# update on cell drift of my pack



## dtbaker (Jan 5, 2008)

I just thought I would start a thread on MY pack to record results, and see if it is what other people are seeing.

First.... my pack is 38 x 100ah Thundersky cells. I am using an Elcon pfc-1500 charger with simple CA->CV curve set to finish at 139v which gives a target average of 3.658vpc at 'finish'. I have no BMS, and leave my dc-dc 'on' all the time. I do use the vehicle almost every day, and charge at night.

Longest period of non-use was 1 week, and I left dc-dc 'on' to see what drain would be.... Kill-a-watt meter measured 1.9kWhr it took to charge back up to 'full'. Confirming that if you are not going to drive for a week, you really ought to flip the main breaker to prevent ANY drain!

I did an initial top balance in parallel to 3.90v with a bench power supply, then installed and wired in series. First series charge I monitored very closely and it didn't take more than a couple minutes since cells were charged from initial parallel balance, and was glad I did because I did have a couple cells head past 3.80 while others were below the CA->CV voltage of 3.65vpc. I then 'knocked down' the high ones with a pair of 50W resistors and repeated charge cycle until I had the whole pack +/- .02v just after end of charge.

The manual final tweaking of the balance in series is amazingly sensitive to small changes, and the voltage rise from flat to starting up the knee happens fast, and doesn't last long. I found that tweaking individual cells with no more than about 30 seconds of either draining with resistors or adding with power supply was the best way to avoid overshooting the target.

The cell voltage measurement I found most practical to do a few minutes AFTER the charger 'finished', and the pack voltage settled from the finish voltage of 139 to 136.0... because it hangs at close to 136.0 long enough for me to buzz around and record voltages from each cell.

The initial balance was +/- .02 on all cell but 4 that were -.03 ... I had to button the car up and do a show and tell and local community college, so I left it at that figuring that the 'high cells' would finish at no more than 3.68vpc, which is still fairly conservative for Thunderskies.

My first balance check was after about 1000 miles.... there was virtually no change in relative balance between cells. I decided not to change balance at all to see if there would be any drift with more use.

I just did second balance check yesterday at just over 3000 miles on the pack. There was virtually no change in relative balance between cells. I decided to spend a little time to get the 4 cells that were -.03 up a little... and after a couple charge/checks, I was able to get all cells +/- .02 at end of charge.

so.... at this point I just wanted to note that I experienced NO significant relative cell drift in the first 3000 miles.


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## Salty9 (Jul 13, 2009)

Reads like you are the BMS for your pack If you had a BMS how much more time would you be spending monitoring the BMS?


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## icec0o1 (Sep 3, 2009)

Salty9 said:


> Reads like you are the BMS for your pack If you had a BMS how much more time would you be spending monitoring the BMS?


He's doing it so we don't have to. Why would you want to reinvent the wheel every time?


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## Salty9 (Jul 13, 2009)

If it were my pack I would be monitoring it just like dtbaker is. Not because I mistrust his results but because I don't trust the uniformity of Chinese manufacture. Given the state of BMS today I trust them even less.


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## GizmoEV (Nov 28, 2009)

It took nearly 100 full cycle equivalents for my cells to be 0.1V different at the extremes. This could have been due to several different factors and likely wasn't much energy difference. I was charging to 3.485vpc and had BlackSheep BMS boards attached which draw only about 1mA. I rebalanced my pack to 4.00vpc using the BMS boards and then removed them. I take a reading each month to see what things are doing and then record and plot the results. I do have a half-pack voltage monitor on my pack but that is it besides the CycleAnalyst.


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## PTCruisin (Nov 19, 2009)

Dan,

As you know, I have a similar setup here and have about the same number of miles on the car. I did an initial top balance and did a "touch up" after about 2,000 miles. There are small differences (0.2 to 0.3) in voltage at the end of a charge cycle, but the batteries settle down nicely in a few hours. I only use 50 to 60% DOD in my commute and never get near the bottom, so the irregularities in cell capacity are not an issue for me. I think that is the key to running without a BMS.

I am considering adding one or two more cells to the string to lower the Vpc to either 3.55V (39s) or 3.47V (40s) to alleviate the differences in terminal voltages and add a mile or two of range.

Ralph


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## dtbaker (Jan 5, 2008)

Salty9 said:


> Reads like you are the BMS for your pack If you had a BMS how much more time would you be spending monitoring the BMS?


yes, I am the BMS.... but only checked twice in 3000 miles of use, so that seems a lot less expensive than the electronic version. 

I am posting my results as a matter of general interest along with my methods for those people evaluating their own needs. I know that when I was designing and installing my Li system, I found very little actual use reports from EV users to support arguments one way or the other on many points, so.... I thought I would post my situation and experiences so far.

My conclusion at this point, supported by MY data on MY pack is that a careful initial top-balance (within +/- .02vpc at end-of-charge) and charging with a simple CA-CV curve set to a fairly conservative 3.65vpc average exhibits virtually no relative cell drift after 3000 miles of use. 

My goal is simply to add a data point for people like myself to establish 'reasonable' frequency to check the balance on cells, and reasonable methods to manually re-balance if required.


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## dtbaker (Jan 5, 2008)

PTCruisin said:


> I did an initial top balance and did a "touch up" after about 2,000 miles. There are small differences (0.2 to 0.3) in voltage at the end of a charge cycle,


I hope you mean .02-.03 vpc differences?



PTCruisin said:


> but the batteries settle down nicely in a few hours. I only use 50 to 60% DOD in my commute and never get near the bottom, so the irregularities in cell capacity are not an issue for me. I think that is the key to running without a BMS.


agreed... but I would like to say that I think .02-.03vpc irregularity at end-of-charge is about all I want to tolerate to avoid possible overcharge issues on any one cell at end of charge. I have been surprised how a tiny difference in SOC can make a rapid and significant difference in final voltage. applying a 6amp charge or drain on a single cell for 30 seconds seems to change the final charge voltage close to .05v if the rest of the pack is 'balanced'. That gives me an idea just how steep that curve is, and how important that end-of-charge balance is.




PTCruisin said:


> I am considering adding one or two more cells to the string to lower the Vpc to either 3.55V (39s) or 3.47V (40s) to alleviate the differences in terminal voltages and add a mile or two of range.
> Ralph


I would be concerned that if you target a final voltage closer to the flat, you may start giving up a significant amount/accuracy of capacity in the charge cycle because the 'final' voltage may be hit when a lot of the cells are not that close to full. I am figuring that with Thunderskies and target of 3.65 +/- .03vpc is pretty safe. They seem to only be at that final voltage of a couple minutes, and settle down rapidly after the charger turns off.


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## T1 Terry (Jan 29, 2011)

I have conducted a number of experiments with my 360ah 12v house power pack. 16 X Winston LYP90ah. The first pack I carefully balanced at 4v per cell till they accepted no further charge using solar and an adjustable PWM float voltage regulator. I took it up in 1v steps from 3.4v per cell till the amps in dropped below 5amps and moved to the next stage, just made it easier and less likely to have a run away cell. this pack has remained balanced with every day cycling for 60 cycles so far and shows no sign of changing. I charge to 3.45v per cell, gives a good safety margin just in case and I have built a simple charge cut off/reset if any cell goes over 3.85v, just in case. i use a simple Jusi cell logger and the alarm triggers a 30 min timer circuit that turns off the solar positive input before the solar regulator allowing the house load to discharge any over voltage from a rogue cell. Similar to DT Baker leaving his DC to DC on.

The next pack of 16 cells I charged by a method but no balancing. The difference with these cells was they are all older Thundersky TS cells, been it the packing case for 2 1/2 yrs, production date 03/07/2009. I didn't do the 4.0v conditioning first but rather let them charge to 4.45v per cell and put them to work. Initially there was over a 300mv difference and the 3.85v cut out operated a few times but over a few mini cycles during the day the cells started to creep closer together. Within a few days the 3.85v cut didn't operate at all and after 2 weeks the pack as within 17mv, sometimes within 4mv. I decided to do the 4v conditioning, the pck stay together all the way to 4v per cell, I returned it to 3.45v per cell float and let it run for a week, still in balance.
these packs haven't cycled below 70% DoD much yet but on the few times they have (rain for a few days) they have remained in balance.

Just a side note, when under a 1C or higher load they do go out of balance, up to 60mv and it's never the lowest cell before the test starts that drops low under load so the relevance of off load balancing becomes a bit questionable but as hat is the only real method we have it will have to do.

T1 Terry


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## PTCruisin (Nov 19, 2009)

dtbaker said:


> I hope you mean .02-.03 vpc differences?


I did mean 0.02-0.03V



> I would be concerned that if you target a final voltage closer to the flat, you may start giving up a significant amount/accuracy of capacity in the charge cycle because the 'final' voltage may be hit when a lot of the cells are not that close to full. I am figuring that with Thunderskies and target of 3.65 +/- .03vpc is pretty safe. They seem to only be at that final voltage of a couple minutes, and settle down rapidly after the charger turns off.


At one time I monitored all the cells in my pack with Cell logs and observed that the "knee" of the curve is near 3.5V. The charger is already reducing current at that point in the profile, so by the time the cells get close to 3.6V, there is very little current (charge) going into the pack anyway. 

Before I did the "touch up" I would see the lower capacity cells get up to 3.8 - 3.9V before the charger would shut down. I don't think this is all that harmful given that the current is around an amp or less, but I rebalanced them anyway.


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## GizmoEV (Nov 28, 2009)

PTCruisin said:


> > I would be concerned that if you target a final voltage closer to the flat, you may start giving up a significant amount/accuracy of capacity in the charge cycle because the 'final' voltage may be hit when a lot of the cells are not that close to full. I am figuring that with Thunderskies and target of 3.65 +/- .03vpc is pretty safe. They seem to only be at that final voltage of a couple minutes, and settle down rapidly after the charger turns off.
> 
> 
> At one time I monitored all the cells in my pack with Cell logs and observed that the "knee" of the curve is near 3.5V. The charger is already reducing current at that point in the profile, so by the time the cells get close to 3.6V, there is very little current (charge) going into the pack anyway.


My testing and use shows that if the current at end of charge is low (like 0.005C or so), charging to only 3.45-3.5vpc doesn't sacrifice more than about 1% if that much. I would say that 3.65vpc is not as safe as charging to 3.5vpc.


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## EVfun (Mar 14, 2010)

I've noticed interesting behavior toward the end of charge and it makes me wonder how to accurately measure the balance. I have a 32 cell pack of 60 amp hour Thunder Sky cells made in February 2010.

I'm quite good at walking back into the garage just after the charger hits the constant voltage stage. The charger will be at about 10 amps at 111.6 volts. The pack will have 4 cells at 3.47, 2 cells at 3.51, and 1 at 3.52* volts. The rest of the cells will be between 3.48 and 3.50 volts. If I walk back out 30 minutes later near the end of the charge the cells will be between 3.48 and 3.50 volts and the one that was at 3.52 is one of the cells at 3.48 volts. The charger voltage will be 111.8 volts and the amps will be to low to accurately read, perhaps 2 amps.

If I check the pack 1 to 2 hours after the charge is complete the cells will all read either 3.38 or 3.39 volts except that one cell will read 3.37 volts. By the next morning the DC to DC has bled off about 1/2 amp hour and all the cells read 3.33 volts. In fact, each block of 8 cells will read the same, this morning they where all 26.68 volts (depending on how long "overnight" is they might all be 26.64 volts.)

I'm paranoid enough that I keep checking near the end of most charge cycles. These current numbers have held for about 1 week, prior to that I was still doing small balance adjustments at the end of most cycles. These adjustments where measured in amp seconds and I doubt any cell was adjusted by 3 amp minutes (0.05 amp hours.) I have been comfortable enough to monitor the finish charge only every other cycle. I've never seen a cell get above the others by 0.1 volts, even before balance adjustments. Most of the adjustments where knocking down that cell that pops up to 3.52 volts. I want to leave it alone for awhile and see if it creeps up.

* The one at 3.52 volts is the largest cell in the pack. After a deep test cycle it was at 3.18, the low one was at 3.06 and the rest where between 3.08 and 3.16 volts.


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## Guest (Sep 21, 2011)

What are you worried about. That is just fine. There will always been some variance near the end of charge and discharge. None are even in the 3.6 volt range. I have one that goes into the 3.8 volt range and it is consistent and always rests at 3.34 or 3.35. What more can you ask for. My pack is also bottom balanced as you know. 

I am in Cape Girardeau, Mo right now. I will be posting during the evening after each day of the conference. 

EV, you have nothing to worry about. Keep watching until your just plain sick of doing so and sure in your own head that all is OK. Don't get too anal about it but do what you must do. We all check. Except on my Leaf. 

Pete


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## GizmoEV (Nov 28, 2009)

EVfun said:


> I'm quite good at walking back into the garage just after the charger hits the constant voltage stage. The charger will be at about 10 amps at 111.6 volts. The pack will have 4 cells at 3.47, 2 cells at 3.51, and 1 at 3.52* volts. The rest of the cells will be between 3.48 and 3.50 volts. If I walk back out 30 minutes later near the end of the charge the cells will be between 3.48 and 3.50 volts and the one that was at 3.52 is one of the cells at 3.48 volts. The charger voltage will be 111.8 volts and the amps will be to low to accurately read, perhaps 2 amps.


I've noticed a similar behavior with my pack. What I think might be the cause is the cell which goes high at first has a higher internal resistance so its terminal voltage will be higher when the current is higher. As the current drops the IR doesn't factor as much in the terminal resistance so the voltage drops. I think that this points to a reason to only top shunt balance when the charge is basically finished and then only at very low current meaning less than 0.5A. That is assuming you are going to top balance. The behavior you see is the reason I take cell measurements at the end of charge immediately after the charger quits pumping in current. If I can't wait for that and need to get the readings I only make them when my charger is pulling 20-30W out of the wall.

Like Pete said, I wouldn't worry about it especially since you are charging to a low vpc like I am.


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## dtbaker (Jan 5, 2008)

EVfun said:


> If I check the pack 1 to 2 hours after the charge is complete the cells will all read either 3.38 or 3.39 volts except that one cell will read 3.37 volts.



this is why I think that part of 'check and balance' procedure has to be structured rigorously just after the end of charge when the pack starts settling down, but has slowed down enough to allow you time to zip around and take all your measurements, but not an hour after charge when the pack has settled to resting voltage and hides the small differences.

My pack 'finishes' at 139v. I notice that about 3 to 5 minutes after the charge stops, the pack slowly drops as the surface charge dissipates... and when it hits 136.0, I start my measurements (always with the same cell and in same direction ) and try to get thru as quickly as possible while pack voltage is still above final 'resting' voltage. At final rest the cells are too close to spot differences. The intention is to try and spot any cells getting out of line at the very end of charge... the problem is that the pack only stays at that peak for less than a minute, not enough time to get all cells measured!


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## PTCruisin (Nov 19, 2009)

GizmoEV said:


> My testing and use shows that if the current at end of charge is low (like 0.005C or so), charging to only 3.45-3.5vpc doesn't sacrifice more than about 1% if that much. I would say that 3.65vpc is not as safe as charging to 3.5vpc.



I'm beginning to believe that as well. I charge and drive the car daily and don't worry much about the cells being balanced anymore. I think I'll add one or two more cells to the string to lower the Vpc to approx. 3.5V and just keep driving.


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## dtbaker (Jan 5, 2008)

PTCruisin said:


> I'm beginning to believe that as well. I charge and drive the car daily and don't worry much about the cells being balanced anymore. I think I'll add one or two more cells to the string to lower the Vpc to approx. 3.5V and just keep driving.



I would hesitate adding new cells to an existing pack if it could be helped.... I think the time to re-balance, and possible difference in cell chemistry from one batch to the next might introduce more risk of imbalance that you prevent by charging to a lower vpc.

anyway, I just wanted to share my case, with procedures I use, to present as controlled 'check' as possible... to add a data-point in deciding whether or not relative cell-drift is even a significant factor over time in real use.

At this time (3000 miles), I'd say that little if any relative drift is evident. Supporting the hypothesis that a top-balance, non-BMS system is pretty safe from over-discharge relying on initial balance and functional charger with simple CA->CV charge curve.


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## dtbaker (Jan 5, 2008)

8-2-2012.... finally made time to check balance on my pack in the Swift again. It went a little further than I had intended, but all is well. I now have 8431 miles on my pack (38 x 100aH Thunderskies). my usual daily use is about 20 miles (50% DOD or less), so this represents about 425 partial charge cycles. I have taken pack down to 75%-85% DOD about twice I think.... and the wimpy Curtis 1221 can only maintain 2C, and max 3C, so pretty gentle use.

I am pleased to report that all cells are still +/- .03 vpc from theoretical average vpc target at end of charge after letting pack settle from 139. to 136. and starting measurements just after surface charge dissipates. I re-charged and double checked the highest ones right at end of charge and found that the very highest was at 3.80v for about a minute at tail end of charge until pack settles and voltages drop closer together.

There were 7 cells on the high side by .03, so I applied a 50 watt resistor for 10 seconds to each of the highest ones as long as I had battery covers off. Probably didn't need to tweak balance, but why not!

tests are bearing out hypothesis that top-balance without BMS stays in balance just fine. at this point I plan to do just annual checks/tweaks.... about every 5000-7500 miles.


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## GizmoEV (Nov 28, 2009)

Kind of a boring thing to do, isn't it? After a while you just don't worry about it. I just checked my pack today and the voltage spread is 0.033V. The low was 0.020V, high of 0.047V. It is interesting to look at a graph of the spread. The cells change places frequently. The attached graph is for my 40 cell TS-LFP100AHA pack in a 2p20s arrangement.


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## dtbaker (Jan 5, 2008)

GizmoEV said:


> Kind of a boring thing to do, isn't it?


I am loving this particular boredom! I thought it was time to check terminal nuts and figured I might as well go thru and check balance. Thrilled that balance is basically unchanged without having any BMS.

The terminal studs/nuts were all good too. I did not use regular bolts; I switched out to stainless set screws (with loctite), toothed washers and nuts. I did this primarily to be SURE I was getting full thread engagement in those soft AL and Cu terminals.

I did look at your graph, and am a little surprised to see the spread opening up a little, and the specific cells showing relative drift/change with respect to each other. It also appears that you wait until after the charge cycle has settled almost completely to check (by the voltage shown)... what is your measurement procedure, and how did you do your initial balance?


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## Yukon_Shane (Jul 15, 2010)

dtbaker said:


> I did look at your graph, and am a little surprised to see the spread opening up a little, and the specific cells showing relative drift/change with respect to each other. It also appears that you wait until after the charge cycle has settled almost completely to check (by the voltage shown)... what is your measurement procedure, and how did you do your initial balance?


I notice the same thing, this data seems to clearly show that these cells are not only drifting around a bit but that the final charger voltact is spreeding appart.

DT, you've certainly shown that a BMS isn't a necessary peice of equipment but if this data is consitent with all packs of this size I'd say it atleast shows that the occassional rebalance is appropriate.


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## dtbaker (Jan 5, 2008)

Yukon_Shane said:


> I notice the same thing, this data seems to clearly show that these cells are not only drifting around a bit but that the final charger voltact is spreeding appart.
> 
> DT, you've certainly shown that a BMS isn't a necessary peice of equipment but if this data is consitent with all packs of this size I'd say it atleast shows that the occassional rebalance is appropriate.



my hypothesis in this case would be that I did my initial top balance to a higher voltage than gizmo, which would narrow the band of variation. The further down the knee you go with either the initial balance or final vpc in charge, the harder it is to tell where you're at...

I would recommend annual check up regardless, just to check terminal connection torques, and spot any cells drifting toward the high side at the very end of charge.

At this point, I think I am going to change my future checks to try and catch cell voltage at the very end of charge. I have anout a minute, or two, when charger hits CA->CV voltage and starts cutting back on amps. I am thinking I will get thru as many cells as possible while it is in the max voltage stage, and 'rest' with hedlights on for a couple minutes, and do another cycle checking end of charge vpc unless I get them all right at max voltage..... THEN tweak down any that are a little high.


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> I did look at your graph, and am a little surprised to see the spread opening up a little, and the specific cells showing relative drift/change with respect to each other. It also appears that you wait until after the charge cycle has settled almost completely to check (by the voltage shown)... what is your measurement procedure, and how did you do your initial balance?


My old Zivan NG1 only terminates based on time, not ending current. As it reaches the saturation voltage the current continues to taper toward zero. Frequently it tapers to the point that the charger actually pulses the current on and off. Watching a Kill-a-Watt meter the power drops to 17-19W and the charger turns off the current. At this point the Kill-a-Watt reads 3.4-4W. After a short while the charger kicks on again for a few minutes while the current quickly tapers off again. It repeats this until it times out which is anywhere from about an hour to significantly longer than that based on how long the charger had to charge. This is a hold over from the lead acid charging algorithms. Apparently it can't be changed.

Typically it is during these "off" times that I take my voltage measurements. It usually takes me 3 of these cycles to get all of the measurements. Sometimes the current doesn't actually do this either due to cold batteries or I don't have the time to wait. In the spreadsheet I've attached you will see a line indicating wall power. If it is 4W or less then the readings were taken when no current was going into the batteries. The only load on them during this time is my Batt-Bridge circuit and the DC-DC converter which draws less than 10mA in standby mode so likely no more than 50mA keeping radio presets and the Batt-Bridge meter powered.

The last measurements were taken with wall power at 22W. Assuming that the standby power of 3.4W stays constant that leaves at most 18.6W going into the pack. With a pack voltage of 69.1V this would be at most 269mA less the parasitic loads. Remember I have a 200Ah pack so this current is quite small for this pack.

I chose to take measurements this way because this is when a shunting BMS would be shunting current. They won't care about the settled voltage, only the voltage during end of charge. For comparison I went out and took measurements again and attached them in the pdf file. Note that the difference is 0.019V which is lower than the first charge after I balanced my pack over a year ago. See the spreadsheet in the zip file for all of my data.

I did note that the low cell is the same low cell I had when I still had my Black Sheep Technology BMS boards installed but was not charging to the shunting voltage of 4.00V. One of the cells may have a slight self discharge.

For initial balance I charged until all the BMS boards were shunting and held that voltage for a while to make sure that no boards shut off. I then removed all the boards. Since I was balancing at 4.00V even slight variations between boards wasn't going to be noticeable at my regular eoc voltage of about 3.46V.

Hope that helps. I'm interested in any feedback you may have.


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## EVfun (Mar 14, 2010)

I thought it was time to check on my top balanced pack without a BMS. Since my previous post in this thread I added 6 cells to my pack, taking it from 32 cells to 38 cells. All my 60 amp hour Thunder Sky cells are from the same Feb. 2010 batch. I purchased 42 cells. In 2010 40 of them where in a Datsun EV. In 2011 32 of those where in the EV Buggy. This year 38 of them are in the EV Buggy (it runs much stronger with the voltage increase.) I have 4 cells on the shelf in the garage. No cells have showed signs of damage or failure. 

It was a good thing I checked...

I found the 6 cells I added have creeped up a bit. I charge the pack to 3.5 vpc (133 volts for 38 cells.) The 32 cells that have been used for 3 years where all between 3.43 and 3.50 volts. Most of them where at 3.44 volts. The 6 added cells ranged from 3.61 and 3.99 volts at the peak during charge. These cells matched the pack in the spring. I top balanced the 32 in the car and added the 6 cells that had been top balanced by charging in parallel. I installed these cells in the car as 2 blocks of 3 cells. Each block of 3 has only been treated as a unit (no individual balancing within each block of 3.) I had removed a fraction of an amp hour from each block of 3 to make them match the 32 in the car. 

The right hand block ranged from 3.74 to 3.99 volts. I removed 0.15 amp hour for now which lowered the high one to 3.93 volts. The left hand block ranged from 3.61 to 3.82 volts. I removed 0.1 amp hour from that block of cells which lowered the high cell to 3.80 volts. I will pull more out of both banks before recharging the pack again. I may do a capacity test to see if the bottom is getting more ragged or if the smallest cell is different. I will certainly make sure the top is balanced again, or is at least not more than 0.1 volt high at the end of the next charge. 

I'm not so sure adding cells is a good idea, even when they come from the same batch. I may end up installing shut regulators to nudge the top in line. It wouldn't take more than 20 milliamp hours each cycle. Even though all 38 cells have seen the same number of amp hours go into and out of them the less used cells are ending up at a higher state of charge over time.


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## dtbaker (Jan 5, 2008)

EVfun said:


> I thought it was time to check on my top balanced pack without a BMS. Since my previous post in this thread I added 6 cells to my pack, taking it from 32 cells to 38 cells.


so.... did you ever put the WHOLE pack in parallel and top balance to something like 3.8 or 3.9? then put back in series and check end of charge to knock down any high cells at very end of charge? if not, then it sounds like initial re-balance was not quite 'complete'.


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## EVfun (Mar 14, 2010)

I had the 6 cells at a slightly higher SOC than the 32 cells. After I installed them I put a resistor around each block of 3 added cells to pull it down while charging for several cycles. After working the SOC down on the added cells I verified they where matched to the rest of the pack by monitoring the voltage during the end of charge for 2 cycles. The cells finished in the same voltage range (3.47-3.54 volts in the last 10 minutes of the charge) without intervention for 2 charge cycles. I think I had the added cells matched to the rest of the pack. I checked several more times this spring and everything looked normal.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> The cells finished in the same voltage range (3.47-3.54 volts in the last 10 minutes of the charge) without intervention for 2 charge cycles.


So what were the voltages at the moment the charger shut off or just a few seconds before? As you know, even 10 minutes at a low charge rate can have a drastic effect on the ending voltage.

Edit: Also, if you look at the graph of my cell voltages in the .zip file I posted earlier you will see that the voltages do move around a bit. Maybe after the "new" cells get cycled a little more they will match up.


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## EVfun (Mar 14, 2010)

They are not getting closer. They where all between 3.47 and 3.54 in the early spring right after I worked the 6 additional cells into the pack and got them agreeing at the end of charge. I would just go around in circles checking the cells continuously after fitting the new cells into the pack. Must have been 4 or 5 cycles where I would burn off a little charge with resistors to knock the added cells down to match the other 32. Then I watched all 38 cells stay in the same range for 2 charges without intervention. Today my total pack range was 3.43-3.99 volts! 32 of them where between 3.43-3.50 volts. The added 6 cells ranged from 3.61 to 3.99 volts. I only knocked them down down a little and will tighten up the added cells to the rest of the pack when I charge tomorrow (at least get close, balancing may take several cycles.)


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## GizmoEV (Nov 28, 2009)

I wonder if maybe the "new" cells are slightly more efficient than the ones which have more cycles on them. If so, the 32 would heat slightly more than the other 6 which would store more of the energy.


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## Ziggythewiz (May 16, 2010)

Yukon_Shane said:


> this data seems to clearly show that these cells are not only drifting around a bit but that the final charger voltact is spreeding appart.


The drift is pretty clear...but it has increased by a max of only .025 V total variance over 5000 miles. Looks like very reassuring data to me.


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## dtbaker (Jan 5, 2008)

EVfun said:


> They are not getting closer. They where all between 3.47 and 3.54 ....The added 6 cells ranged from 3.61 to 3.99 volts. I only knocked them down down a little


if you are top balancing, my belief is that for at least the balancing you need to be checking and adjusting right at the tail end of charge... thats what you are trying to control after all! If they are balanced close to 3.65vpc at end of charge, they'll be even closer if you set your regular cycles to end lower.

I took my initial balance in parallel ABOVE intended pack voltage (to 3.9), the checked and tweaked a little after putting them in series until all were 'finishing' very close to 3.65 vpc.

trying to balance at lower voltages is a waste of time as you can't tell where you are if the curve is flat.


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## EVfun (Mar 14, 2010)

GizmoEV said:


> I wonder if maybe the "new" cells are slightly more efficient than the ones which have more cycles on them. If so, the 32 would heat slightly more than the other 6 which would store more of the energy.


I think that is right, but that points to some type of self-discharge rate, even if it is only when charging. After all, exactly the same numbers of amp hours have passed through all of them since spring when I balanced the pack.


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## jeremyjs (Sep 22, 2010)

I'm wondering if it has anything to do with the how the cells degrade. I remember seeing a 500 cycle test put up over at evtv that he managed to get out of thundersky. While the decline in capacity was slow and straight for the most part, other than the gain within the first 50 cycles, there was also the occasional spike or valley in the chart for one charge then a continuation of the slope. Is it the same cells showing variation or does it move around?


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## EVfun (Mar 14, 2010)

dtbaker said:


> if you are top balancing, my belief is that for at least the balancing you need to be checking and adjusting right at the tail end of charge... thats what you are trying to control after all! If they are balanced close to 3.65vpc at end of charge, they'll be even closer if you set your regular cycles to end lower.
> 
> I took my initial balance in parallel ABOVE intended pack voltage (to 3.9), the checked and tweaked a little after putting them in series until all were 'finishing' very close to 3.65 vpc.
> 
> trying to balance at lower voltages is a waste of time as you can't tell where you are if the curve is flat.


I am top balanced and do check at the end of charge while the charger is still running. I balanced them at 3.50 volts and very low current (held them at 3.50 for a couple hours.) I figured that the tweaking you mention wouldn't be needed after balancing if I balanced to the peak charging voltage. I don't think 3.50 is to low, it is certainly past the flat part of the curve.


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## EVfun (Mar 14, 2010)

jeremyjs said:


> I'm wondering if it has anything to do with the how the cells degrade. I remember seeing a 500 cycle test put up over at evtv that he managed to get out of thundersky. While the decline in capacity was slow and straight for the most part, other than the gain within the first 50 cycles, there was also the occasional spike or valley in the chart for one charge then a continuation of the slope. Is it the same cells showing variation or does it move around?


It is consistently the same cells that are high at the end of charge. It is the 6 I added to the pack. I'm going to discharge the pack today, constantly scanning with the volt meter to find out how the bottom of the pack numbers have changed.

After a drive to take the cells down about 1/2 way I let them rest a couple hours then checked the voltage. All 38 where 3.297 +/- 0.001 volts. I'm doing a discharge test now to see what I see at the bottom. I'll post back whatever I find.


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## MN Driver (Sep 29, 2009)

jeremyjs said:


> I'm wondering if it has anything to do with the how the cells degrade. I remember seeing a 500 cycle test put up over at evtv that he managed to get out of thundersky. While the decline in capacity was slow and straight for the most part, other than the gain within the first 50 cycles, there was also the occasional spike or valley in the chart for one charge then a continuation of the slope. Is it the same cells showing variation or does it move around?


Pretty sure it was Sky Energy/CALB that provided that data and he put it into the spreadsheet.


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## EVfun (Mar 14, 2010)

I pulled my pack down until a cell hits 3.000 volts. My main 32 cells where between 2.996 and 3.104 volts. The added 6 where between 3.107 and 3.125 volts. I pulled 0.2 amp hour from them. I added 0.06 amp hour to the smallest cell because I know it isn't one of the highest voltage at the end of charge. The pack has remained consistent, the smallest cells are still the smallest and the largest cells are still the largest. Once I get the 6 added cells in line I will figure out where they are compared to the rest of the pack.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> I think that is right, but that points to some type of self-discharge rate, even if it is only when charging. After all, exactly the same numbers of amp hours have passed through all of them since spring when I balanced the pack.


I wouldn't call it self-discharge. Self discharge is losing charge just sitting. If the efficiency of getting the Lithium ions into and out of the crystal structure is different between the cells then that could account for the difference but that does not mean that the cell will self discharge any differently as it ages. It doesn't mean that it doesn't start to self discharge either.

Given that the "new" cells were higher in voltage when you discharged the whole pack it appears that they were not quite balanced at the top. However, if the efficiency hypothesis is correct, then those cells would also not lose as much charge for a given output as the "old" cells.

It will be interesting if you continue to see this same behavior. If it continues then it does point to the issue of putting cells with a different cycle age into a pack in a series arrangement. If they were added in parallel, however, the problem would not likely exist but that would mean doubling the number of cells in the pack so each cell got a new buddy.


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## EVfun (Mar 14, 2010)

It was easy to get the pack back in balance and it is surprising just how little it takes to run a cell from 3.5 volts to 3.9 volts. The 6 cells had 0.2 amp hour removed and now they finish with the rest of the pack. I had a total range from 3.46 to 3.53 volts. The 2 cells at 3.46 where both the added ones that had been finishing at around 3.8 volts. The 2 cells finishing at 3.53 where from the older 32 cells and have always been the highest ones at finish. 

Just 0.2 amp hours was all the difference between one cell going to 3.99 volts and now only going to 3.51 volts at the end of charge. It makes me wonder what really happens in the last 30 minutes of a charge with a bottom balanced pack.


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## GizmoEV (Nov 28, 2009)

Now you can find out if those two creep up in voltage at the end of charge after several cycles. If they do there might be some merit to the efficiency hypothesis.

For a bottom balanced pack the ending voltage has to be lowered. As long as the charger shuts off when the current drops to about 0.05C then a cell ending up at 3.65V is likely ok and not overcharged. I'm sure that if the cells' capacities were spread a long ways out then it could be a real problem.


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## mizlplix (May 1, 2011)

I have chosen a different cell strategy, one I'm sure that will ignite many critics and cause a lot of "You're a dummy" posts. I don't care. It works for me. I only present it here as an abstract idea and mental exercise.


I named it -Middle balanced-:

I paralleded my pack in their as-is delivered state. I do not much care as to their real charge state just as their not full 3.9 or 3.0

I left them for a week. After disconnected another week, they were all 3.310

They were installed, series, and brought up to full charge. (recorded) 

I had one wild cell, #12. I put a cell voltmeter on it. It was 3.90

Driven, recorded, charged, driven, charged....4 complete 35-40 mile cycles.

The figures all repeat, every-time accurately.

The same cells charge and drain in an accurate, predictable manner.

The basis of my "middle charging" idea is that when it charges it only gains 50% of the deviation of a bottom balanced pack. It also drains at 50% of the deviation of a top balanced pack.

In short, It uses a 50% median line to limit the deviation to 50% above or 50% below instead of the full 100% as of top or battom balanced.

The top charge is repeatable and "watched by the charger. The problem cell that will drain first has a cell monitor on it and will give me a really obvious red light on the dash.

Yes, it has been only 4 cycles so far, but it has repeated accurately.

Miz


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## onegreenev (May 18, 2012)

So what is your end of charge target? That one cell could have been 5 AH out of whack from the others and could very well be exactly the same AH in the cell as the others and would be sitting happy with the others if you had lower balanced your pack. If you even take your pack to 3 volts per cell you know your not on the bottom but pretty much on the down swing. All should easy be within an AH or so from each other. Then on the upper charge your swing won't be so far as long as you stay off the very bottom which is pretty easy to do with a smart controller and AH counter. I would not just do a middle because it is hard to KNOW what the MIDDLE is. It is easier to do the balance on the upswing or downswing. Downswing being the safer but either works if you so choose. My last balance is more in the range of 2.7 volts rather than 2.4 volts like I did before. Still no cell goes over 3.8 by the time the charger cuts off. All cells just before termination are 3.7 and lower. I now use 3.55 volts as a cutoff voltage since I can choose any termination voltage with my controller when its used as a charger. 

Pete 

I might suggest you bring your one cell down a touch to better match the others so your not charging it to such hight voltages. 3.9 is in the range where you may not want your cell to live and if it has the capacity you surly don't want to leave it there. 

The purpose of balancing is to also find which cell is the lower capacity one or ones and to find out by how much. If your termination voltage is like 3.7 and lower for your highest voltages your doing well.


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## onegreenev (May 18, 2012)

No matter how you set up your pack your still charging in series single cells so each will charge accurately and repeatably every time. That is not in question. The question is how well balanced is your pack of cells to each other cell in the pack. The closer together they are the better. Top, Bottom, or Middle. Just harder to balance accurately in the middle vs the top or bottom swing.


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## mizlplix (May 1, 2011)

For the last 4 cycles, my pack has done the exact same thing. They all come back to .014/volt of each other when in that mid-balanced area. Even that high one. 

I do not intend to further alter any even if I lose that strange one. That is the premise of this method.

I am aware I am leaving some capacity on the table with the way my charger is set, but that is another story.

When I get my transmission put back in, I will get some lower discharged cycles on them. If that follows predictions, it will bear me out. If not then I will just top balance like everyone else and keep them that way.

In my world, when a cell tops out first AND bottoms out first, it is lower in capacity than the rest. I have 14 that deviate from the rest of the pack when on the top end. I still need to check them on the bottom end to say for sure.

The problem with this forum is that there are too many different opinions. All state they are right, making it very confusing for anyone seeking knowledge.

Miz


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## onegreenev (May 18, 2012)

> The problem with this forum is that there are too many different opinions.


The information given is factual and I see no opinions. Information is already out there that supports what is said, including mine. Believe it or not some still do things based on what has been done and not what is done on a typewriter.

If you don't know where the top or bottom truly lies then how do you propose to properly balance the pack in the MIDDLE.


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## DIYguy (Sep 18, 2008)

mizlplix said:


> I have chosen a different cell strategy, one I'm sure that will ignite many critics and cause a lot of "You're a dummy" posts. I don't care. It works for me. I only present it here as an abstract idea and mental exercise.
> 
> 
> I named it -Middle balanced-:
> ...


I did exactly this when I got my pack new over a year and half ago. I wanted to get driving and didn't want to take the time to bottom balance accurately. I never even go below 50 -60% so, it worked fine and never drifted or had any issues until I got around to bottom balancing.
After my rebuild, I re-did everything, so I decided to redo the bottom balance a bit lower. I used the Power Lab 8 and discharged each cell to 2.7 volts. It took a bit of time, but I did it while I worked on other things. 
After seeing that the finish charge in CV at 3.45 volts per cell did not cause any shooters at the top above 3.6, I upped the charger to 3.5 vpc. I have 3 cells that I watch with slightly lower capacity . . about 1% is all. They climb to about 3.65 and interestingly enough, they start to go back down before the charge clicks off. It's perfect. 
So, ya, it can work and perhaps if you have a pack that is not so perfectly matched, maybe is a good compromise.


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## ricklearned (Mar 3, 2012)

DIYguy said:


> I did exactly this when I got my pack new over a year and half ago. I wanted to get driving and didn't want to take the time to bottom balance accurately. I never even go below 50 -60% so, it worked fine and never drifted or had any issues until I got around to bottom balancing.
> After my rebuild, I re-did everything, so I decided to redo the bottom balance a bit lower. I used the Power Lab 8 and discharged each cell to 2.7 volts. It took a bit of time, but I did it while I worked on other things.
> After seeing that the finish charge in CV at 3.45 volts per cell did not cause any shooters at the top above 3.6, I upped the charger to 3.5 vpc. I have 3 cells that I watch with slightly lower capacity . . about 1% is all. They climb to about 3.65 and interestingly enough, they start to go back down before the charge clicks off. It's perfect.
> So, ya, it can work and perhaps if you have a pack that is not so perfectly matched, maybe is a good compromise.


I have a very similar situation with my pack, although I only have one shooter. So now there are three of us that "middle balance". Another description I have heard is ragged ends, to reflect the fact that on either end there is some uneveness. I stay away from the ends and my cells are within 10mv of each other at various states of charge.


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## EVfun (Mar 14, 2010)

Are you sure they vary by "about 1%" DIYguy? I ask because the 6 cells I had there where getting a little high near the end of charge (3.61-3.99 volts) only needed 0.2 amp hours removed to knock them back in line, now finishing between 3.46 and 3.51 volts. I have put a second cycle on them since pulling the newer 6 down and got the exact same finishing numbers.

It is a shame nobody makes a simple 100 milliamp 3.5 volt shunt reg (no BMS function, simple shunt reg.) That would be more than enough for slight variation, even for added cells of the same type. A red LED would make inspection easy.


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## DIYguy (Sep 18, 2008)

ricklearned said:


> So now there are three of us that "middle balance".


Just to be clear, I only did this for a few months. I have been bottom balanced since then. Thanks.


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## dougingraham (Jul 26, 2011)

mizlplix said:


> The problem with this forum is that there are too many different opinions. All state they are right, making it very confusing for anyone seeking knowledge.


There is the right way, the wrong way, and the military way.

Here is a helpful (or not) guide for grading opinions.

1) If the opinion comes from a manufacturer or vendor it is suspect. They want you to spend your money and will tell you what they have to to get you to do so. There are exceptions to this rule.

2) If the opinion comes from a product user their opinion will be against if they have had problems with a product but will be in favor if they have had no problems even if the product does nothing of value for them. So their opinion cannot be trusted. There are exceptions to this rule.

3) If the opinion comes from an "Expert" it is suspect. You don't know what they do and don't know and what is just their opinion. Experts are good at "typing themselves smart". There are exceptions to this rule.

4) If the opinion comes from a noob then who cares. But it doesn't mean their opinion is wrong. It just doesn't mean much.

5) If the opinion comes from me then it isn't an opinion, it is a fact! There are exceptions to this rule. (I usually, but not always, will tell you I don't know when I don't know.)

So whats the answer? There is no correct answer. There is only what works for you.


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## DIYguy (Sep 18, 2008)

dougingraham said:


> There is the right way, the wrong way, and the military way.
> 
> Here is a helpful (or not) guide for grading opinions.
> 
> ...


HAHAHA I like it...


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## DIYguy (Sep 18, 2008)

EVfun said:


> Are you sure they vary by "about 1%" DIYguy?


Yep. blah blah blah (Yup was too short)


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## DIYguy (Sep 18, 2008)

mizlplix said:


> The problem with this forum is that there are too many different opinions. All state they are right, making it very confusing for anyone seeking knowledge.
> 
> Miz


Just like life Miz. U get to sort it all out eventually. 
The key is understanding perhaps. Like the batteries for example. If you know their behavior, you can be successful with or with a BMS. . . with top or bottom balancing. You just need to understand it and mitigate risks as much as possible.


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## dougingraham (Jul 26, 2011)

onegreenev said:


> If you don't know where the top or bottom truly lies then how do you propose to properly balance the pack in the MIDDLE./QUOTE]
> 
> Middle balance is what Jack R was originally promoting. Take the batteries out of the box as they were shipped and use them. As he points out the problem is replacing or adding cells later. If you think bottom balancing is difficult it is a lot more difficult to middle balance for the purpose of adding or replacing a cell because you need to balance to better than 0.001 volt or the ends will be off significantly.
> 
> So if you have to add cells it is quite a bit easier to bottom balance and add in the cell or easier still to charge up the new cell and charge up the top balanced pack and add in the cell.


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## EVfun (Mar 14, 2010)

DIYguy said:


> Yep. blah blah blah (Yup was too short)


Well I'm a bit confused. You have your charger set to 3.5 vpc average, bottom balanced, and none go above 3.6 volts with a 1% variation in capacity. I have my charger set to 3.5 vpc average, top balanced, but 6 where about 0.3% more charged and they all went above 3.6 (but not over 4.0) volts. 

You are measuring while charging, but near the end of charge, right?

3 cycles now and the 6 there where finishing higher are staying with the rest of the cells now that I have removed 0.2 amp hours. My pack is 38 Thunder Sky LiFeYPO4, 60 amp hour cells, and my charger is set to 133.0 volts peak, holding that for 15 minutes before shutting off.


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## drgrieve (Apr 14, 2011)

EVfun said:


> I added 0.06 amp hour to the smallest cell because I know it isn't one of the highest voltage at the end of charge.


Just in case you don't know or if someone reading this equates charging voltage to SOC.

Charging voltage doesn't equal SOC. The only way to measure SOC is using static voltage or counting amp hours from a known point.

Using charging voltage as a point to balance your pack is wrong and is a failing of using an active BMS to balance your pack.


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## dtbaker (Jan 5, 2008)

I didn't start this thread as an opinion rantplace.... it is intended as a thread to post my actual results over time of running a top-balanced BMS-less configuration for those wanting to follow how that is working out.

Net is that so far ... with initial balance in parallel to 3.9 vpc, and 'tuned' after installing in series at end-of-charge until within .02v... cell drift has been un-noticable after more than 10k miles without ANY bms. Average day is about 25 miles (50% DOD), so thats about 400 cycles.


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## JRP3 (Mar 7, 2008)

EVfun said:


> Well I'm a bit confused. You have your charger set to 3.5 vpc average, bottom balanced, and none go above 3.6 volts with a 1% variation in capacity. I have my charger set to 3.5 vpc average, top balanced, but 6 where about 0.3% more charged and they all went above 3.6 (but not over 4.0) volts.
> 
> My pack is 38 *Thunder Sky* LiFeYPO4, 60 amp hour cells,


You can't compare TS voltages to CALB voltages. Plus, if you have cells going higher during charging, they aren't really top balanced


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## EVfun (Mar 14, 2010)

JRP3 said:


> You can't compare TS voltages to CALB voltages. Plus, if you have cells going higher during charging, they aren't really top balanced


 I'm not sure there is a squirrel fart worth of difference between CALB, Thunder Sky, Winston, and Sinopoly cells. They all seem to be working very similarly using the same controllers, chargers, and sometimes BMS systems. 

What do you mean "they aren't really top balanced"? I just got told you can't tell based on charging voltage.


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## JRP3 (Mar 7, 2008)

CALB has a different charge/discharge curve and a different voltage profile. CALB max charge is 3.6V and most people use 3.45 max. They also seem to have better QC and more consistently matched cells than TS/Winston.
Voltage is of course a good indicator of SOC, especially near the ends, but you also have to consider the influence of current and temperature.


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## EVfun (Mar 14, 2010)

JRP3 said:


> CALB has a different charge/discharge curve and a different voltage profile. CALB max charge is 3.6V and most people use 3.45 max.


I know they list that max, but perhaps they are just picking a different part of the curve. I'm not so sure there is a definite "full charge" for LiFePO4 so much as a choice between just how many Li ions to drive across vs. lifespan. Has anybody been taking CALB cells to 3.8-4.0 volts routinely often enough to know it has a disproportionate impact of cell life? I would think that at least Jack Rickard would have found out if that is a problem -- he runs more or less bottom balanced packs without any battery management hardware. I would think some must be seeing over 3.65 volts at the end of charge.


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## JRP3 (Mar 7, 2008)

Not if they set their charger right  There is just no effective capacity above 3.6 with CALBS, and not really any above 3.5.


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## EVfun (Mar 14, 2010)

There is no effective capacity with Thunder Sky cells above 3.5 either. After all, removing 0.2 amp hour from one of my TS cells took it down from 3.99 volts to 3.52 volts at the end of charge (charger set to 3.50 vpc.)


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## PStechPaul (May 1, 2012)

I have not followed all of this thread, but it seems like a valuable resource as evidence of charging and balancing issues. Is this pack LiPo or LiFePO4? AAIU the former are about 3.7Vpc and the latter are 3.2Vpc.

I found some detailed information on batteries and charging methods:
http://www.mpoweruk.com/chargers.htm


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## EVfun (Mar 14, 2010)

I think everyone discussing their EV pack in this thread are using either CALB or Thunder Sky cells. Both are LiFePO4 cells.


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## JRP3 (Mar 7, 2008)

EVfun said:


> There is no effective capacity with Thunder Sky cells above 3.5 either. After all, removing 0.2 amp hour from one of my TS cells took it down from 3.99 volts to 3.52 volts at the end of charge (charger set to 3.50 vpc.)


 Probably the easiest thing to do is compare curves of TS and CALB at the same C rate. Jack has done the work for us. You can see the differences the TS 160 voltage climbs much higher while still adding significant capacity, the TS 200 less so, and the CALB 180 even less.

http://1.bp.blogspot.com/_i_c2BM_uB...AABXQ/bj_KhONGd8w/s1600/+0416chargecurve2.jpg

During discharge the CALB's don't sag as much and have a flatter curve.

http://1.bp.blogspot.com/_i_c2BM_uB...BXY/EGz2UfRkWWE/s1600/+0416dischargecurve.jpg

To me those are fairly obvious differences in behavior.


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## EVfun (Mar 14, 2010)

I remember seeing those curves. If I remember right, the TS cells where all older than the CALB cells, and the LFP160 was the oldest of the cells tested. Internal resistance has clearly been going down for years in small incremental steps. On the charge curve, if you assume (yeah, that's dangerous) that the TS 200 and CALB 180 are actually about the same thing and so expand the TS curve 10% to using only the middle 180 amp hours (about 4% down at the the bottom and 6% up at the top) you find they almost lay on each other. On the discharge curve I see a somewhat higher internal resistance and about a 10% exaggeration of capacity again. LiFePO4 behavior clearly dominates over slight manufacturer differences.

Has anybody tested a CALB cell by repeatedly running it up to 3.8 volts at the end of charge to see if that negatively impacts life more than for other brands? When charging a bottom balanced series string some are going to rise more than others at the end (I would think someone is unintentionally doing this test right now.)


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## JRP3 (Mar 7, 2008)

Hopefully no one with a bottom balanced CALB pack is seeing any cells go to 3.8V, which is above the 3.6 factory recommended max.


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## EVfun (Mar 14, 2010)

In the graph you show Jack took the CALB cell up to 4.25 volts.


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## JRP3 (Mar 7, 2008)

Yes, it was a test. CALB says no higher than 3.6, who am I to argue  Especially since we know keeping cells at lower SOC makes them last longer.


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## GizmoEV (Nov 28, 2009)

Even with all the data I've seen, including JR's graphs, there is nothing saying that there is a difference in the charge profiles between the different LiFePO4 prismatic cells. Just because one manufacturer says 3.65-4.00V and another says 3.6V doesn't make them different. The discharge tests JR posted doesn't either. It merely shows that the internal resistance of one cell is different than another. That difference could entirely be because of the thickness of the coating on the plates. It is entirely possible that CALB is being more conservative than TS, et al.


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## tomofreno (Mar 3, 2009)

JRP3 said:


> Probably the easiest thing to do is compare curves of TS and CALB at the same C rate. Jack has done the work for us. You can see the differences the TS 160 voltage climbs much higher while still adding significant capacity, the TS 200 less so, and the CALB 180 even less.
> 
> http://1.bp.blogspot.com/_i_c2BM_uB...AABXQ/bj_KhONGd8w/s1600/+0416chargecurve2.jpg
> 
> ...


 Those curves look a bit strange to me - smooth toward low V end, jerky toward high V end. 

Keep in mind you are comparing two "populations" of cells, TS and CALB, based on a sample size of 1. Charging curves from 16 of my 180Ah CALB (2009 SkyEnergy) cells are below. The spread in cell voltages over most of the charging time is around 0.03V, but would be quite a bit larger if charging at 1 C as Jack did rather than at the 0.11C used here. Maybe on the order of the spread shown between TS and CALB on Jack's graph, if the spread scales linearly with charging current.
View attachment Cell V while charging.pdf


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## JRP3 (Mar 7, 2008)

Yes it is a small sample group, but the results are similar to curves published by the cell manufacturers. I suppose we'd need to test a few of the newest cells from each company, but the recent CALB CA cells appear to be even stiffer than the SE cells, so CALB may still do better than the others.


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## JRP3 (Mar 7, 2008)

As if on cue



> Well it looks like jack finally put the screws to some 40ah cells. The results are definitely encouraging. Roughly half the sag at 2x the load compared to the old SE cells.


http://www.diyelectriccar.com/forums/showthread.php/calb-ca-p319149.html#post319149


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## EVfun (Mar 14, 2010)

Can you spare me the time of watching one of Jack's videos and just tell just where to look or what the results where?

I followed that test on the CALB CA 180 amp hour cell and it is impressive. It looks like around 0.3 milliohms of internal resistance. That is an incremental improvement that may be worth paying for. For comparison, my TS 60 amp hour cells are 1/3 the capacity and I would love the internal resistance to be 0.9 milliohms. The measured internal resistance is 1.4 milliohms. That is good for 6C, but not for 10C.

For Drgrieve, here is a video from a Jay Whitacre, a professor who really knows a bit about what goes on inside a LiFePO4 cell. A number of us here can share our experience with them, Jack and tell you about his too, but none of us are scientists or professors on the subject.


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## Ziggythewiz (May 16, 2010)

EVfun said:


> That is an incremental improvement that may be worth paying for.


Paying what? The new cells are the same price as the old.


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## EVfun (Mar 14, 2010)

"Paying for" in terms of consider replacement of perfectly working existing cells for more power. Still, by reports I've heard the CA cells are a little more than $1.21 per amp hour, which has been a common price for the SE cells.


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## Ziggythewiz (May 16, 2010)

Oh sure, I wouldn't replace a working pack for the upgrade unless the old is getting used in the wife/daughter's e-mobile, but your price isn't far off. Most people seem to be getting the greys through Jack, and should know they're paying a decent markup but seem happy to do so.


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## JRP3 (Mar 7, 2008)

EVfun said:


> Can you spare me the time of watching one of Jack's videos and just tell just where to look or what the results where?


Nope, you have to suffer through it  Actually I haven't yet seen it myself, just read it in the link I posted. Jack will reference it in his blog and post charts I'm sure eventually.


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## tomofreno (Mar 3, 2009)

JRP3 said:


> As if on cue
> 
> 
> 
> http://www.diyelectriccar.com/forums/showthread.php/calb-ca-p319149.html#post319149


 Yes, the CALBs seem to have a lower internal resistance compared to TS cells, but I thought you were saying they ran at different voltages compared to TS - something like the difference between lithium cobalt and LFP, but less. I think Gizmo and I are saying the TS and CALB are the same chemistry, LiFePO4, so operate at similar voltage levels, but yes there are some differences, such as internal resistance. There are also differences in internal resistance between different capacity CALB cells.

The battery stuff I've read recently makes me think that there aren't going to be any significant changes for quite a while, so performance and cost will likely float around about where they are now, with tweaks like lower internal resistance of the CALB gray cells. I don't much care about higher specific power, I want higher specific energy and lower cost.


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## PStechPaul (May 1, 2012)

I've heard the figure of $1.20 per Ah cited a lot recently for LiFePO4 cells, which works out to $0.375 per watt-hour. That is significantly less than the cost I found for Calb cells 40 A-h for $54 or $0.42/wH, but still in the ballpark. So what's the best source for these?


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## JRP3 (Mar 7, 2008)

Yes I agree, though the higher C rates of these cells make the prospect of messing with A123 pouch cells even less attractive, and not necessary for most applications. Also the lower internal resistance could lead to longer cycle life, and therefor lower cost per cycle.


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## Ziggythewiz (May 16, 2010)

EVfun said:


> Can you spare me the time of watching one of Jack's videos and just tell just where to look or what the results where?


Covered from 9:22 to 44:05. Basically new cells are good to at least 12C, and sag much less (17%) at 12C than the SEs did at 5.5C. Cell heating was minimal. ~ 7 deg F IIRC. 

Some great tests, especially if you have cells on order  

Grain of salt though, as these tests are 40Ah cells and being compared to tests on 180? The SEs had higher specs for small vs large, and I wouldn't be surprised if the same held for CAs. My other grain is that temp tests were done throughout, but these cells are sitting slightly spread on a workbench, not in a (possibly insulated) enclosed battery box.


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## DIYguy (Sep 18, 2008)

I have no doubt that the new CA cells have some significant improvements over the SE cells. I would like to make one point though, based on my ability to draw significant amounts of current. I've noticed that ambient temperature makes a significant difference on these cell's ability to give up current. I don't recall what the temp was when the test was done on the SE cells, but I do remember Jack mentioning 94F for the latest round. I doubt it would change any of the results in general terms. . . but it's worth mentioning. I can really tell the difference when my cells are warmed up vs a colder day. Voltage drop changes a lot. Point being, there are some differences in the testing parameters here. . . both in cell size (as previously noted) and perhaps in temperatures. . ???


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## JRP3 (Mar 7, 2008)

Certainly hotter cells have lower internal resistance and can put out more current with less sag, and yes Jack said it was hot in the shop. Don't know what the temperature was at the dyno during the initial testing but it probably wasn't as warm, so that is a mitigating factor. Jack does get a little sloppy in his testing when he sees a result that he likes.


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## EVfun (Mar 14, 2010)

EVfun said:


> I charge the pack to 3.5 vpc (133 volts for 38 cells.) The 32 cells that have been used for 3 years where all between 3.43 and 3.50 volts. Most of them where at 3.44 volts. The 6 added cells ranged from 3.61 and 3.99 volts at the peak during charge. These cells matched the pack in the spring.
> 
> I'm not so sure adding cells is a good idea, even when they come from the same batch. I may end up installing shut regulators to nudge the top in line.





EVfun said:


> It was easy to get the pack back in balance and it is surprising just how little it takes to run a cell from 3.5 volts to 3.9 volts. The 6 cells had 0.2 amp hour removed and now they finish with the rest of the pack. I had a total range from 3.46 to 3.53 volts. The 2 cells at 3.46 where both the added ones that had been finishing at around 3.8 volts. The 2 cells finishing at 3.53 where from the older 32 cells and have always been the highest ones at finish.


After the re-balance I checked the cells regularly during the last 15 minutes of the charge cycle 3 times over the following week. They stayed together and charged as a pack with no intervention. A few weeks have passed with lots of cycles so I decided to check again, this evening near the end of charge.

The 32 more used cells where between 3.47 volts and 3.52 volts, very nice. The 6 added cells, from the same batch but with about 1 less year of use, where now between 3.51 volts and 3.58 volts. Clearly, they are once again diverging.

???


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## GizmoEV (Nov 28, 2009)

EVfun said:


> The 32 more used cells where between 3.47 volts and 3.52 volts, very nice. The 6 added cells, from the same batch but with about 1 less year of use, where now between 3.51 volts and 3.58 volts. Clearly, they are once again diverging.
> 
> ???


I wonder what the mechanism is. Could it be an efficiency difference? Maybe the more used cells have a slightly lower efficiency and are heating more on charge and discharge. The difference in energy between the lowest cell at 3.47V and 3.58V is minimal. On the couple of tests I did on a Nov 2009 TS-LFP100AHA cell I got 0.15Ah and 0.17Ah between those two voltages.

I wonder if over time the differences will start to narrow a little.


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## JRP3 (Mar 7, 2008)

Possible theory: Some graphs have shown an initial gain in capacity of cells over the first 50 cycles. Maybe the cells with fewer cycles are still "breaking in" and have slightly higher internal resistance?


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## dtbaker (Jan 5, 2008)

EVfun said:


> The 32 more used cells where between 3.47 volts and 3.52 volts, very nice. The 6 added cells, from the same batch but with about 1 less year of use, where now between 3.51 volts and 3.58 volts. Clearly, they are once again diverging.
> 
> ???



perhaps just a tiny difference in resistance from the year of use?

regardless, I would suggest just knocking a little juice out of the 6 high ones, and check again in a month. say.... 30 seconds with a 50 watt resistor. Then do a charge cycle to see where they finish compared to older cells. If still high, drain more in 10 second shots. I found that toward the end tweaking, just 10 seconds with a 50 watt resistor made a difference in where a cell ends compared to others.


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## tomofreno (Mar 3, 2009)

EVfun said:


> The 32 more used cells where between 3.47 volts and 3.52 volts, very nice. The 6 added cells, from the same batch but with about 1 less year of use, where now between 3.51 volts and 3.58 volts. Clearly, they are once again diverging.???


 Yes, but that voltage difference on that part of the curve is not much difference in charge. Also, I've noticed that cells sometimes behave differently on this part of the curve than they do at lower voltages. Their charge/discharge curves may match fairly well at lower voltages but diverge at these higher voltages. They have very different resistance on this part of the curve, likely due to different mechanisms, than on the rest of the curve. You might track them with a cell log8 during charge/discharge to see how they compare over the rest of the curve. With that small amount of charge difference a shunt balancer would easily keep them in balance.


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## rwaudio (May 22, 2008)

JRP3 said:


> Possible theory: Some graphs have shown an initial gain in capacity of cells over the first 50 cycles. Maybe the cells with fewer cycles are still "breaking in" and have slightly higher internal resistance?


Just throwing this out there.... since the new cells may increase in capacity over the first X number of cycles how does that affect the SOC of a group of cells that are top/bottom balanced. If you have cells at the same SOC relative to todays capacity, but the capacity of some cells increases the SOC might change (or appear to change from your point of view from voltage measurements at end of charge), my first guess would be to say that they would decrease in voltage at the end due to the new larger capacity since they would no longer be "full" however as mentioned by others, if they have a lower equivalent series resistance now they could be charging just slightly more efficiently and the SOC might slowly be creeping up relative to the old cells which might be on the downward slope of decreasing capacity and increasing equivalent series resistance.

To also add to the side conversation in this thread about the performance of the new CA cells, with the help of another local EV'er, I'm going to be testing a 60Ah, 100Ah, and maybe even a 40Ah in series with my A123 pack to see what they can do relative to each other as well as the rest of the pack which is made up of 3P groupings of A123 20Ah pouches (measured at ~56.6Ah at 2C discharge)
The A123's do 1000A, the 100Ah CA should do 1000A easily, the 60Ah cell would likely be at it's limit close to 1000A, and the 40Ah cell... I just have no idea, it does 480A easily, but 1000???


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## Ziggythewiz (May 16, 2010)

rwaudio said:


> The A123's do 1000A, the 100Ah CA should do 1000A easily, the 60Ah cell would likely be at it's limit close to 1000A, and the 40Ah cell... I just have no idea, it does 480A easily, but 1000???


Sounds fun! I'm interested in the results.


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## tomofreno (Mar 3, 2009)

GizmoEV said:


> I wonder what the mechanism is. Could it be an efficiency difference? Maybe the more used cells have a slightly lower efficiency and are heating more on charge and discharge. The difference in energy between the lowest cell at 3.47V and 3.58V is minimal. On the couple of tests I did on a Nov 2009 TS-LFP100AHA cell I got 0.15Ah and 0.17Ah between those two voltages.
> 
> I wonder if over time the differences will start to narrow a little.


 I don't see how lower efficiency, say due to loss in some internal resistance (ir), can affect the amount of charge into the cell and its SOC. The charger just has to supply higher voltage to move the charge into the cells due to the voltage drop across this extra series resistance. More work is done, but the charge transferred in is the same whether the resistance is there or not. All that determines final SOC is beginning SOC, capacity, and the product of charging current and charging time.

I wouldn't infer much about relative cell ir from the exponential part of the charge curve since their behavior there is not necessarily representative of behavior over the rest of the charge/discharge curve. I have cells that have a lower slope on the rest of the curve, but a larger slope on that exponential part relative to some other cells.

It may be that capacity of the older cells has increased a bit and that is the cause for the difference, but I think it is very hard to tell from that data. I have had cells that were just a bit higher in voltage near end of charge like that, so I discharged 1/2Ah from them and then they become one of the lower cells. If they are well- balanced to start with, draining or adding just a small amount of charge will throw their voltages off that much on that part of the curve. I would keep an eye on them for further drift though.


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## EVfun (Mar 14, 2010)

I'm a bit perplexed myself. You are right on tomofreno, batteries store amp hours, not watt hours. That is to say, you move an electron around the outside and move a Lithium ion across the inside, and you only have so many Lithium atoms to move from the carbon to the Iron Phosphate. Since my cells are in a series string, and there is nothing tapping into to it, there should be no way for any cells to gain on the rest of them. There is about 1.35*10^24 Lithium atoms available to be moved between the anode and cathode (and the other way around) in a 60 amp hour cell. 

The problem is, something seems to be moving more Lithium ions across on the newer cells compared to the older cells. Either that, or the capacity of some of the cells are changing compared to others, by making either more Lithium atoms available to the older cells or fewer Lithium atoms available to the newer cells. 

In the first case, we either have a coulombic efficiency less than 100% on the older cells when charging, or less than 100% on the newer cells when discharging, or some slight self discharge. Generally, LiFePO4 cells behave very well in this regard, so able to avoid side reactions (unlike Lead acid), that they are pretty easy to charge in a series string without shunt regulators so long as you don't want the very top few percent (which isn't good for their lifespan anyway.) 

In the second case, something is strange because clearly the newer cells have just slightly more capacity, showing as a few hundredths of a volt at the end of a discharge test under a 0.2C load, a few thousandths of a volt with that load removed. I suppose the newer cells could be shrinking slowly, but since we are only talking a years worth of use difference between them and the older cells I don't see why they would age noticeably faster than the rest of the cells. All the cells are from a batch made in late February 2010, with the "older" ones about 200 cycles more used than the "newer" ones. 

Of course, there may be other theories out there about what has happened and they may be right. I'm not quite sure what is going on, but this creep is noticeable and increasingly undeniable.


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## tomofreno (Mar 3, 2009)

Couple things from Jay Whitacre's video:

If cells are overcharged you loose Li from the cathode to lithium carbonate deposition on the anode, but coatings can be added to the anode during manufacturing to prevent or reduce this.

Heat, 50 - 60 C, breaks down the electrolyte which coats the electrode with lithium carbonate and LiCFx species, causing a loss in capacity due to less volume for lithium ions in the films.

Wouldn't think your cells have seen such effects though.


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## GizmoEV (Nov 28, 2009)

tomofreno said:


> I don't see how lower efficiency, say due to loss in some internal resistance (ir), can affect the amount of charge into the cell and its SOC. The charger just has to supply higher voltage to move the charge into the cells due to the voltage drop across this extra series resistance. More work is done, but the charge transferred in is the same whether the resistance is there or not. All that determines final SOC is beginning SOC, capacity, and the product of charging current and charging time.





EVfun said:


> I'm a bit perplexed myself. You are right on tomofreno, batteries store amp hours, not watt hours.


So where does the heat come from on discharge or charge? Is it merely from the difference in voltage and not the current? Say two cells one with a higher resistance (not ESR) and one with a lower in the same string that since both will receive/give out the same current that the one with higher resistance will do it with a larger voltage difference across the terminals to compensate for the lost energy?


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## tomofreno (Mar 3, 2009)

GizmoEV said:


> So where does the heat come from on discharge or charge? Is it merely from the difference in voltage and not the current? Say two cells one with a higher resistance (not ESR) and one with a lower in the same string that since both will receive/give out the same current that the one with higher resistance will do it with a larger voltage difference across the terminals to compensate for the lost energy?


 Yes, the energy of each electron is the product of its charge and potential (voltage). If there is additional internal resistance (ir), the charger output voltage must be a bit larger to drive the same current into the cell at constant current (cc) operation. The product of that extra bit of voltage and the charge on the electron is the additional work done by the charger due to the voltage drop across the additional ir. If the cc setting is the same, then the charger has to put out a bit more power since the energy per charge per unit time is a bit larger. That extra bit of work done is Joule heating due to the resistance.

Similar Joule heating occurs during discharge, and there is an additional voltage drop across the cell terminals (sag) due to the additional ir. 

The charge into/out of each of the cells must be the same due to conservation of charge (or current continuity) unless there are sources or sinks internal in the cell like the ones mentioned by Whitacre. Such reactions likely do occur, just at much lower rates at somewhat lower cell temperatures, resulting in the long term loss of capacity. The higher the average temperature of operation, the greater the rate of loss of capacity I assume. I wouldn't think one year of operation at moderate temperatures would cause an amount of capacity loss outside the normal variability of capacity observed in a sample of new cells, so wouldn't explain evfun's observations, but that's nothing more than an uniformed guess.


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## EVfun (Mar 14, 2010)

EVfun said:


> After the re-balance I checked the cells regularly during the last 15 minutes of the charge cycle 3 times over the following week. They stayed together and charged as a pack with no intervention. A few weeks have passed with lots of cycles so I decided to check again, this evening near the end of charge.
> 
> The 32 more used cells where between 3.47 volts and 3.52 volts, very nice. The 6 added cells, from the same batch but with about 1 less year of use, where now between 3.51 volts and 3.58 volts. Clearly, they are once again diverging.


I have not intervened and the slow separation between the more used and the less used cells is increasing near the end of charge, when the charger is holding the pack to 133 volts. The 32 that have been in the buggy a year longer are between 3.47 and 3.51 volts. The 6 added cells are between 3.52 volts and 3.66 volts.

I have turned the current limit down for winter. The cool evenings are slowly taking their toll on internal pack temperature. The 38 cell pack was starting to dip a little below 105 volts at 360 amps (6C.) I've cut the peak current down to 300 amps (5C.)


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