# First charge cycle in new cells



## mizlplix (May 1, 2011)

So, when I got the new pack togather, they averaged about 3.310 vdc per cell.

I turned on the Elcon charger and about 6-8 hours later it went green.

128.7 pack volts and after sampling a few cells, 3.39 per cell.


Is this not kinda under charged?

I'm on step 8. Step 9 will be 132.21 pack / 3.48 per cell.

I'm thinking this is ok.

Comments please.

Miz


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## Ziggythewiz (May 16, 2010)

Sounds like a good charge. 3.4V resting is full, and they come at 60% SOC.


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## GizmoEV (Nov 28, 2009)

Do you know what the charge current was at the end of charge? If that 3.39vpc is after sitting for a while then the cells were fully charged. As was stated, 3.4V is fully charged. There is very little energy above 3.4V and it isn't worth stressing your cells to get it.

On the 200Ah pack in my Gizmo, before I knew better, I used to charge to 4.00V. I discovered that accelerating up to about 15mph was all I gained by charging to 4.00vpc compared to what I do now which is charge to 3.465vpc. This is on a 20 cell pack. My cells are usually resting just below 3.4vpc a couple of hours after a charge. My Zivan chargers taper the current to about 100mA before timing out so charging to such a "low" voltage is preferred.

Remember that the standard charge PROCEDURE is to charge to 3.6-3.65vpc and terminate the charge when the current drops to 0.05C. If the ending charge current is lower than that then lower the ending voltage.


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## mizlplix (May 1, 2011)

This 3.39 /cell was immediately after unplugging. Now a day later, they are at 3.348/cell.

my next step on the charger would be about 3.43 after sitting.

So, judging by all of your answers, I will leave everything as-is and just run it for a while.

Thx to all. I dont feel so bad now.

Miz


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## GizmoEV (Nov 28, 2009)

mizlplix said:


> This 3.39 /cell was immediately after unplugging. Now a day later, they are at 3.348/cell.
> 
> my next step on the charger would be about 3.43 after sitting.


How are you judging that it will be 3.43v after sitting? The voltage curve isn't linear so it will likely be just under 3.4v after sitting. If your cells are top balanced charge them up with the other curve (#9) and see where they end up. Before you do, however, I'd make sure to have an Ah counter attached. that will give you a good idea of how much energy there is between the two points.


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## mizlplix (May 1, 2011)

"About": The state of being in the near area of something. 
(dictionary)

I don't claim an absolute if there is some doubt and it is just my guess.

Regards, Miz


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## GizmoEV (Nov 28, 2009)

mizlplix said:


> I don't claim an absolute if there is some doubt and it is just my guess.


Right, I understand that, but you chose a number above 3.4V rather than at 3.4V or just under so that is why I asked. A cell sitting over 3.4V after resting for a while has been overcharged. This is assuming it is in the 20-25°C range.


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## mizlplix (May 1, 2011)

Ah, OK./

If I re set the charger to the 9th step, it is setting it for 39 cells, which would be about 3.3 volts above where it is now, divided into the 38 cell pack = .086/cell

3.39 (now) + .086 = 3.476 when just unplugged. the 3.43 was a guess after sitting.

But still too high, so I am stuck with the way it is set currently.(I think)

Miz


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## tomofreno (Mar 3, 2009)

mizlplix said:


> This 3.39 /cell was immediately after unplugging. Now a day later, they are at 3.348/cell.
> 
> my next step on the charger would be about 3.43 after sitting.
> 
> ...


Yeah, it takes at least several hours for them to stop dropping in voltage after charge. For a couple years I charged to around 3.45V per cell. Last few months I've been charging until most shunts come on at 3.5V so I can get some balancing. For the former the "relaxed" voltage would be around 3.39V, now it is about 3.4V. The V versus Ah curve is fairly flat at 3.4V, so a few Ah difference in SOC won't show much voltage difference.


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## GizmoEV (Nov 28, 2009)

mizlplix said:


> Ah, OK./
> 
> If I re set the charger to the 9th step, it is setting it for 39 cells, which would be about 3.3 volts above where it is now, divided into the 38 cell pack = .086/cell
> 
> ...


That depends on what the ending current is. I don't recall if you said what it was. If it is low then you might not gain much, if it is high then you might be undercharging by quite a bit. As tomofreno says below, the SOC difference won't show much in voltage at that point.



tomofreno said:


> Yeah, it takes at least several hours for them to stop dropping in voltage after charge. For a couple years I charged to around 3.45V per cell. Last few months I've been charging until most shunts come on at 3.5V so I can get some balancing. For the former the "relaxed" voltage would be around 3.39V, now it is about 3.4V. The V versus Ah curve is fairly flat at 3.4V, so a few Ah difference in SOC won't show much voltage difference.


What was/is your ending current (and pack size if you don't give it in terms of a C rate)? Also, do you see much balancing needed? I'm not seeing the need to balance but at most once/year for my pack.


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## mizlplix (May 1, 2011)

I really cant say as the charger was totally off when I got there, the next morning. (Well, there was no light on.)

The cells were within a hundredth volt of each other and again after 12 more hours.

Miz


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## GizmoEV (Nov 28, 2009)

Do you have a CycleAnalyst or something? You can hook up a computer to read the data stream from it to see what is going on and not have to baby sit it. The other thing I've done is be there at the end of charge. Of course I use my CA to know how long the charge will likely take.

That is great that the cells are staying that close to each other. Hopefully they continue to do so after they get a few cycles on them.


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## tomofreno (Mar 3, 2009)

GizmoEV said:


> What was/is your ending current (and pack size if you don't give it in terms of a C rate)? Also, do you see much balancing needed? I'm not seeing the need to balance but at most once/year for my pack.


 Charge current is usually 7 to 10A (0.39 to 0.056C) when charging overnight. It remains at that until the pack reaches about 124V (3.44V/cell) then it starts tapering back fairly quickly. It is less than 1A in about 10 minutes or so, then tapers down to about 0.2A if I let it run until it times out in about 35 minutes. Figure around 5A average for 10 minutes, 0.6A average for 25 minutes, so about 1.1 Ah added during that time, so no overcharging. Shunts generally start coming on when charge current is around 1A. Not all shunts come on, but all cells are between about 3.47V and 3.51V at end of charge (shunt turn-on varies from about 3.49V to 3.51V).

No, I don't see much need to balance, though cell voltages did start moving around more after around 2 years. I have one cell which apparently has higher leakage current than the rest (has always been that way) as it drifts down to lower voltage than the rest. I thought I would try keeping it charged up enough so it shunted each charge and remained balanced with the rest. Doesn't work, its leakage is too high, so it looses enough charge in just one cycle so it doesn't reach shunt voltage next charge. Guess I'll have to keep individually charging it every 3-4 months like I have been. Not so bad, I just set the power supply in constant voltage mode with V = 3.5V and let it run. Usually the current is down to a fraction of an amp, the shunt comes on within 45 minutes or so, and I put away the power supply for another few months.


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## GizmoEV (Nov 28, 2009)

tomofreno said:


> No, I don't see much need to balance, though cell voltages did start moving around more after around 2 years.


Sounds like your charging is similar to mine. I have BMS boards but currently they are hardwired to 4.00V shunt voltage. I'll be sending them back for that voltage getting lowered. I haven't decided what to set them to, however. They aren't on my pack right now. I'm really wondering how my pack will stay together as it ages.

BTW, have you checked the current drain of your BMS? Mine are all really close to 1mA. Maybe the one on that rogue cell has a higher current drain when not shunting.

It will be interesting how Miz pack does over time. More data points.


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## dougingraham (Jul 26, 2011)

tomofreno said:


> I have one cell which apparently has higher leakage current than the rest (has always been that way) as it drifts down to lower voltage than the rest


LiFePo4 cells don't have any leakage. I suspect that the battery monitoring device you have measuring the voltage of the cell is the source of the leakage. If you swap the BMS modules around I bet your bad cell will move with the module. Alternatively you could swap your lowest leakage cell with the highest leakage cell and see that it isn't the cell that has leakage when nothing changes.

You only need a little over 1ma additional load on a cell to unbalance the cell by 1AH over the course of a month.


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## EVfun (Mar 14, 2010)

How long have you used a LiFePO4 pack Doug? After 2 years I am pretty sure they do have a self discharge rate, it is just very slight. About once a year I have to bump a few cells up a little and one down a little to keep my top balanced no BMS pack in tight alignment. It would appear that the difference in this slight self discharge is less than 50 microamps, but even that would be 0.4 amp hour in a year. The drift shows up as the end of charge voltage spreading from the expected 0.04 volts up to 0.12 volts (3.45-3.57 end of charge before the charger shuts off.)

I am considering reinstalling my EVworks BMS modules. I would remove the cell monitoring loop wires so they where just functioning as shunt regulators. They would start to turn on at at 3.63 volts and shunt about 500 milliamps. I find I generally leave the DC to DC running all winter anyway and it is a 36-40 milliamp drain, far larger than the BMS units at 2.7 to 2.8 milliamps. It would be nice if someone made simple shunt regs for LiFePO4 cells instead of buying BMS units just to not use most of the functionality.


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## dougingraham (Jul 26, 2011)

EVfun said:


> How long have you used a LiFePO4 pack Doug


I've been doing pretty careful testing for about 10 months now on 6 cells out of my 60. With instruments that are quite a bit better than the A/D converters in most microprocessors.

I've postulated that a difference in temperature could affect charge acceptance rate. I might set up a test to cycle a cell in a refrigerator wired in series with one outside at room temperature and see if there is noticeable drift.

I wouldn't hook up a BMS that has different loads on the cells. You could design a BMS that would apply only a couple of microamps load to the cell but calibrating 50+ units once a year is way more work than I want to do.

I tested three cells to find their capacity, discharged one to 2.5v and left it sit. Discharged one to 3.30v and left it sit. And the last one I charged so its resting voltage is 3.40v and let that sit. At six months the only one that had any measurable change was the one left stored after a discharge to 2.5v. All three were still at the same voltage within a few ten thousandths of a volts. Charge state was still the same within measurement accuracy. But the one left stored discharged had lost about 0.25% of its capacity. Now this is admittedly a sample size of just one and there could be cells in the lot that behave differently.

I am hoping to have them in the car by the end of June. At that point I can watch them as a group and see what goes on. My own plan is to set the charger CV point around 3.40 and let it float to give a max charge of around 95%. My typical drive will only use 14% of the capacity so I should get good life.


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## GizmoEV (Nov 28, 2009)

dougingraham said:


> I've postulated that a difference in temperature could affect charge acceptance rate. I might set up a test to cycle a cell in a refrigerator wired in series with one outside at room temperature and see if there is noticeable drift.


That would be a very good test if you can do it. I'd like to know what happens too.



> I tested three cells to find their capacity, discharged one to 2.5v and left it sit. Discharged one to 3.30v and left it sit. And the last one I charged so its resting voltage is 3.40v and let that sit. At six months the only one that had any measurable change was the one left stored after a discharge to 2.5v. All three were still at the same voltage within a few ten thousandths of a volts. Charge state was still the same within measurement accuracy. But the one left stored discharged had lost about 0.25% of its capacity. Now this is admittedly a sample size of just one and there could be cells in the lot that behave differently.


Voltage alone isn't a good measure of SOC in the middle of the SOC curve. Did you do a capacity test on all three cells? That is a couple of full cycles to verify or did you only use voltage?



> My own plan is to set the charger CV point around 3.40 and let it float to give a max charge of around 95%. My typical drive will only use 14% of the capacity so I should get good life.


"Floating" at 3.40V for long enough will give you 100% SOC. Also, sitting at 100% SOC will shorten their calendar life so you really don't want to constantly float them there. If you only need 14% of the capacity for your typical drive then don't charge until you have used 50-70% of the capacity of your pack. This will prolong the time they sit below 100% SOC thus extending their life.


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## dougingraham (Jul 26, 2011)

I am short of time but I am going to do the cycle test where one is at refrigerator temp and one is at room temp. Probably happen after I get the car drive-able.

I did measure the SOC of the test cells by discharging the remaining capacity and then cycled the cells to see if there was any change in capacity. I get a standard deviation of around 150mah on a 100ah cell cycle test. I don't know where this error comes from. Probably several sources. But 0.15% is pretty good repeat-ability. I would have been happy with 0.5%. Voltage as a SOC indicator in the middle is ok if you can read to 0.0001 volts and let it sit for a day and have the same temperature. You can use these batteries as a thermometer. Voltage goes up with temperature rise. No idea how linear it is. I don't have a mapping of voltage change to temperature. But you can see it change. Either that or my meter changes a few ten thousandths of a volt over a few degrees which is also possible. Overcoming that issue is possible but I am not all that interested because it doesn't apply to the use in a car. 

A123 indicates you can float their cells at 3.45v for standby applications. I've left several of the 26650 cells floating for a few months with no ill effects. There isn't much difference between 3.40 and 3.45 volts from a capacity standpoint. I will probably play with the CV point so the worst cell rests at 3.40 volts or a little less. I've cycle tested all 60 of my batteries and sorted them by capacity. The idea is to bottom balance and then charge to somewhere between 90 and 95% so I will be playing with the CV point to get that.


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