Lifep04 percent of charge?

I looked up not sure what to call it, percent of charge? And I keep getting stuff like this showing as little as 3.4 volts per cell as 100% state of charge. But I’m dumbfounded Bec I was told the 100% was 3.65 volts. I know lifep04 have a steep curve at the ends but that’s still conflicting numbers. Is it 3.65 volts per cell or 3.4 volts to 100%?

3.65 volts is the absolute max charge voltage for a cell.
Once the cell has been charged to 3.65 volts per cell and the charge source is disconnected the cell will settle to ~3.4 volts.

3.65 volts is max charge voltage
~3.4 volts is full resting voltage

smoothJoey said:

3.65 volts is the absolute max charge voltage for a cell.
Once the cell has been charged to 3.65 volts per cell and the charge source is disconnected the cell will settle to ~3.4 volts.

3.65 volts is max charge voltage
~3.4 volts is full resting voltage

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I understand. So I have charged the batteries to 3.44 volts, what percent of charge is that? Bec this picture is showing the “resting state of charge” or whatever. I wana know what percent did I charge my batteries to. Is that possible with just using a voltmeter? Or is a shunt that’s purpose? If lifep04 voltage was linear I would not be having so much trouble.

FYI - It's "state of charge" (SOC), not "percent of charge".

Also, it's LiFePO4 - Lithium (Li) Iron (Fe) Phosphate (PO₄). While many write it as lifepo4 or LFP, technically it's LiFePO4. But never lifep04 (with a zero).

That's your helpful, if not pedantic, lesson for today.

GGameBoy said:

Or is a shunt that’s purpose?

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A shunt is much more useful than trying to judge based on voltage.

GGameBoy said:

I understand. So I have charged the batteries to 3.44 volts, what percent of charge is that? Bec this picture is showing the “resting state of charge” or whatever. I wana know what percent did I charge my batteries to. Is that possible with just using a voltmeter? Or is a shunt that’s purpose? If lifep04 voltage was linear I would not be having so much trouble.

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Are we talking about top balancing cells or charging a pack?

I am top balancing but not wanting to go to the maximum of the 3.65 volts due to some cells already expanded prior to me doing anything. I have them under compression but I understand charging to the max voltage makes them want to expand further. I was aiming for about 90% full charge or top balance. I was thinking that would be a good compromise. It was confusing to see 3.4 volts per cell as 100% lol. I’m like I’m at 3.44 now

GGameBoy said:

I am top balancing but not wanting to go to the maximum of the 3.65 volts due to some cells already expanded prior to me doing anything. I have them under compression but I understand charging to the max voltage makes them want to expand further. I was aiming for about 90% full charge or top balance.

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Voltage corresponds best to SOC at the extremes of 2.5 and 3.65 volts.
If your cells expanded before you did anything to them I would suggest you see about returning them.
Balancing your cells to 90% is not a top balance.

smoothJoey said:

Voltage corresponds best to SOC at the extremes of 2.5 and 3.65 volts.
If your cells expanded before you did anything to them I would suggest you see about returning them.
Balancing your cells to 90% is not a top balance.

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So to nail it in, u have to top balance at 3.65 per cell. Anything less then that is not a proper top balance?

GGameBoy said:

So to nail it in, u have to top balance at 3.65 per cell. Anything less then that is not a proper top balance?

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Some would argue that 3.6 as measured at the cell terminal is good enough.
Whatever you do, do it consistently for each cell in the pack.
You want the cells to a common point very high in the upper knee.
I did mine to 3.625 and its been fine for >6 months.

Since your pack is already questionable you should top balance them to 3.65 volts in a compression rig and capacity test each cell individually to determine if your cells are viable.

smoothJoey said:

Some would argue that 3.6 as measured at the cell terminal is good enough.
Whatever you do, do it consistently for each cell in the pack.
You want the cells to a common point very high in the high knee.
I did mine to 3.625 and its been fine for >6 months.

Since your pack is already questionable you should top balance them to 3.65 volts in a compression rig and capacity test each cell individually to determine if your cells are viable.

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Ok thanks

You may want to download THIS resource: The PDF File is High Rez chart.
General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

Also this will help you with additional details and information with regards to assembling & setting up your packs. (Applicable to all LFP)
Luyuan Tech Basic Lifepo4 Assembly Guide

Steve_S said:

You may want to download THIS resource: The PDF File is High Rez chart.
General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V

Also this will help you with additional details and information with regards to assembling & setting up your packs. (Applicable to all LFP)
Luyuan Tech Basic Lifepo4 Assembly Guide

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Sweeeet thank you!

3.4v rested near 25degsC, no cell current, is just about full charge.

To get current to flow in a cell there requires an over-potential voltage to create and move ions. This overpotential is terminal voltage increase over rested voltage for charging and lower then rested for discharge. As cell approaches full charge the charging over-potential to support demanded charge current gets greater because most of available lithium ions have be swept up into anode so more 'kick' over-potential energy is needed to create and collect the remaining scarce lithium ions.

Losses in a cell is over-potential voltage X cell current plus the cell impedance X cell current squared. LFP have about the best charge-discharge efficiency of all battery types.

Since over-potential is based on demanded cell current, when charging at low current (approx <0.05C) the overpotential is lower so the increase in terminal voltage is lower over rested cell voltage. Charging takes longer due to lower charge current but cell achieves full charge at a lower terminal voltage and 3.65v is not necessary. At higher current ( approx >0.3C) the cell will not be at full charge when terminal voltage first reaches 3.65v due to higher over-potential for higher cell current. At high cell charge rate, the cell must be held at 3.65v for a while to fully charge cell. The 3.65v limit on terminal voltage is a value that curtails damaging effects on cell, primarily electrolyte degradation.

Because of the logarithmic overpotential voltage to cell current relationship, an overpotential greater than 10-20mV is required on a LFP cell to get any significant current to flow through cell. This is why just passively paralleling cells will not achieve equilization of their state of charge. You will only achieve a rested cell voltage variance of 20-40 mV which can be as much as 30-40% difference in state of charge between the paralleled cells.

I probably should mention surface charge. When cell is fully charged and charge current is allowed to drop to low level, a surface charge voltage of up to couple tenths of a volt can develop, primarily on graphite anode side of cell. This is like a supercap but the charge storage on LFP cell is very low, only about 0.01% C of cell. It can be burned off within a minute with a load resistor of about 3 ohms. If not burned off this increase in terminal voltage can take several days to bleed down. It can screw up your estimation of charge state by cell voltage reading.

If the cells are never going to be taken to 3.65 what's wrong with a balance at say 3.50 volts and use maximum charge voltage = cell number x 3.45 volts

Mike

mikefitz said:

If the cells are never going to be taken to 3.65 what's wrong with a balance at say 3.50 volts and use maximum charge voltage = cell number x 3.45 volts

Mike

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3.5 is less than 3.65 and is a little less accurate to SOC.
Imagine 4 people of different heights and you want to build boxes to stand on so they can all see some distant point from the same eye level.
If you measure them all at the shoulder and then build the boxes there will be likely be slight differences at eye level.
Strange analogy but I hope you take me point.

That analogy was not my best work.
It should have mentioned something about the short guys feet.

Anyway...
The capacity of a battery is determined by its weakest cell.
We want to synchronize the "necks" of the cell so that we can fill the battery while minimizing the possibility for one or more cells to pop up out of spec.

The bottom end is determined by the weakest cell.
Cells stay better synchronized if the weak cell does not get drawn far down into the knee.

The practical way to sync the necks(more or less) is by top balancing to highest safe voltage.

mikefitz said:

If the cells are never going to be taken to 3.65 what's wrong with a balance at say 3.50 volts and use maximum charge voltage = cell number x 3.45 volts

Mike

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You can top balance to a lower voltage ( but >3.45v) but need to let the cells dwell longer at the peak voltage to ensure full state of charge. The amount of time they need to dwell at peak voltage depends on charge current rate which is different for all paralleled cells when they are not balanced yet.

Most folks would not like making the top balancing process take longer time.

There is no single right or wrong answer. There is an optimum answer for your particular use case.

Understanding the factors can help you make the best choices for your use case.

Cell aging factors are primarily two things. Re-growing protective SEI layer and electrolyte decomposition.

1) Approaching full charge state fattens up negative graphite anode which puts stress fractures in SEI layer that will regrow. Regrowth occurs on subsequent recharge cycles and consumes some lithium and electrolyte reducing capacity and thickens up SEI layer which increases cell impedance. This process is small per cycle but cummulatively adds up over time. Doing it a few times for top balancing is not going to have much impact. I would not top balance then let them sit for five or six months in a warm garage while you wait for the rest of system components.

2) Higher cell voltage reduces barrier for electrons to escape electrode and enter electrolyte which causes some electrolyte decomposition. Gummy tar compounds of electrolyte decomposition plug up some of electrode ion flow paths increasing cell impedance.

3) High cell temperature either due to ambient temp or high rate charge/discharge creating internal cell heating accelerates electrolyte breakdown. High temp and high state of charge have compound degradation acceleration. For thick electrode cell designs (blue cells), internal heating rate becomes an issue above about 0.5 CA discharge or charge rate. For a 280 AH blue cell, at 0.2 CA there is less then 3 watts of internal heating. At 0.5 CA rate the internal heating goes up to about 10-12 watts. At 1 CA rate the internal heating is about 35 watts per cell creating about a 20 degC rise in cell temp from ambient temp. Cells in the center of a clamped together bundle have less ability to dissipate heat.

You also have to consider what it takes for cell balance maintanence based on your particular BMS/balancer and your use case. Three primary factors effect rate of balance divergence. Self discharge variations in cells, temperature increases self discharge rate and amplifies cell difference in self discharge rate, and high discharge/charging currents. I assume there will be a 1% C per month variance among cells for minimum balancing requirements. If you routinely run high C rate discharge or charging the minimum balancing amount should increase. Running with diverse state of charge for a long period, besides aggrevation of early high/low end BMS shutdowns, will cause cells to age at different rates for the reasons outlined above. This increases mismatch between cells making it more difficult to keep cells in balance.