# Charge at 3.4, 3.5, 3.6, or 4.2 Volts Per Cell



## Sunking (Aug 10, 2009)

Ran across a manufacture Power Stream that released some non-conclusive charge/discharge test results on 26650 LiFePO4 cells aka LFP from 4 different manufactures. Basically what they did is take LFP cells and CC/CV charged them 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6,.... 4.2 volts per cell, and then discharged them to 2.6 vpc and measured Amp Hour Capacity.

The results are interesting as you will see. In a Nut Shell you could draw two conclusions. 

1. It takes at least 3.3 vpc to charge which gets you to about 20 to 30% SOC range.

2. No point in going higher or at least much higher than 3.4 vpc to charge them. 3.4 vpc saturated (.01 to .03C) gets you to 96 to 99% SOC. Almost nothing is left above 3.4 volts to 4.2 volts other than unnecessary STRESS.

I tend to agree, but I am biased because I do Float Charge my batteries pretty dang close to 3.4 vpc on a 16S at 54.2 volts or 3.39 vpc. I do use a calibrated Coulomb Counter, my pack is Bottom Balanced, and when charged reads 80 to 83 AH on a 100 AH battery. Not saying it is right or wrong way to do things, but I will say charging to 3.4 vpc can do no harm if tried. 

*More Here. *


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## dcb (Dec 5, 2009)

simply being charged (esp if hot) is detrimental to life cycle AFIK. Most are stored at about 30% charge for this reason. A float charge implies long periods on the charged end of the spectrum. So it makes more sense to charge right before you need it to retain AH capacity, rather than leaving it sitting there charged and loosing long term capacity.

At least that is my understanding of the situation.


re: 3.4-4.2, it is the terminal voltage that is important there, again afaik, there is little problem getting there quickly (if you then discharge quickly) as that minimizes the time @ charge.


Also worth noting that lithium polymer is a very different set of curves from the cylindrical cells.


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## Sunking (Aug 10, 2009)

dcb said:


> A float charge implies long periods on the charged end of the spectrum.


Miscommunication on my part. My bad. *Float* was the wrong term but means the same thing as CC/CV at some C-Rate like C/2 or 1C whatever floats your boat. Terminate charge when current tapers to C/33. AFIK in practice is a 3-hour to 3.5 hours to charge from 100% DOD to fully charged. So even a C/10 charge rate getterdone over night.

I will note before I just charged at C/2 until the first cell topped 4.1 volts and terminated. Once rested and confirmed with Coulomb Counter took me to 90% SOC and an average resting voltage of 2.389. IMO it is the fastest method I know of to get to 90%, but requires a cell monitor to terminate the charge. Only thing I did not like was the heating that took place while charging. Baked off CC/CV at C/2 set at 54.2 volts seems to work pretty darn good after 8 months of fine tuning.


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## EVfun (Mar 14, 2010)

That is very informative, but I wish they had done some of the same tests at a more normal cutoff current. They terminated charge at around C/70. I only charge to 3.42 volt per cell but terminate charge at about C/20.


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## jonescg (Nov 3, 2010)

Probably also worth starting all of your cells at the same voltage - i.e. balancing them to 3.45 V or something. Simply connect them all in parallel (yes, even if you have 50 of them) and find a 3.45 V power supply and leave them hooked up for a few days.

Then when you put your pack together you know they are all starting at exactly the same state of charge. The more I learn, the more inclined to agree that charging an LiFePO4 cell past 3.5 V is achieving little extra capacity, but shortening the life of the pack.


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## BVH (Jul 4, 2014)

Based on what I've read here, I made the following post over at EndlessSphere in a thread on LiFeP04 termination Voltage:


"Members at this EV forum (Link to DIYELECTRICCAR) are very savvy on LiFeP04 chemistry batteries. Rooting around over there, you'll find the general consensus is there's no benefit to charging over 3.45 - 3.50 per cell and definitely don't go below 80% DOD. Many use a "Bottom Balance" method of cell balance. It's an interesting concept and makes for some good reading.

Some are into building their own very sophisticated, high KW chargers."


This resulted in the following response:


"that is not correct.

you get more efficient performance and less internal heating of the cell when it is discharged if you charge the cell to full charge before use.

so charging the cell up only partially before use forces it to get hotter during discharge than if you charged it up to full charge before discharging it.

your assumption is incorrect. you will cause your battery to decay in performance and life cycles if you charge it up only part way before use."


To which I responded:


A while back, I performed a test on my two, almost new 51V/12 Ah LiFeP04 packs. One battery at a time, I rode and stopped and rode and stopped with my Cycle Analyst until the resting voltage of each pack was 3.05 Volts per cell. Using the Satiator, I charged them at a 3.5 Amp rate to a set 3.5 Volts per cell termination. I then let them rest overnight. I then used a second profile in the Satiatior to charge them further to a set termination Voltage of 3.65 Volts per cell. On battery 1, an additional .1 Ah was recorded on my in-line Watt meter and on battery 2, an additional .12 Ah was recorded. Is it conceivable that a less than 1% difference in increased charged capacity is going to induce any significant additional heat during the subsequent discharge of those packs?"


Is there any significant merit to what the responder is saying? Based on my very small sample test, it doesn't seem so. And I would think that a lower termination Voltage would stress the cells less adding to their life.


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## Ampster (Oct 6, 2012)

No merit based on what Tesla and other manufacturers are doing. They reportedly don't charge to the top. The default mode for a Tesla is less charge. There is an option for range charge but we don't even know if that is at the top.
Note:
I saw that poster's comment on Endless Sphere and he definately has strong opinions. He claims transconductance is the reason. I looked up transconductance and there are examples for vacuum tubes, FETS and transistors but not batteries.


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## Karter2 (Nov 17, 2011)

Some of the guys on ES have done considerable tests on "Capacity Profiling" of 18650 LiCo cells.
Those results , whilst varying from make to make, and different chemistries, the general result is that if you only charge to 4.0 volts, you are unlikely to get more than 65-70% of available capacity.
The final 0.1- 0.2 volts can provide significant extra capacity ....but at the cost of reduced cycle life.
https://endless-sphere.com/forums/viewtopic.php?f=14&t=54202


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## Sunking (Aug 10, 2009)

Karter2 said:


> Some of the guys on ES have done considerable tests on "Capacity Profiling" of 18650 LiCo cells.
> Those results , whilst varying from make to make, and different chemistries, the general result is that if you only charge to 4.0 volts, you are unlikely to get more than 65-70% of available capacity.
> The final 0.1- 0.2 volts can provide significant extra capacity ....but at the cost of reduced cycle life.
> https://endless-sphere.com/forums/viewtopic.php?f=14&t=54202


OK but I am not talking about LiCo cells. I am talking about LFP which behave differently. From my own experience charging to 3.4 and allow it to saturate gets you fully recharged or close enough.


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## EVfun (Mar 14, 2010)

BVH said:


> [snip]
> This resulted in the following response:
> 
> "that is not correct.
> ...


It looks like they are trying to apply lead acid learning to LiFePO4 batteries. I have seen no change in internal resistance and I charge my 60 amp hour cells to about 3.423 vpc and hold for 40 minutes (133.5 volts for 39 cells.) No cell ever reaches 3.45 volts, all cells reach at least 3.41 volts. They are top balanced and have had no further attention for 2 years (except occasionally checking to see that they are still together near the end of charge.) I have not lost capacity and my cells are still happy to dish out 360 amps from spring through fall (300 amps in the cool winter.)


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## Ampster (Oct 6, 2012)

This thread has caused me to reevaluate my charging strategy. I have been using a JLD5740 to terminate charging voltage at 3.45v per cell. What I have just realized is I am getting no benefit from the CV stage of my charger as current would taper off during that phase and more effectively "saturate" the cells. It is probably time to get my Elcon reprogrammed or consider one that my BMS can control with Canbus.


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## pdove (Jan 9, 2012)

Ampster said:


> This thread has caused me to reevaluate my charging strategy. I have been using a JLD5740 to terminate charging voltage at 3.45v per cell. What I have just realized is I am getting no benefit from the CV stage of my charger as current would taper off during that phase and more effectively "saturate" the cells. It is probably time to get my Elcon reprogrammed or consider one that my BMS can control with Canbus.


I can reprogram it for you or show you how


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## pdove (Jan 9, 2012)

Most of what you read in forums about lithium batteries is made up on the spot.

Charge cycles cc/cv were developed by scientists attempting to get the most capacity with the least damage. It's folly to think you will find a better method unless you have the ability to dissect or X-ray cells to see how the are affected.

Charge cycles were not picked at random.


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## Duncan (Dec 8, 2008)

_Charge cycles cc/cv were developed by scientists attempting to get the most capacity with the least damage._

If you really believe that I have a bridge I would like to sell you


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## pdove (Jan 9, 2012)

Duncan said:


> _Charge cycles cc/cv were developed by scientists attempting to get the most capacity with the least damage._
> 
> If you really believe that I have a bridge I would like to sell you


http://www.che.sc.edu/faculty/white... of Commercial Ramadass Haran White Popov.pdf


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## jonescg (Nov 3, 2010)

pdove said:


> http://www.che.sc.edu/faculty/white... of Commercial Ramadass Haran White Popov.pdf


From the authors conclusions - "Cycling studies were done for Cell-Batt cell under several modes of charging namely charging the cell potentiostatically, CC–CV charging to several end potentials and CC–CV charging with different dc currents during the CC part. Cells charged to an end potential of 4.17 V gave better performance in terms of utilization and capacity loss with cycling when compared with cells charged to other values of end potentials"

..."For Sony 18650 cells charging was done with CC–CV protocol with a charging current of 1 A and 4.2 V as the cutoff value and discharge current used 1 A. Capacity fade was found to be about 30%for 800 cycles of charge and discharge. Impedance measurements showed an overall increase in the cell resistance with cycling."

I trust the scientists findings too  Here they found that 4.17 V for one brand, and 4.20 V for another, were the best end point voltages. I can't find a data sheet for this particular cell, but 4.17 is an unusual figure - most would quote 4.15 or 4.20. 

I guess the point of this discussion is what voltage should you use _in different applications_. A solar array charging a LiFePO4 pack might call for a lower Vmax than an EV because the end goal is longevity. In an EV it's range per charge, so go higher but don't leave it up there for long.

Interesting stuff!


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## Ampster (Oct 6, 2012)

My inquiry is not about what the end voltage should be. It's about whether there is any real benefit of allowing the current to taper at that voltage versus terminating at that voltage with no taper of current.


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## Sunking (Aug 10, 2009)

Ampster said:


> My inquiry is not about what the end voltage should be. It's about whether there is any real benefit of allowing the current to taper at that voltage versus terminating at that voltage with no taper of current.


There are two ways to get a LFP battery to 90% SOC.

Fast charge is CC at C/2 until first cell reaches 4.1 volts and terminate.

Slow at C/4 or less with a CV @ 2.38 vpc until current tapers to 3% or less. 

Fast charging causes heating and stress. Slow charging is kind and gentle. I haver done it both ways. I started fast until I wised up and slowed down. Today I charge at C/4, 20 amps on a 100AH pack. I set the charger to 54.1 volts before I go to bed. Next day unplug. I end up with 85 to 91 AH capacity and a resting voltage of 54 volts. 

Generally I recharge at 50 volts but can go down to 48 safely at 3.0 vpc. My neighbor duplicated my setup. Only difference is he disconnects at 42 volts or 2.5 vpc although he has never gone that lower than 50. Even at 42 has a margin of safety built in, about 6 volts of safety. .


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## Ampster (Oct 6, 2012)

Sunking said:


> There are two ways to get a LFP battery to 90% SOC.
> 
> Fast charge is CC at C/2 until first cell reaches 4.1 volts and terminate.
> 
> ...


I am currently charging at C/6 and terminating at 3.45v with a JLD5740.

My question is would it make a lot of difference if I reprogrammed my charger and charged at CC of C/6 and when voltage hit 3.45 go to CV until current tapers to 3%. I have always thought the difference was insignificant but this thread got me to questioning my assumption. So far I haven't heard anything that supports the notion that the difference would be significant in terms of cycle life or capacity. Therefore I may not go through the effort to reprogram my Elcon. 

Secretly I was looking for a way to justify selling my Elcon and getting the new Thunderstruck TMS 2500. It is programmable via a terminal program on a PC or can be controlled by Canbus. That way I could have up to four switch selectable profiles at various C rates, plus J1772 interface built in. I know that way I could charge slow most of the time, unless I needed a quick charge.


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## PStechPaul (May 1, 2012)

I think you also need to consider the internal impedance of the cells, and the resistance of the connections, unless you have a Kelvin connection to the terminals for voltage reading. My test of an 1800 mAh LiFePO4 cell showed about 50 mOhms so I would expect a 100 A-h cell might be at least 800 uOhms. So at a 1C charge that would be 80 mV, and so 3.48 Vpc would be effectively 3.40V. For a fast charge at 10C it would be 800 mV, so a terminal voltage of 4.2 Vpc would be equivalent. This assumes that the internal impedance remains constant, but it probably varies with temperature and SOC. 

My actual measurement of capacity was 1100 mA-h so a true 1800 mA-h 18650 cell may have lower ESR. And it may not be directly proportional especially compared to larger prismatic cells. But I think my figures are good for a first order approximation and example.


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## Sunking (Aug 10, 2009)

Ampster said:


> My question is would it make a lot of difference if I reprogrammed my charger and charged at CC of C/6 and when voltage hit 3.45 go to CV until current tapers to 3%. I have always thought the difference was insignificant but this thread got me to questioning my assumption.


According to the article results charging at 3.4 vpc until saturated gets you 96 to 99% SOC tells me there is not really anything left except unnecessary stress to the cell. 

The beauty of Lithium Batteries is they are made to operate and thrive operated in the PSOC range (partial state of charge). The exact opposite of Pb which must remain at 100% SOC or they deteriorate. Lithium deteriorates at 100% SOC, so it only makes sense to stay away from the knees at either end of the curve.


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## Karter2 (Nov 17, 2011)

PStechPaul said:


> .
> ....My actual measurement of capacity was 1100 A-h so a true 1800 A-h 18650 cell may have lower ESR. .


Paul..minor niggle....but your odd/mixed units had me thrown for a minute 
An 1800Ah 18650 cell .....nice !


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## PStechPaul (May 1, 2012)

It's really a flux capacitor, charged with lightning!


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## Ampster (Oct 6, 2012)

Sunking said:


> According to the article results charging at 3.4 vpc until saturated gets you 96 to 99% SOC tells me there is not really anything left except unnecessary stress to the cell.
> ......


The thrust of my question is about the term "until saturated". The answer may be that it makes no difference if there is a taper of current until saturated or an abrupt cut off of charging with no taper. The asumption being that the cutoff voltage and the CV voltage are the same. Any thoughts?


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## pdove (Jan 9, 2012)

Sunking said:


> According to the article results charging at 3.4 vpc until saturated gets you 96 to 99% SOC tells me there is not really anything left except unnecessary stress to the cell.
> 
> The beauty of Lithium Batteries is they are made to operate and thrive operated in the PSOC range (partial state of charge). The exact opposite of Pb which must remain at 100% SOC or they deteriorate. Lithium deteriorates at 100% SOC, so it only makes sense to stay away from the knees at either end of the curve.



Wouldn't that rule out the use of balancers?


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## Sunking (Aug 10, 2009)

pdove said:


> Wouldn't that rule out the use of balancers?


Does for me, I don't use them.


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## Ampster (Oct 6, 2012)

pdove said:


> Wouldn't that rule out the use of balancers?


Not necessarily, if your BMS has an adjustable balance voltage it could be set for 3.35V and it will start at that voltage. It will rule out many non adjustable balancers if they start balancing above the cutoff voltage.


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## Sunking (Aug 10, 2009)

pdove said:


> Wouldn't that rule out the use of balancers?


Now that I have had time to think about it, if you Top Balance a proper strategy actually would work quite well. Example even if the Balance Boards were fixed say at 3.55 volts, there is no rule or anything forcing you to use them. Setting your charger to 3.4 vpc they will never turn on. They will only do so if you raise your charger voltage high enough to balance the pack as part of routine maintenance, or in the event they should drift. 

So no it does not eliminate the Balance Boards all together. You just would not be using them frequently.


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