible. Analogous to lead acid, which might be 13.0V at rest must be charged to above the known 100% resting voltage to get to 100%. The popular choice of 3.375V/cell for float is chosen because it is believed to be a point at which you can't get to 100% and thus avoid over-charging while still maintaining a high state of charge for overnight needs.

 

[Partimewages]

Off Topic!
I love this forum so many smart people helping us think out technical ideas.

 

[Checkthisout]

Overpotential voltage is the overhead voltage above zero cell current necessary to push lithium ions through cell. It is key to understanding charging or discharging. Overpotential has a exponential time decay.

With cell current, there is also internal conductive resistance voltage drop. This is the approx. 0.2 milliohms for a 280 AH prismatic cell. Compared to overpotential voltage bump, conductive resistance voltage drop is usually a small part of the total voltage bump during charging.

At 0.2C bulk charge rate for a 280AH cell the conductive resistance voltage drop is about 11 mV of charging voltage bump at cell terminal voltage compared to about 50-60 mV of overpotential voltage bump after 1-3 minutes of 0.2 CA constant charging current.

Overpotential voltage goes up as cell current goes up. Greater charging current requires greater overpotential overhead.

As a cell approaches full SoC the required amount of overpotential voltage increases for same amount of cell charging current. Most of the 'parking spaces' within the negative graphite electrode have been taken and it takes greater overpotential voltage to drive the lithium ions deeper into the negative graphite electrode in search of an available parking space within the graphite lattice.

As cell approaches a fixed charging voltage, the demanded cell current drops off and the overpotential overhead drops off.

An LFP is fully charged at a rested open circuit voltage of 3.43v.

Overpotential voltage vs cell current has a logarithmic relationship meaning there will be 5-15 mV of overpotential required for small amount of cell current. This is why just passively paralleling cells will not fully balance cells. Their rested open circuit voltage will not get any closer than 10-20 mV which can be up to 5-10% SoC difference.

The reason to have a higher absorb voltage is to make up for overpotential voltage loss and speed up charging. You can fully charge a battery at an absorb voltage of 3.45v but it will take a long time of a day or two. Remember when you get down to 0.01C charging current or less you will still have 10-20 mV of overpotential overhead above an open circuit rested cell voltage.

The primary longevity normal wear degradation to an LFP cell is degradation to the negative graphite anode and electrolyte of cell. The negative electrode graphite expands and contracts about 11% over full SoC range. This fractures the protective solid electrolyte interface shell around graphite making electrolyte more vulnerable to degradation and can fracture graphite granules possibly causing them to become electrically isolated from cell operation.

The SEI protective layer is regrown during subsequent recharging, but it consumes a small amount of free lithium and electrolyte to rebuild the SEI protective shell reducing cell capacity over lifetime of cell and thickens the SEI layer which increases cell internal resistance.

The SEI protective shell around graphite helps to keep electrons from escaping graphite into electrolyte during charging where they degrade electrolyte.

Higher charging current rate accelerates this degradation process rate. Exercising cells over wider SoC range accelerates the degradation process rate from greater graphite expansion/contraction.

So what would you do for longest service life vs what is reasonable in day-to-day use?

 

[hwy17]

You mentioned something earlier - simply charging at 3.35V to full. I don't think this is possible.

I am prone to getting these cell voltage numbers mixed up so maybe it was that, or maybe it was talking about the theoretical time unlimited ideal. There's a lot of 3's and 4's and 5's and 7's in different decimal places.

I'm taking about charges to 3.4-3.45 as the realistic practical target for this one voltage charge profile that I'm imagining here.

 

[Checkthisout]

I am prone to getting these cell voltage numbers mixed up so maybe it was that, or maybe it was talking about the theoretical time unlimited ideal. There's a lot of 3's and 4's and 5's and 7's in different decimal places.

I'm taking about charges to 3.4-3.45 as the realistic practical target for this one voltage charge profile that I'm imagining here.

What's the anticipated service life using the standard charge profile?

What's the anticipated service life using the "one voltage" method you're proposing?

Assuming your objective is to increase service life.

 

[hwy17]

Assuming your objective is to increase service life.

It's not. I think that if anything it would be criticized as shortening service life due to higher float voltage.

I don't know how to explain the goal really other than something clear and more simple. Imagine how succinct it could be to say to a new user "bulk, boost, absorb, float, and balance at 55v".

And, what if, maybe, this could result in an easier more reliable straight forward balancing method that wouldn't leave people turning to active balancers right away. If you float and balance at 55 you have all the fully charged day to balance.

 

[RCinFLA]

So what would you do for longest service life vs what is reasonable in day-to-day use?

Biggest thing is do not use high charging current. Keep it less than 0.2 C(A).

To a lesser degree, don't completely use the whole SoC range in routine use. It stresses the graphite and SEI layer more.

Lastly, higher average cell voltage accelerates electrolyte degradation. Don't continually keep at full state of charge. Beware of PV charge controllers that continually re-bulk battery to absorb level charging voltage. LFP positive cathode is very rugged compared to nickel based 4.2v Li-Ion batteries used in cell phones and EV's. LFP can be fully charged with little negative impact, but higher cell voltage also causes slightly greater breakdown rate of electrolyte.

Cell degradation is most prevalent when a cell approaches full SoC during charging. This is when 'parking spaces' get sparce. The longer lithium-ions meander between graphite granules trying to find an available parking space, the more likely they will get attacked by an escaping electron from graphite turning into pure lithium metal which no longer contributes to battery operation and is a conductive metal creating possible cell shorting dendrites.

 

[hwy17]

Haven't read the rest of the thread but counter argument- forget float voltage entirely and just charge to full- sun goes down and battery starts discharging and the voltage will automatically drop down to where it would have been at float anyway so why even bother with float?

To me float is where you want the battery voltage at if you are going to be out of town for a few days. You don't want to keep it at the full voltage for days on end but for everyday use i don't see why two voltage are needed when the battery voltage is going to drop as soon as the sun goes down

That's basically what I'm getting at, yeah. When the CV absorption current tapers all the way down to near zero it's a matter of terminology if you want to call that the float or just the tailed off end of an absorption phase. I'm still calling it the float. And it does end after a few hours anyway, like you say if this was a 24/7 charging standby power application the considerations would be different.