# Voltage Settle After Charge



## Joey (Oct 12, 2007)

I have 60 cells, CALB CA 180 Ah, the gray ones. My initial charge was done with the cells connected in parallel, with 12 AWG wire and crimped ring terminals. I broght the cells to 3.42 volts, charging at16 amps for 16 days and then tapering the current to 1.5 amps for the last 2 days. A resting voltage of 3.4 volts is considered full. The manufacturer says to charge at 3.6 volts, until the current drops to 0.05C, or 9 amps for a 180 Ah capacity and the voltage will settle to 3.4 in a day or two. However, with 60 cells in parallel, my 0.05C current is a bit beyound what my supply can provide - 180*60*0.05 = 540 amps. So at lower current, 0.0001C in my case, you do not want to bring the cells up to 3.6 volts, or the cells will overcharge. 

About an hour after the charge termination, the average voltage dropped from 3.42 to 3.4005 volts, and the cell variation dropped from 40 mV to 0.3 mV. Over a 6 day period my, voltage has dropped to 3.389 volts. if the trend continues, an exponential curve fit indicates the voltage will slowly approach 3.385 volts. The cells are still in parallel, just hanging out. Because I plan to slightly under charge my pack (I'm thinking of 3.5 volts/cell and terminating at 0.05C or 9 amps), I think I will just leave the cells alone, and not worry about trying to get the resting voltage up to 3.4 V. Soon I will remove the parallel connections and see how well the cells stay in balance.

Here is the chart of the cell voltage for 6 days, starting 1 hour after the charger turned off.


----------



## Ziggythewiz (May 16, 2010)

I don't think the actual full voltage is 3.4, but 3.38 IIRC from Jack R. I've also heard 3.33, which is where mine settled after a couple days (charged to 3.55 @ C/20).


----------



## kennybobby (Aug 10, 2012)

i have a confidential test report in which the battery manufacturer let their cells sit for 10 days after charging to measure the resting or Open Circuit Voltage (OCV) for several charge levels from 50% up to 100% SOC. By far the greatest "settling" of voltage occured in the 100% SOC cells, dropping 23 to 70 mv over 10 days resulting in 3.384 to 3.447 OCV. The lesser %SOC cells only fell 5 mv over the 10 day period: 50% SOC to 3.296, 80% to 3.3315, and 90% to 3.333 OCV.

Why don't you give it four more days and see where the voltage ends--it looks like you are up in the 100% SOC region by comparison.

Did you leave the charger on 24 hours a day for the 16 + 2 days of charging? If i counted up all the electrons correctly it looks like you put 103.6 A-Hrs into 60 cells of your pack--do you have the initial pack or cell voltage before you started charging, that would be an interesting data point.


----------



## Joey (Oct 12, 2007)

Ziggythewiz said:


> I don't think the actual full voltage is 3.4, but 3.38 IIRC from Jack R. I've also heard 3.33, which is where mine settled after a couple days (charged to 3.55 @ C/20).


Cool then. That would be right in line with what I'm seeing. Near the end of charge, I saw the voltage turn north at 3.388 V.


----------



## Joey (Oct 12, 2007)

kennybobby said:


> i have a confidential test report in which the battery manufacturer let their cells sit for 10 days after charging to measure the resting or Open Circuit Voltage (OCV) for several charge levels from 50% up to 100% SOC. By far the greatest "settling" of voltage occured in the 100% SOC cells, dropping 23 to 70 mv over 10 days resulting in 3.384 to 3.447 OCV. The lesser %SOC cells only fell 5 mv over the 10 day period: 50% SOC to 3.296, 80% to 3.3315, and 90% to 3.333 OCV.
> 
> Why don't you give it four more days and see where the voltage ends--it looks like you are up in the 100% SOC region by comparison.
> 
> Did you leave the charger on 24 hours a day for the 16 + 2 days of charging? If i counted up all the electrons correctly it looks like you put 103.6 A-Hrs into 60 cells of your pack--do you have the initial pack or cell voltage before you started charging, that would be an interesting data point.


Awesome. Thanks for the data.

The charger wasn't on continuously the first 3 days. I wasn't sure what would happen at first and only charged while I could actively watch. Once I figured out the Volts per day rise, I felt confident leaving the charger on. About half way through, I installed a voltmeter with relay control (JLD5740) to terminate the charge at 3.42 volts. I calculate that I put in about 84 Ah per cell.

I will wait the extra 4 days and see where I settle out. In 6 days, I have observed a drop of 35 mV, but I didn't follow the proceedure for terminating at 3.6 V and current of 0.05C, and I think that will affect the amount of voltage drop after charge.

When I got the cells the average voltage was 3.3002 with a st. dev of 0.0011 V, and a range of 0.0047 V.


----------



## Joey (Oct 12, 2007)

So according to the curve kennybobby posted, 3.300 V is about 55%. The 84 Ah I put into the pack is 47%, so I probably am at 100%, or a little over.


----------



## Ziggythewiz (May 16, 2010)

Joey said:


> So according to the curve kennybobby posted, 3.300 V is about 55%. The 84 Ah I put into the pack is 47%, so I probably am at 100%, or a little over.


CALB ships at ~60% SOC. Also, if you look at the spec sheets your cells are likely all over 180 so that takes some extra, and the charging isn't 100% efficient, so there's a bit of juice lost there (though nearly nothing compared to lead).


----------



## mizlplix (May 1, 2011)

> CALB ships at ~60% SOC.


My cells were all close to 3.10/cell when uncrated.

I am using 34 cells in series. I have the charger set to charge to 3.597/cell (129.5 total)

After sitting a few days, the pack is at 3.308/cell (119.1-total)

Miz


----------



## kennybobby (Aug 10, 2012)

Howdy Mizl,
How many amps and for how long did you charge? Are you using constant current charging, or what is your charging procedure?


----------



## mizlplix (May 1, 2011)

Sorry but I didn't really pay attention...

A guess: 10 hrs. @ 13.7 amps

Then 2 hrs @ 1.6 amps

On an Elcon PFC2500 on 240VAC using curve 501V on the 7th step (129.5)-3.597 per cell

I will watch closer the next time...

Miz


----------



## kennybobby (Aug 10, 2012)

What size are your cells--A-Hr capacity? And how many cells are in your pack--you have 36 cells, not 34 that you said above, right?

If you have 36 cells and your charging time estimate is correct it looks like you put about 140.2 A-Hr into your pack.

Your final pack voltage of 119.1 suggests that you are up to about 50% SOC.

If you only have 34 cells in your pack, then you have overcharged them and likely have damage.


----------



## mizlplix (May 1, 2011)

sorry, you are correct, 36-130AH cells.

Trying to do too many things at once...LOL

Miz


----------



## Elithion (Oct 6, 2009)

Ziggythewiz said:


> charging isn't 100% efficient, so there's a bit of juice lost there (though nearly nothing compared to lead).


Yes and no. Depends on whether you're talking about charge efficiency (which is 100 % for Li-ion) or energy efficiency (which, naturally, is always less than 100 %).

How can that be?

Charge efficiency is 100% because, to a very great degree, every electron you put into the - terminal while charging stays on that side of the cell; no electron crosses over to the + terminal side through the separator. (Unlike some other chemistries.)

Energy efficiency is < 100 % because you're heating the cell as you charge and discharge it (internal resistance). That is related to the fact that, at the same SOC, the terminal voltage is higher than the OCV during charging, and lower during discharging.


----------



## Ziggythewiz (May 16, 2010)

Elithion said:


> Charge efficiency is 100% because, to a very great degree, every electron you put into the - terminal while charging stays on that side of the cell; no electron crosses over to the + terminal side through the separator. (Unlike some other chemistries.)
> 
> Energy efficiency is < 100 % because you're heating the cell as you charge and discharge it (internal resistance). That is related to the fact that, at the same SOC, the terminal voltage is higher than the OCV during charging, and lower during discharging.


That makes no sense whatsoever. Charge efficiency is the efficiency at which you charge and includes losses in the charger, the cells, and all connections.


----------



## EVfun (Mar 14, 2010)

It makes perfect sense. Coulombic efficiency is 100% (or very close to it.) Cell energy efficiency is less than 100% because the charging voltage is higher than the discharging voltage. Charger and wiring losses add to total losses, but they are outside the cells and vary based on parts chosen.


----------



## Elithion (Oct 6, 2009)

Ziggythewiz said:


> That makes no sense whatsoever.


Yes, I get that a lot. It does take some effort to grok that. Please let me try to explain.



Ziggythewiz said:


> Charge efficiency is the efficiency at which you charge and includes losses in the charger, the cells, and all connections.


Yes, absolutely. And charge efficiency (charger DC output, connections, cells, fuses, contactors...) is 100 %. 

Note, I said "charge" not "charging". 
"Charge" is a specific number of electrons, or a specific number of Coulombs. "Charging" is a process. 

Here we are talking about charge: electrons and holes. Every single electron that leaves the charger's DC "-" output will end up in the atoms in the "-" electrode in the cell, no matter how much resistance it has to go through. Every single electron that leaves the the atoms in the "+" electrode in the cell will end up in the charger's DC "+" output , no matter how much resistance it has to go through. There is no other place for the electrons to go.

You may be thinking of is "energy" efficiency during charging; and, yes, that is < 100 %.


----------



## Ziggythewiz (May 16, 2010)

Yes, what you call ""energy" efficiency" normal people call "charge efficiency".

What you are going on about is called *Faraday efficiency* (also called _faradaic effiency_, _faradaic yield_, _coulombic efficiency_ or _current efficiency_).


----------



## Elithion (Oct 6, 2009)

Ziggythewiz said:


> Yes, what you call ""energy" efficiency" normal people call "charge efficiency".
> 
> What you are going on about is called *Faraday efficiency* (also called _faradaic effiency_, _faradaic yield_, _coulombic efficiency_ or _current efficiency_).


Duly noted. I thank you. Faraday efficiency


----------



## GizmoEV (Nov 28, 2009)

Ziggythewiz said:


> Yes, what you call ""energy" efficiency" normal people call "charge efficiency".
> 
> What you are going on about is called *Faraday efficiency* (also called _faradaic effiency_, _faradaic yield_, _coulombic efficiency_ or _current efficiency_).


I'm obviously not in the "normal people" class then, I knew exactly what Davide was saying and he did properly use the terminology. The problem is the "normal people" who slaughter the meaning of words to the point that then can't understand what is being said when the term is used properly.


----------



## MN Driver (Sep 29, 2009)

Either way, we were just getting a little mixed up over terms, he stated both parts.


----------



## Joey (Oct 12, 2007)

kennybobby said:


> i have a confidential test report in which the battery manufacturer let their cells sit for 10 days after charging to measure the resting or Open Circuit Voltage (OCV) for several charge levels from 50% up to 100% SOC. By far the greatest "settling" of voltage occured in the 100% SOC cells, dropping 23 to 70 mv over 10 days resulting in 3.384 to 3.447 OCV. The lesser %SOC cells only fell 5 mv over the 10 day period: 50% SOC to 3.296, 80% to 3.3315, and 90% to 3.333 OCV.


kennybobby, is there any chance the 80% open circuit voltage is 3.315? It would fit a better curve with the other SOC data points and have the same significant digits as the other voltages. Looks like a typo with an extra 3 getting in there. 

If so that would mean my shipped voltage of 3.3 volts is closer to 62% SoC. Everyone says CALB ship at 60%.

My pack is still settling. After 8 days resting, I'm at 3.383 Volts and the exponential curve fit shows the pack should reach 3.372 V, but not for a very long time - another 23 days, if trend holds. 

Everything indicates that I am between 99% SoC, or slightly overcharged. My datasheets with the cells stated capacities between 193 and 195 Ah, but I only have data for 75% of the cells I recieved.


----------



## Ziggythewiz (May 16, 2010)

Joey said:


> I only have data for 75% of the cells I recieved.


Just ask Keegan for the missing sheet(s). I've missed one each time but they're quick to send if you ask.

I like to have 'em in digital form anyway and OCR doesn't like tables or Chinese.


----------



## kennybobby (Aug 10, 2012)

hey Joey,

That's not a typo, it is the average of two cells at 80% SOC after 10 days, one was 3.332 and the other was 3.331. The two 90% SOC cells settled to 3.333 each. Not sure what sort of curve fit would be used as there is also a small temperature fluctuation to consider (about 4 mV from -20 to +60 C). 0% SOC was considered to be at 2.5 V.

For longest life/highest number of cycles the engineer said that to only charge to 90% and only discharge to 10% SOC was better than doing 100% and 20%.


----------



## Joey (Oct 12, 2007)

Update: After nearly 20 days, my cells are at 3.3768 V. An exponential curve fit shows they will settle out to 3.3756 V. Every 6.6 days, the voltage drops 63% of the way toward the final voltage. The first hour of voltage drop does not fit my curve at all, but the rest of the data is a good fit. I terminated at 3.42 Volts, and within an hour I was at 3.4005 V. So the total voltage drop is 44.4 mV.

I suspect that the settling curve depends on the termination voltage and the terminal C rate. In my case that would be 3.42 V and C/10000 (1.5 amps).


----------



## GizmoEV (Nov 28, 2009)

Joey said:


> I suspect that the settling curve depends on the termination voltage and the terminal C rate. In my case that would be 3.42 V and C/10000 (1.5 amps).


That is why LiFePO4 charging is not just by voltage alone. It is a charging procedure. It includes a target voltage and current falling to a target value such as 3.6V and ending when current falls to 0.05C. Over the past nearly 3 years I've come to the conclusion that if the ending current is really low the ending voltage needs to be really close to 3.4V. I'm currently using 3.455V but may go a little lower. I haven't let my cells sit with no load for 20 days to see what the settling voltage is but over night with only a few mA of parasitic load my 200Ah pack usually sits at just under 3.4vpc. I've heard 3.40-3.38V as 100% SOC voltages after sitting open circuit for a long time. I think you are quite close to that so I'd call it good.


----------



## Joey (Oct 12, 2007)

GizmoEV said:


> That is why LiFePO4 charging is not just by voltage alone. It is a charging procedure. It includes a target voltage and current falling to a target value such as 3.6V and ending when current falls to 0.05C. Over the past nearly 3 years I've come to the conclusion that if the ending current is really low the ending voltage needs to be really close to 3.4V. I'm currently using 3.455V but may go a little lower. I haven't let my cells sit with no load for 20 days to see what the settling voltage is but over night with only a few mA of parasitic load my 200Ah pack usually sits at just under 3.4vpc. I've heard 3.40-3.38V as 100% SOC voltages after sitting open circuit for a long time. I think you are quite close to that so I'd call it good.


Agreed. I'm thinking of charging them to between 3.45 and 3.5 volts and 0.05C for their normal charge cycling. And I hope the cells won't sit 20 days at a time, once I finally get this project on the road.


----------

