# Recovering overdischarged LiFePO4 pack



## Guest (Sep 16, 2011)

Chances your cells will be fine. The ones below 1 volt may just bounce back. I have a bunch that have been sitting below 1 volt for a couple years and bounced right back. Bring up the low ones to 2.600 and bring down the others to 2.600 static. 2.600 or as near as possible to that. This is maximum bottom and you should set your controller to keep you above or at least 2.8 static that should then be considered your empty voltage. Static voltage is not the same as running voltage or running sag. This is classic bottom balancing. This is the safe way. When you leave your car sit you need to remove any draw from your pack. Any and all. No compromise. Unbalanced cells are bad. Top balanced are even worse. Bottom balance and keep them bottom balanced. If you find you have a dead cell then just remove it and replace it and balance it to the others.


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## drgrieve (Apr 14, 2011)

Is bottom balancing to 2.6 easy? I've heard that 2.85 is easier to bottom balance too. There wouldn't be much ah between 2.6 and 2.85?


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## Guest (Sep 16, 2011)

You can do 2.8 but I said based on what I have done. Balance them then charge them up and check them then drive with care and check along the way your cells to be sure you don't have any that are not holding a charge. Don't drive much then check then do it again until you are OK then charge and continue enjoying your EV. Turn off everything if you plan on letting it sit for more than a week and if you forget your cells will remain balanced on the bottom if you loose capacity again. The lower you balance the the more balanced they remain if you really drive them down but for reality 2.6 or 2.8 would be the level of choice. The further down your bottom balance is the more ragged the top becomes at charge. End your charge at 3.65 at the most. 

Pete  No need to go more as you won't gain but maybe a mile or three at best.


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## GizmoEV (Nov 28, 2009)

I concur with Pete. I charge to 3.485vpc and it works just fine. I'd recommend charging to no more than 3.5vpc if your charger tapers the current really low. In any case I wouldn't go over 3.65vpc.

You didn't, however, mention if you had a BMS installed. Some of those will really drain your pack if it is sitting for a while. Furthermore, they are unlikely to be exactly the same draw on each cell.


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## mora (Nov 11, 2009)

Do you have Winstons/Thunderskies? Charge them slowly to 4.00V and discharge at 1C. Watch terminal temperature and voltage. Measure internal resistance after draining off the surface charge and compare to previous values (if you have them recorded somewhere).

I've been told Winstons/Thunderskies like occasional charge to 4.00V / cell. You don't have to charge them to that point every time though. Mine revived from 0.01V/cell voltage levels. Ones at 0.00V/cell didn't revive though.


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## GizmoEV (Nov 28, 2009)

mora said:


> Do you have Winstons/Thunderskies? Charge them slowly to 4.00V and discharge at 1C. Watch terminal temperature and voltage.


Why discharge at such a high rate? How do you maintain that rate too?



mora said:


> I've been told Winstons/Thunderskies like occasional charge to 4.00V / cell. You don't have to charge them to that point every time though.


I have heard the same thing but never heard how they "like" it. What happens if they don't get charged that high? Both the theory and the practical reasons appear to be a secret. Also, why is this only for TS/WB cells?


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## Guest (Sep 16, 2011)

There has NEVER EVER EVER EVER been any official information about occasionally charging to 4v per cell. That is bogus information and we pretty much know where that came from. Long ago the original specs were to charge to 4.2 volts. Those have been revised since the early days but nothing ever stated that they like an occasional charge to 4v. It's just bogus bull shit.


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## mora (Nov 11, 2009)

GizmoEV said:


> Why discharge at such a high rate? How do you maintain that rate too?
> 
> I have heard the same thing but never heard how they "like" it. What happens if they don't get charged that high? Both the theory and the practical reasons appear to be a secret. Also, why is this only for TS/WB cells?


Topic starter was going to discharge at 3C. Now that's high. I wound some steel wire into shape of loose coil and submerged it into water. Then I simply attached wire ends to cell terminals and monitored current using clamp meter. From 4.00V to 2.5V amps went from 91 to 88 during the test. Wire used might play a role in that range though.

My information of occasional charging to 4.00V per cell came from engineer who has worked for TS. I also got my set of "bad" cells from him. He could say beforehand which cells wouldn't revive at all. I believe him as those cells didn't hold charge even if I tried to charge them multiple times. Apparently TS/Winston has a bit different chemistry than others.

I haven't heard of what happens if one doesn't charge to 4.00V once in a while either. I charged mine to 4.00V once and put them in my car. Been charging to 3.65V per cell since. I know TS/Winston recommends charging them to 4.00V before use but doesn't say they must always be charged to 4.00V per cell.


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## ev4me (May 10, 2011)

Thanks for the replies guys.

I like the idea of the bottom balance though I'm curious how quickly we can bring the low cells up to 2.6/2.8 V. Is 0.5 A like I said a good rate or can we safely go faster (or need to go slower)?

We have a kilowatt adjustable resistor along with several bench power supplies so implementation isn't really an issue; I just need to decide what charging/discharging current and voltages to use.

Glad to hear that most of the cells may be okay! 

Oh, and there is no BMS in this car, FYI. We're a nonprofit organization trying to address the lack of quantitative data about various EV design tradeoffs. BMS vs. no BMS is one of the most prominent ones; this is one of the no BMS cars. Not because we think that's better, but because we want to do it both ways and compare results.


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## GizmoEV (Nov 28, 2009)

I would charge them slowly since it doesn't take much charge to bring the voltage up when below 3V.

On the monitoring front, do you have a half pack voltage comparison circuit in place? I posted how I built mine on my blog. It is very simple to make and very inexpensive. Also, please publish your no BMS results. There isn't enough data on it. At this point I don't think they are required based on my 1 year of experience without using it's features. Now it is completely removed from my rig.


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## icec0o1 (Sep 3, 2009)

You can easilly bring them up but the ones that dropped under 2V would have an increased internal resistance and moreso, increased self discharge rates. It could be a very, very small difference but it'll mean that your pack will get unbalanced faster than a brand new one. 

As Gizmoto suggested, have a half pack voltage comparison warning and pay more attention to the cells.


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## dtbaker (Jan 5, 2008)

you may have already decided what you are doing..... but I know I would rewire pack in parallel, and re-balance. Top or bottom your choice. 

I happen to be a top-balancing non-BMS person, so I would wire in parallel and re-balance to 3.80 using my Mastech bench power supply and just let it sit until amps drop to zero.

The REASON I think top-balance is a better choice, without a BMS, is that it sets up the pack to hit the finish voltage as near to the same time as possible with your standard charger and not rely on a secondary BMS system or timer to shut down the charge when the first cell hits target.

my .02


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## ev4me (May 10, 2011)

Update for anyone who's interested:

We aren't having much luck here. After taking the pack out of the car we recorded the static voltage of each cell. They were really, really unbalanced. We had about thirty cells < 0.5 V, twenty cells 1 - 2 V, twenty cells 2 - 3 V, and thirty cells at the nominal 3.2 V. I didn't know a deep discharge would pull them this far apart.

We took the thirty worst ones (< 0.5 V) and wired them in parallel with a power supply. Not even 70 amps would push them up to 2.7 V, and after a while the voltage stopped rising. Some of the cells bulged and got really hot, seemingly just dissipating power like a resistor.

So for a few days we would identify a hot/bulging cell, remove it, then wait and see how the power supply responded. With each removal the voltage would rise a bit, and eventually we got to CV mode at 2.7 V. At one point we had the current down to 2.5 A at this voltage and left it overnight, but the next day the current was up to 20 A and another cells was hot.

After removing twenty of the thirty cells we gave up on this strategy and decided that maybe we had in fact managed to kill $2,000 of batteries. (Regrettably, they've never been treated very well.) At this point we are charging one cell at a time, all the way to 3.65 V, then CV mode for 24 hours. If the current doesn't taper off we mark the cell bad. Then we wait another 24 hours and see if the cell has held a static voltage; if not, we mark it bad. Then we load test and record the internal resistance. It's a slow process, but I don't see another way to feel confident about the condition of the cells, given the way they've been acting.

If we find more than a third of the cells to be bad we will probably just replace the pack. This was the first car we built, before we understood proper battery maintenance, and we haven't treated the batteries well.


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## Guest (Oct 19, 2011)

Don't parallel the cells. Charge up each cell, one at a time. A pain in the butt but in parallel they will act as one cell and if you have cells that are fuller than the others those will over charge during the charge process. Yes even in parallel. Bring up the low cells one at a time to 2.8 volts static and bring down the higher cells to 2.8 volts static. Put them in series and charge but watch your cells during the entire charge and have your charger only charge to like 3.65 or maybe even 3.5 volts per cell max and have it hold that voltage until the amps drop to like 5 amps then shut down the charge process. You must watch each cell and if any go out of bounds you need to remove them. Some will be, near the end of charge, above 3.5 volts or higher than 3.65 volts and some may even be less. If you have any cells in the 4 volt range static (sitting for 12 hours or more) then those cells need to be rebalanced or replaced. Do not put your badly out of balance cells in parallel. Done correctly and carefully you should still be fine.


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## dtbaker (Jan 5, 2008)

ev4me said:


> After removing twenty of the thirty cells we gave up on this strategy and decided that maybe we had in fact managed to kill $2,000 of batteries. (Regrettably, they've never been treated very well.) At this point we are charging one cell at a time, all the way to 3.65 V, then CV mode for 24 hours. If the current doesn't taper off we mark the cell bad.



what was your initial balance procedure?


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## EVfun (Mar 14, 2010)

Could someone please explain the mechanism whereby some cells in parallel could be overcharged before other cells in the parallel string are fully charged?


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## dtbaker (Jan 5, 2008)

EVfun said:


> Could someone please explain the mechanism whereby some cells in parallel could be overcharged before other cells in the parallel string are fully charged?



only way I can think of would be if they were charging at CA rather than CV.


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## Guest (Oct 19, 2011)

Easy, Each cell is made from multiple plates in parallel. Each plate needs to be in the same state of charge before being put in parallel. If not it would be like adding an empty cell and full cell together and then charging them. So the further out of balance your plates are the greater the chance you'd loose a plate. It can be shown with two cells were one has a greater SOC than the other. Do you want me to show you by destroying another cell? I can take a full cell and empty cell and parallel them and then charge the pair up and show you the results. Once the plates or cells are in balance with each other you can charge them safely. That will after all prove the point right? Why do you think the cells would be safe and not be ruined? I'd be happy to do this if it would make you a happy camper. If wrong the results will show the truth. I have no problem doing this. I have plenty to play with. 

Lets play.


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## icec0o1 (Sep 3, 2009)

gottdi said:


> Easy, Each cell is made from multiple plates in parallel. Each plate needs to be in the same state of charge before being put in parallel. If not it would be like adding an empty cell and full cell together and then charging them. So the further out of balance your plates are the greater the chance you'd loose a plate. It can be shown with two cells were one has a greater SOC than the other. Do you want me to show you by destroying another cell? I can take a full cell and empty cell and parallel them and then charge the pair up and show you the results. Once the plates or cells are in balance with each other you can charge them safely. That will after all prove the point right? Why do you think the cells would be safe and not be ruined? I'd be happy to do this if it would make you a happy camper. If wrong the results will show the truth. I have no problem doing this. I have plenty to play with.
> 
> Lets play.


If you have a charger which limits the voltage, you can't overcharge a lithium cell no matter how many you have in parallel and at what state of charge they are at. Think of voltage as pressure or potential. If a charger limits the voltage to 3.5V and one cell in a parallel string is at 3.5V, no energy will go into that cell as there's not enough pressure to make the electrons flow through. Feel free to try it out.


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## GizmoEV (Nov 28, 2009)

gottdi said:


> It can be shown with two cells were one has a greater SOC than the other. Do you want me to show you by destroying another cell? I can take a full cell and empty cell and parallel them and then charge the pair up and show you the results....If wrong the results will show the truth. I have no problem doing this. I have plenty to play with.


Then test this. So far my tests prove you wrong. Take a cell at 80-100% SOC and parallel it with a cell at 0%SOC. With a 40Ah TS cell with a 50A 50mV shunt in parallel you will see ~28A initial current. The full cell is draining into the empty cell. Your premise assumes this does not happen. The voltage of each cell will be forced to remain the same as every other cell when in parallel. That is, after all, the whole point. Now charge or discharge the cells in parallel until they are in the upper or lower knee of the voltage curve and leave them there for a while. They will all be at the same voltage and hence same SOC, either 100% or 0%.

Please go try it and prove my data wrong. You won't be able to. In fact, if you have different sizes of cells then parallel say a 40Ah with a 100Ah and do the same thing. It will work just fine.


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## Guest (Oct 20, 2011)

Test is in process. Video camera is on and our two cells are at start one is .9 volts and the other is 3.25 volts. One is totally empty and we begin our charge routine. What happens is we put in faster than the two cells can balance each other. With that the two cells will show on a volt meter the average of the two cells. Being an average the one cell that is at 3.25 will now show less than 3 volts. If the combined average reaches 3.6 volts the one full cell will actually be higher in voltage than the emptier battery. I will reach the capacity of the one cell faster than the other even though they are paralleled. 

If I first allow the time needed to balance the two cells to equal the same SOC then the problem does not exist because the cells or plates are now in balance which is what I stated you need from the beginning. You'd need to leave a pack of imbalanced cells paralleled for a long time to self balance. But once balanced your good to go. I am not saying they won't self balance but they won't do so quickly. 

Pete 

Video will be up later when the test is done.


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## MN Driver (Sep 29, 2009)

If you have a good connection between both cells, the terminal voltages should be identical. If they aren't there is a connection problem or wire that is too thin. One being at 3.25 and another at 3v? Are your connections or wires a bit hot? How much current is going from the empty one to the one that was higher right when you started(if you checked)? Your average business doesn't make sense if they are connected in parallel with a good solid connection, amperage flows to equal out voltage.

It may be possible to have slightly different SOCs however outside of their curves where a cell would take a certain amount of amperage to get to, say 3.4v and charging(but sucking more amps, also consider this is mostly an internal resistance thing) and a full cell is sitting happy at 3.4v because it had recently finished the charge before being connected. By the time its all at 3.6v and current tapered down the SOC on both should be full.

I agree with you about cells not self-balancing if in the middle of their SOC because the voltage difference when they are connected between the curves would be small enough to where they aren't going to draw much amperage to meet and end up from, say a 30% cell and a 70% cell, even though their voltages would be the same while connection. The knee of the curve is what would induce them to be completely full or empty.


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## Guest (Oct 20, 2011)

Took two more cells, one at 0.00 volts and the other at 3.28 or some where close. I have it on video. Connected a shunt between the two cells and connected them in parallel with an amp meter. The amp meter instantly jumped to 100 amps then within a few minutes was at 50 then a few more minutes at 25 and right now sitting around 10 amps. The full cell showing a lower voltage reading and the empty one showing a much higher reading. I am amazed at how fast the balancing really is. So with that the two cells that were imbalanced at the beginning of the charge cycle will more than likely be at or near the same SOC when I terminate the charge. I will let it sit over night before saying any more but what I saw in front of me and on video says that the cells if way out of balance will quickly begin balancing each other until they are nearly the same and as they get closer in balance the amperage will drop. At some point they will self balance. It is a much faster process than I had expected.

Pete 

Video will be shown. Not sure how late I will be up doing this but I will show the video. 

Show and tell is better than just I saying so. Because you can't refute the showing. I was shown that I was incorrect that the self balance will be slow. It is not slow at all.


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## JRP3 (Mar 7, 2008)

One thing to consider is a cell at 0 volts may be damaged and not actually behave properly.


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## GizmoEV (Nov 28, 2009)

gottdi said:


> What happens is we put in faster than the two cells can balance each other.


This won't happen if you are charging with a CCCV charge procedure because the charge current will taper at the end of charge so that the parallel cells will end up being full at the same time. If you actually can get the scenario you claim the fuller cell will be dumping into the lower cell anyway as it will have a voltage ABOVE the charge voltage.



> With that the two cells will show on a volt meter the average of the two cells. Being an average the one cell that is at 3.25 will now show less than 3 volts. If the combined average reaches 3.6 volts the one full cell will actually be higher in voltage than the emptier battery.


What current can actually do this without going over 3.6V? The current will be tapering back by this point and the lower SOC cell will be taking more of the current. Do you have a volt meter on each cell? With the standard straps and properly done connections you won't see a voltage difference beyond the mV range if at all. Early in the charging this won't matter, at the end the difference will be gone.



> I will reach the capacity of the one cell faster than the other even though they are paralleled.


This isn't a problem because since the two cell's terminals are held at the same voltage the fuller one will take less current or actually stop charging. That is the beauty of the LiFePO4 charge curve.



> If I first allow the time needed to balance the two cells to equal the same SOC then the problem does not exist because the cells or plates are now in balance which is what I stated you need from the beginning. You'd need to leave a pack of imbalanced cells paralleled for a long time to self balance. But once balanced your good to go. I am not saying they won't self balance but they won't do so quickly.


I don't know what balancing current you use but this isn't a problem because if you top balance you are going into the upper part of the knee of the voltage curve where the current will be tapered back. Any low cells will pull the voltage down below the upper part of the curve until it starts up. In fact, charging a parallel set is the fastest way to top balance a set of LiFePO4 cells. Much faster than doing each one individually. If you have a properly set charger it is really a set and forget type of process. Remember that no matter how out of balance the cells are the ending current is quite low. That is why it works.



> Video will be up later when the test is done.


Looking forward to it.


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## ev4me (May 10, 2011)

We're now charging the cells individually, one at a time, to 3.65 V. We intend to load test each cell. This project will last a few months due to the nature of the organization. We'll probably be doing one cell per day.

We spoke to someone from Manzanita Micro about the bulging. He said the solution for that is to use a clamp to squeeze them back in.  Okay...



gottdi said:


> With that the two cells will show on a volt meter the average of the two cells. Being an average the one cell that is at 3.25 will now show less than 3 volts. If the combined average reaches 3.6 volts the one full cell will actually be higher in voltage than the emptier battery. I will reach the capacity of the one cell faster than the other even though they are paralleled.


A single point (okay pair of points) in space can't have two different potentials. If you put a voltmeter on the battery terminals and it reads 3 volts...well...it is at 3 volts.



gottdi said:


> The amp meter instantly jumped to 100 amps then within a few minutes was at 50 then a few more minutes at 25 and right now sitting around 10 amps. The full cell showing a lower voltage reading and the empty one showing a much higher reading.


That's a resistive drop across the connecting wire. To make sure that isn't a problem we put the power supply sense wires on the lowest cell. Not that it matters at this point though, because we are charging individually.


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## JRP3 (Mar 7, 2008)

Yes people have squeezed cells back together, but I would assume some damage has occurred.


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## EVfun (Mar 14, 2010)

First off to icec0o1, I'm pretty sure you can overcharge a cell or any number of cells in parallel with just a 3.5 volt regulated power supply. You can apply that voltage to the point where the current is to low. If you leave a cell on a 3.5 volt supply I don't think it will last very long. At least that is true if I am to believe the manufacturers. At that voltage the current and the end of charge would be quite low, but I can only guess the exact number based on a few manufacturer data points for "full charge." The manufacturers are quite clear that the cells can't be left floating at 3.500 volts.

Now, as far as overcharging some cells in parallel *before* others, I'm not quite sure how. Each cell in that string is basically a series of smaller cells (the many separate plates within it) already in parallel and we don't seem to have issues with cells being partially overcharged and partially undercharged. In fact, LiFePO4 cells seem to parallel quite well, behaving like a larger cell. It would seem that each "more full" plate simply doesn't take the electrons from the charge because overcharge is a slightly harder to push than full (that rapid rising end of charge voltage curve, or rapidly declining end of charge current when using a fixed voltage.) 

Please Pete, I would like you to outline the method of causing this in detail. I'm one of these picky people so you won't see the positive and negative charge wires come off the same end of a parallel string when I'm charging in parallel. I work to count the milliohms, because they are the millivolts at the end of charge. Perhaps my idea of opposite end wiring a parallel string (+ and - wires come off opposite ends) is enough to prevent this?


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## ev4me (May 10, 2011)

EVfun said:


> First off to icec0o1, I'm pretty sure you can overcharge a cell or any number of cells in parallel with just a 3.5 volt regulated power supply. You can apply that voltage to the point where the current is to low. If you leave a cell on a 3.5 volt supply I don't think it will last very long. At least that is true if I am to believe the manufacturers. At that voltage the current and the end of charge would be quite low, but I can only guess the exact number based on a few manufacturer data points for "full charge." The manufacturers are quite clear that the cells can't be left floating at 3.500 volts.


Wait, what? You're saying that if we put a battery on a supply at 3.65 V, constant voltage mode, then come back the next day, it will be ruined? Won't the current taper off to zero once the charge is done?


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## JRP3 (Mar 7, 2008)

Remember a LiFePO4 cell is fully charged at 3.45V or so. Continuing charging above that will eventually over charge it.


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## Guest (Oct 20, 2011)

JRP3 said:


> Yes people have squeezed cells back together, but I would assume some damage has occurred.


Warm the cell first to soften the plastic first then squeeze and hold and while holding let cool. Should do fine. Some damage maybe.


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## Guest (Oct 20, 2011)

In a nut shell at this time the cells DO self balance FAST. Amazingly fast. I assumed wrong that that process would be much much slower to the tune of days if not weeks. I was so wrong. My two cells where one was nearly full and the other was at 0 volts are now sitting happy at 2.89 volts. Both of them. Seems like that process is quite quick. I will separate the cells and let them sit for a few hours then recheck the voltages. If they remain the same I will then charge them up in series to see how much each one takes. I can dump in up to 20 amps charge current if needed. The two cells that were out of balance at the beginning of the charge and in parallel are now sitting pretty much the same voltage too. So if you put your badly out of balance cells together your cells will pretty much balance out within 24 hours or less. The further out of balance the cells before placing in parallel the faster and at higher amperage they will balance. Im duly impressed with the cells ability to self balance FAST when placed in parallel. I never knew how fast. Placing the shunt and amp meter in line tells the story. No arguing that point any longer. I will have video posted later. It won't be until tomorrow. Pretty simple to check yourself if you have a shunt and amp meter. 

Pete


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## Guest (Oct 20, 2011)

JRP3 said:


> Remember a LiFePO4 cell is fully charged at 3.45V or so. Continuing charging above that will eventually over charge it.


Are you stating charging voltage or static voltage? If your at static voltage then your over charged at 3.45 volts. While charging it won't hurt if you hold at 3.65. Holding at 3.5 is better since there is little power from 3.45 to 3.65. I'd not go above 3.65. I know every one is lowering the top voltage but my charger is set for 3.65 volts and to hold there where it does that just fine. 

Pete


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## EVfun (Mar 14, 2010)

The manufacturers are saying that the cells cannot be floated above some quite low voltage (like 3.4 volts.) I highly doubt the "next day" would be an issue, though if the "next day" had been going on day after day for another week after the current had dropped to near zero ("near" a moving target based on how many amp hours are in parallel) it would end up damaging. 

It seems to be a "fill a container thing." Overfull is not desirable and ages the cells quicker. Under full isn't taking full advantage of the full capacity, but harmless.



ev4me said:


> Wait, what? You're saying that if we put a battery on a supply at 3.65 V, constant voltage mode, then come back the next day, it will be ruined? Won't the current taper off to zero once the charge is done?


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## MN Driver (Sep 29, 2009)

"My two cells where one was nearly full and the other was at 0 volts are now sitting happy at 2.89 volts. Both of them. Seems like that process is quite quick." 2.89 volts is a going to be a very low SOC though, you've got two cells that are still very empty. If you had a cell that was completely full and connected it to a 2.8 volt cell you'd get tons of current until their voltage stabilized, the initial current could make the wires quite hot depending on how thick the wires are, the internal resistance of the cells, and the capacity of the cells. You'd want to lose the shunt in a test like that and go hall effect though as that shunt is added resistance which induces its own voltage drop by design, which is more significant when going between just two cells.


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## Guest (Oct 20, 2011)

MN Driver said:


> "My two cells where one was nearly full and the other was at 0 volts are now sitting happy at 2.89 volts. Both of them. Seems like that process is quite quick." 2.89 volts is a going to be a very low SOC though, you've got two cells that are still very empty. If you had a cell that was completely full and connected it to a 2.8 volt cell you'd get tons of current until their voltage stabilized, the initial current could make the wires quite hot depending on how thick the wires are, the internal resistance of the cells, and the capacity of the cells.


Wires did get warm while at 100 amps. But since it did not stay at 100 amps that did not last long at all. The cells was nearly full. I intend to redo this one tomorrow morning with a different set of cells. One that will be full and one nearly empty. I will have the camera on the amp meter from the start so you can see. I will leave it that way until the cells balance pretty much. You will see it will happen fast. It will balance FAST. So because the one cell is totally empty and the other full the combined after balancing will not be full at all. Draining a single cell at 100 amps is pretty impressive. Its a cool process to watch. Better than watching paint dry. So if one is full and the other is empty I'd expect to see a 50% SOC when done. That is plausible to assume with this information.


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## JRP3 (Mar 7, 2008)

gottdi said:


> Are you stating charging voltage or static voltage? If your at static voltage then your over charged at 3.45 volts. While charging it won't hurt if you hold at 3.65. Holding at 3.5 is better since there is little power from 3.45 to 3.65. I'd not go above 3.65. I know every one is lowering the top voltage but my charger is set for 3.65 volts and to hold there where it does that just fine.
> 
> Pete


I was talking static voltage and meant 3.4V. Point being that once the cell is actually full and resting at 3.4 volts holding it at higher voltage will eventually overcharge it, at 3.6 or even 3.5V.


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## Guest (Oct 20, 2011)

My charger holds my cells to 3.65 volts per cell or 3.65 x number of cells. That is the voltage it holds at. So for my pack 138.7 volts until the amperage reaches 2 amps then terminates the charge and after resting my cells are all at around 3.35 volts per cell. Which is exactly dead on. None of my cells have ever been at 3.4 volts resting voltage. None. Maybe before I got them but not since I got them. I won't charge over 3.65. Heck I even had a cell at 4 volts charging but after resting dropped to nearly 3.38 volts. I charged the cell to 4 volts but did not let the current taper off. I stopped the charge at 4 volts. No damage. Not over full.


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## JRP3 (Mar 7, 2008)

Terminating the charge at 2 amps is not holding the voltage. If you actually continued to hold the voltage it would eventually overcharge the cell at some point.


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## dtbaker (Jan 5, 2008)

gottdi said:


> Easy, Each cell is made from multiple plates in parallel. Each plate needs to be in the same state of charge before being put in parallel.



huh?

if multiple cells are put in parallel with a constant voltage applied, it doesn't matter what the relative state of charge is, they will all be brought up to that voltage (assuming it is within range the cells can attain). Only problem is if a cell is damaged or shorted internally, it may not be able to attain the set voltage, and act more like a resistor and heat the cell rather than charge it.

assuming cells are not physically damaged this is the definition of top-balancing. It doesn't matter what the initial charge is on the cells. you put them in parallel, set power supply to something like 3.70v, and let it go until amps go to zero.

if you suspect cells MAY be damaged, it would probably be safer to do a first pass one at a time. THEN put the survivors in parallel for a final balance charge.


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## dtbaker (Jan 5, 2008)

ev4me said:


> Wait, what? You're saying that if we put a battery on a supply at 3.65 V, constant voltage mode, then come back the next day, it will be ruined? Won't the current taper off to zero once the charge is done?


the current tapers off to zero, and the cell will be fine unless it was damaged before you started.


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> you put them in parallel, set power supply to something like 3.70v, and let it go until amps go to zero.


Do NOT let the current go to zero if the voltage is above 3.4V! The charge PROCEDURE is to charge to the target voltage, ie 3.65V, and hold until the current drops to 0.05C and STOP. If the rest voltage is over 3.35-3.40V then the cell is over charged.

Due to various constraints in my rig I charge to a low current but I also charge to a lower voltage: 3.465vpc. The rest voltage is 3.34vpc so I know I'm not over charging them.

If you are going to put a cell on a CCCV bench powersupply and walk away set the target voltage to 3.4V and you won't have to worry about it. Come back in a week and it will still be fine.


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## JRP3 (Mar 7, 2008)

Exactly. The lower you taper the current at a specific voltage above 3.4V the more likely you are to overcharge and damage a cell. In fact to keep my pack at a lower SOC most of the time I simply charge at constant current at 20 amps till my cells get around 3.42 volts average and just stop there, no constant voltage stage at all. This keeps me around 90% SOC max, depending on temperature.


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## dtbaker (Jan 5, 2008)

I am not suggesting to set the power supply at 3.70 and leave it for weeks. And I should re-phrase 'amps drop to zero' as perhaps not 0.00, but something like .01 or .02 C.

What I AM suggesting is that for an initial top-balance charge you should set the initial target of the power supply to charge the cells (either individually, or all in parallel) to a *little* above your planned normal charger CACV target vpc to be assured that when you put the pack in serial the cells are likely to hit the target vpc all at very nearly the same time.

I am ALSO suggesting that the initial balance voltage should be at least a little way up the knee, , a little farther up than you plan to charge normally, so that the charger is more likely to accurately pick up the rising voltage and cut to CV when nearly full...


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## Guest (Oct 21, 2011)

A quick update. Video is as slow as paint drying for sure. Here is what we did today. I took two cells, one empty and one full. I know the one was full because we charged it to 3.65 volts and then held that voltage until we reached 3.6 volts and 4 amps. Took some time but we did get the cell to charge no problem. The other cell was at 0.01 volts and that was at that voltage for at least 2 plus years. I then paralleled the cells together and had an amp meter between them to show the initial amperage from the full cell to the empty cell. It jumped straight up to 150 amps then within a few minutes it was at 50 amps then at about 1/2 hour it was at 25 and slowly dropping. The voltage of the cells behaved as expected. The full cell voltage dropped while the empty cell voltage rose. After about 4 1/2 hours later the amperage was no longer registering on the meter that we could actually measure. The voltage of the cells were sitting at equal voltages and all seemed just fine. But upon closer inspection when we took the cells apart to check voltages the low cell was actually still low. The voltage did rise considerably but it was still lower than the one that started out full. So there was a decent transfer from one cell to the other but neither were balanced yet. I decided to charge the cells in parallel at that time. I now have the cells on the charger and charging at 20 amps but the cell voltage is still below 3.4 volts for each of the cells and 3.42 volts from the charger terminals. So the low cell is still showing it is the low cell while the high cell is still showing it's the high one. They still remain after 2 1/2 hours of charging the same out of balance voltages. Now that the cell is charging a single 200 ah cell instead of a single 100 ah cell the charging process has slowed by twice as much. It is creeping along in parallel. How long will it take to charge this so both cells have balanced and become as one in voltage and SOC. Who knows. But it is as super slow process. Not that the theory is bad but is it going to actually be a better process to parallel the cells then charge them in parallel rather than to just drain them one at a time and then charge them in series. I do not see this at this time being a time saving or easy method of making them all the same. They may look the same while connected in parallel but they will not be unless ample time has elapsed. I can drain the cells at a constant amperage so no waiting a long time for cells to self balance before charging.

Question is how long do you have to wait at such low amperage transfer before you know that the cells are all in parallel? You don't unless you have proper and very sensitive equipment. Many don't. So at this time the process of putting things in parallel is rather slow and very very time consuming over just using a motor to drain your cells and just be there for them so you don't destroy them until they are all at 2.6 volts static. Then charge them up in series and be happy with the bottom balanced cells and don't take the pack above the pack average of 3.65 volts and hold it at 3.65 or less if you like then taper the charge to like 2 amps or maybe even as high as 5 amps or what ever. 

I do know by watching the process that from 0 volts to 2.4 volts only takes a minute. Really. There is no real usable voltage below 2.4 volts on my cells. Minutes at best if in a string. So I bring my cells to 2.8 volts as my target low voltage for bottom balancing.

I will show video but the process is taking a long time and the video is large. I can cut much out but I am not done yet with video clips. 6 some odd Gigs worth of raw video so far. Not an easy thing to cut and edit for a decent flick. But I will. 

Pete


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## Guest (Oct 21, 2011)

So in a nut shell. Take each cell to the low cut off of like 2.6 or 2.8 volts static. Then put them in series and charge keeping an eye on all of them. When your getting close to the hold voltage of the pack keep a good eye on your cells. No cell should go over 4 volts during charging. Don't worry if the voltages are ragged at the top but you don't want them to be mega ragged. 

This is like I said in the beginning. I stand by that wholly. 

Pete


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## EVfun (Mar 14, 2010)

I would be a little concerned about leaving those cells unattended - can you place them some distance from your house? A cell discharged to a very low voltage may not recover properly, if I'm to believe the published experts. 

I did the parallel balance effort with 2 parallel cells that where close in SOC. I used a pair of 6 inch long 16 gauge jumpers and left them in place for at least 2 weeks on 2 cells that where at 3.28 and 3.30. After separating them and waiting overnight I had 1 cell at 3.29 and the other at 3.30 volts. I could measure current flow on the wires paralleling them down to about 50 milliamps (0.1 millivolt.) The measurable amp flow must have moved about 10 amp hours, and then at least another week left in parallel after I couldn't measure current flow. It still didn't completely balance the cells. Many users have proven that paralleled cells work well together so it must sort itself out effectively at both full and empty. 

Your efforts with very low cells is interesting. Have any of the cells below 1 volt ended up not working properly after being charged?


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## Guest (Oct 21, 2011)

The cells are well away from any structure that could burn just in case the worst were to happen. So far no issues at all with the cells that had been below 1 volt for those amount of years. They just jump right back up. Jack Rickard tested two for me. One that was never allowed to go that low and the other was sent at a level of like 0.56 volts. The one that was low still held full or nearly full ah when charged and discharged at 1C. The cells are rated for .5c so he charged and discharged them at twice the recommended amperage. Pretty impressive little magic boxes if I do say. 

So you let your sit for weeks? Ouch. Way to long for me. I'll just use the manual method of balancing and charging and only parallel cells once they are balanced. 

Pete


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## dtbaker (Jan 5, 2008)

gottdi said:


> ....I know the one was full because we charged it to 3.65 volts and then held that voltage until we reached 3.6 volts and 4 amps.



I would argue that this is not 'full' at all. Thus, not 'balanced'.

My whole series pack (120v nominal) pulls .15amps from the wall when it is 'full' according to the kill-a-watt meter which i think is just feedng the dc-dc loss.

I would say that with a kill-awatt meter ($30-ish) and a bench power supply ($150 ISH) you can charge a single or set of cells in parallel until wall amps drop to .15-ish at a CV=3.6 or 3.7... and consider that 'done'.


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## EVfun (Mar 14, 2010)

Yea, I left them sitting in parallel for at least 2 weeks. I think it was 16 days because I remember it hitting the 2 week target and then ignoring it for a bit. Together the cells read 3.30 volts, and when I first disconnected them I still got 3.30v each. The next day there where reading 3.29 and 3.30 volts. Clearly some moved, but it didn't accomplish an equalization. I wanted to find that out because I'm not comfortable leaving cells in parallel on a charger for days in my garage.

Since a number of people have proven they work well together in parallel this must sort itself out every full charge. If paralleled to equalize it will take a long time to get the voltage up or down. 36, 180 amp hour cells in parallel trying to initially balance is mind boggling. That is a 6480 amp hour cell! Even 100 amps is just 0.015C. People have been sneaking up on this with 10 amps. If someone started with cells at a 50% SOC, similar to how they are shipped, it would take almost 2 weeks to reach empty or full at 10 amps.


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## Guest (Oct 21, 2011)

Argue away but it is full enough to be considered full. Depends upon your version of FULL. Some argue with higher voltages and lower amps. May controller holds the average pack voltage of 3.65 v per cell then current limits until it drops to 2 amps. That is my FULL. The cell here is full. I can assure you that the amount of AH between what I stopped at and what I would have had if I had allowed it to go to 3.65 volts and 2 amps is nearly nothing. I just stopped there because of time constraints. Jeeze. I am so amazed at how much folks argue. No one else here is doing this on video so you don't get to argue.


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## Guest (Oct 21, 2011)

EVfun said:


> Yea, I left them sitting in parallel for at least 2 weeks. I think it was 16 days because I remember it hitting the 2 week target and then ignoring it for a bit. Together the cells read 3.30 volts, and when I first disconnected them I still got 3.30v each. The next day there where reading 3.29 and 3.30 volts. Clearly some moved, but it didn't accomplish an equalization. I wanted to find that out because I'm not comfortable leaving cells in parallel on a charger for days in my garage.
> 
> Since a number of people have proven they work well together in parallel this must sort itself out every full charge. If paralleled to equalize it will take a long time to get the voltage up or down. 36, 180 amp hour cells in parallel trying to initially balance is mind boggling. That is a 6480 amp hour cell! Even 100 amps is just 0.015C. People have been sneaking up on this with 10 amps. If someone started with cells at a 50% SOC, similar to how they are shipped, it would take almost 2 weeks to reach empty or full at 10 amps.



How many cells in parallel are you talking about here? I have my technique and it is faster than 2 weeks. I can bottom balance my pack in a few days one at a time without doing the mega slow parallel technique. Sorry I have no desire to charge a cell that is 6480 AH in size. I just don't have the equipment to do that. Don't have the time either.


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## dtbaker (Jan 5, 2008)

EVfun said:


> Yea, I left them sitting in parallel for at least 2 weeks. I think it was 16 days because I remember it hitting the 2 week target and then ignoring it for a bit. Together the cells read 3.30 volts, ... it would take almost 2 weeks to reach empty or full at 10 amps.


yes.


at 3.3-3.5 vpc my hypothesis is that you have no clue where you actually are w.r.t the cell atual capacity because the voltage curve is still so flat there, especially if still drawing significant amps in charge. my hypothesis is that you need to be further (a little above your planned daily ca->cv trigger) in initial balance where cells are 'full' at less than .02C (.15 amps for a 100 ah cell) to be confident that they will reach the trigger voltage close to the same point at the end of daily charge.

this is a central point to deciding whether you want to trust a charger to stop on a daily basis without BMS cell level monitoring when the first cell hits them limit. Otherwise (without cell -level BMS, and without top-balancing) how do you end the charge?

bottom balancing may protoect the occasion when you run the pack into the ground, but does not protect against overcharge in the daily charge cycle without cell level monitor/control. My whole arguement is that a good top balance eliminates the need for BMS to manage end-of-charge.


....but we're drifting rapidly away from OP in recovering undamaged cells from an over-discharged pack

I will maintian that if a cell can accept a charge to 3.7 and drop amps to .15, then it is still 'good'. If not, then it is questionable for sure.


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## JRP3 (Mar 7, 2008)

What cells are you talking about? With CALB cells 3.5 is well into the steep part of the curve, don't know about TS.


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## Guest (Oct 21, 2011)

3.55 volts to 3.65 volts at 20 amps took a grand total of 10 minutes. I am now holding 3.65 volts and starting to limit the current.


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## dtbaker (Jan 5, 2008)

JRP3 said:


> What cells are you talking about? With CALB cells 3.5 is well into the steep part of the curve, don't know about TS.


with TS, the mid-curve is 3.7 or so according to published curves....


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## Guest (Oct 21, 2011)

The old Hi-Power cells have a max voltage of 3.8 volts per cell. So going to 3.65 and holding that until even 4 amps is plenty full. Well into the curve. Bring them to 3.4 and hold and you will still be in the curve. I am at the upper limits. Plenty full for the tests. I could go up to 3.8 volts per cell then hold that until I reached 1 amp. I could. Won't yield much but I could.


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## EVfun (Mar 14, 2010)

I have tested with 8 cells and some shallow cycling (+10 amp hour one day, -10 amp hours the next day, repeat for 5 days and then leave sitting for 2 days.) That didn't force the 8 cells to agree, with a couple still down by 0.01 volt from the rest. One of those lower cells was discharged to 3.28 volts and the other cell charged to 3.31 (but fell back to 3.30 overnight before I paralleled the cells.) Then those cells just sat together for a good 2 weeks. That didn't work either. I was hoping the passive equalizing would be practical because it couldn't be easier. 

My cells are all 60 amp hour cells, but I know 180 amp hour cells are very commonly used (and 36 is a pretty low voltage pack for most EVs.) I was thinking out loud about just how large this "cell" could be for most EV packs. 

I agree though, this massive parallel charging doesn't seem practical with typical garage equipment. I suppose bottom balancing that way would be fine as I haven't read any information suggesting that very slow discharge rates are in any way harmful. 

My way of top balancing is pretty quick. I throw a BMS modules on each of the cells and hook them in series. I then charge quickly until the first cell comes up. Then I roll the current back to keep any cell from exceeding 3.70 volts. When lots of cells are getting close I cut back to 1/2 amp. Since the regs can bypass 1/2 amp they will just sit there not exceeding 3.65 volts until they all get there. As long as they started out pretty close it is done in an evening. If they where way out of whack I would need a few more old 6v headlamps for loads to speed things up.


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## ev4me (May 10, 2011)

We're now charging individual cells at 3.45 V, constant voltage, for 24 hours. So far all of them have tapered down to near-zero current (~100 mA) at this voltage and read 3.35 V after removal. We allow the current to reach up to 55 A during the constant current stage (if there is one). We've also begun clamping the cells during the charge process to prevent bulging.

I'm working on a computer program to record the voltage and current vs. time curves during the charging, along with the results of subsequent load tests. Maybe we can quantitatively see the difference between how a "good cell" charges and how a "bad cell" charges.

Question: is an internal resistance measurement, taken by measuring voltage and current at two different loads, sufficient to prove that a cell is okay?


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## Guest (Oct 21, 2011)

Charge the cell and count the AH in. Discharge the cell and count the AH out. If you have 100 AH cells you should get out 100 or maybe a bit more. If you do then your cells are fine. That is how you know if your cells are still holding the rate AH. If your looking to see if one has some tiny weeny amount of damage internally I doubt you'll ever know until the end of life which can be a very very long time. If you get 80 Ah out of your cells and they are nearly new then you have damage. If you get 100 AH now and 80 AH in 10 years then your cells are fine. If you get more than 80 AH after 10 years the cells are magic. Discharge and charge according to the manufacturer specs.


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## EVfun (Mar 14, 2010)

dtbaker said:


> with TS, the mid-curve is 3.7 or so according to published curves....


I think TS cells behave like the rest. The voltage starts taking off around 3.45 volts. I charge to 3.5 volts at 12 amps (0.2C) and the current will drop from 12 amps to 3.6 amps in 40 minutes. If I continue charging for another 40 minutes the current drops from 3.6 to 1.2 amps. Let me go grab the note book...

In the first 40 minutes, from 12 amps down to 3.6 amps put 4.4 amp hours into the pack, a little over 7% of the cell capacity. The next 40 minutes, from 3.6 to 1.2 amps put 1.4 amp hours into the pack, about 2% of capacity. I usually stop charging after the first 40 minutes, but ran the second 40 minute window to see what capacity I was giving up.


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## Guest (Oct 21, 2011)

6 hours and counting for the charge cycle of one buddy pair of 100 AH cells.


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## GizmoEV (Nov 28, 2009)

Wow, leave for a short while and the thread grows like a wild fire. 



gottdi said:


> This is like I said in the beginning. I stand by that wholly.


Are you referring to paralleling them won't work or will damage a cell?

If you aren't taking paralleled cells into either the upper or lower knee of the voltage curve they won't balance very rapidly. They don't take too long to balance if you take them into either end.

I top balanced my TS-LFP100AHA cells 20 at a time in parallel because I had two supplies to charge them. It was a set and forget situation. They were balanced in a couple days.

Someone in Florida bottom balanced 20 of his 180Ah CALB cells in parallel by connecting several 3-foot lengths of 1/16" SS welding rod to the string. Just a simple loop worked great and drew about 9A each. Add however many you want but be around to start removing them when nearing the end of the balance process. It took him less than a full day to balance his pack this way. Discharge to 2.8V rest voltage and the pack will be balanced. This is far enough into the knee that any difference in SOC of the cells will be a relatively large voltage difference so the pack will balance automatically. Again, it doesn't matter how out of balance the pack was to begin with except that I wouldn't trust Pete's below 1V cells to behave "normally".

Again, parallel two way out of balance cells. Charge to 3.6V terminal voltage and hold that until the current tapers to 0.05C and the cells will be balanced and fully charged without over charging either cell. Just putting the cells in parallel and leaving them alone will not balance them sufficiently. I never said it would.


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## GizmoEV (Nov 28, 2009)

gottdi said:


> 6 hours and counting for the charge cycle of one buddy pair of 100 AH cells.


What is your charging current?


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## Guest (Oct 21, 2011)

20 amps until I reached 3.65 volts then hold at that until I drop to 2 amps. I was thinking of just going to 3.6 volts but decided to go 3.65. I am near the 4 amp point right now.

Pete 

I can't charge above that current as the charger I am using won't allow it. 20 amp max but I can go all the way up to 150 volts and 20 amps. Circuit breaker cuts out above 20 amps. 20 amps is the limit of the charger. Circuit breaker is working as it should. Perfect manual charger.


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## Guest (Oct 21, 2011)

I am going to shut down the process in an hour. Gotta sleep. I will pull the cells apart so they can't self balance any more. I will check voltages in the morning and continue to fill up after I get home from work. I will then take some more video shots of paint drying. It's 10:30 now my time. 

Pete


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## GizmoEV (Nov 28, 2009)

gottdi said:


> 20 amps until I reached 3.65 volts then hold at that until I drop to 2 amps. I was thinking of just going to 3.6 volts but decided to go 3.65. I am near the 4 amp point right now.


They should be full now since 200*0.05=10 or is the capacity bigger than 200ah.



gottdi said:


> I am going to shut down the process in an hour. Gotta sleep. I will pull the cells apart so they can't self balance any more. I will check voltages in the morning and continue to fill up after I get home from work. I will then take some more video shots of paint drying. It's 10:30 now my time.


Make sure you check individual voltages before paralleling. I think that is what you meant. I think they are both full going by the low current but then if you are trying to simulate a top balance it is probably best to overcharge a bit? If both resting voltages are above 3.4V tomorrow then they are both at or over 100%SOC.

BTW, what charger do you have? I'd like one that can charge at a higher rate than 3.5A and go to a higher voltage than 35V.


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## EVfun (Mar 14, 2010)

You are planning on charging at 3.65 volts to 0.01C? (a 200 amp hour "cell") You are already down to 0.02C at 4 amps so I expect the cells are full. I wonder if they need to be left in parallel as they decline from the charging voltage to the full charge resting voltage (3.34v) in order for the balance to complete.


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## Guest (Oct 21, 2011)

GizmoEV said:


> Wow, leave for a short while and the thread grows like a wild fire.
> 
> 
> 
> ...



Referring to it takes just to damn long to charge in parallel. If it takes this long with two I can't imagine with 48 of them. Can't imagine 48 180 Ah cells either in parallel. One cell 8,640 AH in size would take weeks to charge at 20 amps. I am talking about most people and I guarantee most folks don't have chargers to do one cell at a time let alone at higher than 20 amps. So for your one person doing a bottom balance in one day by self discharging in parallel then the answer is it did not happen. They are not self balanced in one day. If I can't do it in one day then the whole pack will take longer. Maybe a week or two. But like I said that is WAY too long of time to deal with this. Connect a cell to a motor and bring the voltage down to a specific level and let it sit to stabilize then bring it down more and continue until you reach your desired voltage static. 

By the way. My below 1 volt cells do just fine. They were in my MG for some time and never once did I have a problem. They always charged normally and held full capacity. The cells were drained by a faulty BMS system but at a very low amperage. The BMS was a parasitic load and drained two hole packs to zero. The cells have been tested by more than just me. They are fine. 

So until others start to do testing too I will stand by my results. Don't buy it if you don't want and find some friggin thing to bitch about again, that is fine too. But at least I am doing some testing. Every one that does not do testing has something to say to those that do. It happens all the time. If you don't believed the results and I can assure you the results are reproducible. When I pull the cells tonight I will let them sit and stabilize until tomorrow then I will continue. It will take time to process the video clips too. over 6 gigs so far of raw footage of paint drying. 

What do you have?


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## Guest (Oct 21, 2011)

GizmoEV said:


> They should be full now since 200*0.05=10 or is the capacity bigger than 200ah.
> 
> 
> 
> ...


Nope, not charged fully yet. 

http://greenev.zapto.org/electricvw/Electric_VW/The_Albums/Pages/Fair_Radio_Military_Charger.html


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## Guest (Oct 21, 2011)

I am sure the total capacity is going to be over 200 AH total combined. The cells are not bad cells.


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## GizmoEV (Nov 28, 2009)

gottdi said:


> Referring to it takes just to damn long to charge in parallel.


Ok.



> So for your one person doing a bottom balance in one day by self discharging in parallel then the answer is it did not happen. They are not self balanced in one day. If I can't do it in one day then the whole pack will take longer. Maybe a week or two.


Is that because he didn't have a video camera? One day, as in less than 24 hours, was done. 180Ah cells even if full can be discharged in 20 hours at a 9A current. He connected the cells in series and then hooked up the loops of SS wire to each cell since that was a convenient connection point. He had 20 loops each drawing over 9A for a total of 180A on a 3600Ah pack. Once the resting voltage is below 2.8V the pack will be balanced. He did it and his results confirm it worked.




> By the way. My below 1 volt cells do just fine. They were in my MG for some time and never once did I have a problem. They always charged normally and held full capacity. The cells were drained by a faulty BMS system but at a very low amperage. The BMS was a parasitic load and drained two hole packs to zero. The cells have been tested by more than just me. They are fine.


Great. I'm glad. I just wouldn't trust them without testing them individually first.



> So until others start to do testing too I will stand by my results. Don't buy it if you don't want and find some friggin thing to bitch about again, that is fine too. But at least I am doing some testing.


I'm not complaining. I'm stating what I have already tested. That is why I stood by my claim. Sorry I don't have a video camera to show you.



> Every one that does not do testing has something to say to those that do. It happens all the time.


True if by all the time you mean very frequently but it doesn't happen 100% of the time. I'm a Physicist. Questioning everything is the norm. It is part of the scientific method. When not enough information is given to support a claim questions are bound to come up.



> What do you have?


Again, no video  but lots of testing, questioning, research of technical papers from many reputable sources, experience on my own stuff, ...


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## Guest (Oct 21, 2011)

All tucked away for the evening. Back tomorrow after work. Got my new tires on my trailer today. Now I can go get my 69 Ghia tomorrow or Sat.


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## EVfun (Mar 14, 2010)

I guess I would just ask, how do you define full? I'm not sure of the right answer so I'm looking for opinions based on experience.

GizmoEV suggests 3.60v until the current drops to 0.05C. I heard some similar numbers from others. I'm getting 61.4 amp hours between cutting the discharge at 3.000 volts while discharging at 0.2C and charging to 3.50 volts and hold until the current drops to 0.06C.


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## Guest (Oct 21, 2011)

Being a Physicist, what kind of machines to you work with? I understand the questioning. I also understand if the cells were drained in series vs parallel. Series is much faster and I do fully trust the cells. I have many and many were put back in service with no troubles at all. I have sent some off for testing. As for long term damage that will remain to be seen as the cells must actually live a long time and be in service a long time to know the results. So far the cells that were below are actually better than the ones that were not. Regardless of the capacity I can still bottom or top balance the cells even if they are not holding fully 100% of rated capacity at this time. They all charge just fine. The cells I have been charging today were at the same voltage level when I shut down for the evening. Within a minute the low one was a hundredth off the other but basically the same. I expect to see both cells at the same 3.34 or 3.35 volts by the time I get home from work tomorrow and before I begin the finishing of the charge process. I am trying to see if charging them in parallel will balance them closer together. Because of what you mentioned yesterday I began this project. I see that there is a fast initial self balance of cells way out of balance from each other if put in parallel. After a day that out of balance is still present but not nearly as far away. So I am hoping that by charging in parallel it will help get them closer to balance to verify the way they work. I just found that the process is too damn slow for me. I was proven wrong and right that the process of self balancing is slow. With way out of balance cells the beginning is VERY FAST but at the end it is very very slow. So I am both right and wrong. Interesting how things work. 

Did you check out that charger? I purchased it years ago for $50 bucks. Works perfect for my needs. 

Pete


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## Guest (Oct 21, 2011)

EVfun said:


> I guess I would just ask, how do you define full? I'm not sure of the right answer so I'm looking for opinions based on experience.
> 
> GizmoEV suggests 3.60v until the current drops to 0.05C. I heard some similar numbers from others. I'm getting 61.4 amp hours between cutting the discharge at 3.000 volts while discharging at 0.2C and charging to 3.50 volts and hold until the current drops to 0.06C.



I say mine are full at 3.60 and 4 amps. At 1C discharge rate which is twice the rating for continuous current draw the cells are holding if charged to 3.65 and 2 amps 95 AH. If allowed to discharge at .5C then the AH rating would be above 100Ah

You are doing well with your 60 AH cells providing more than 60 AH. I drive my cells hard. I do expect to see some capacity loss at norma .5 Discharge rates from some of my cells but so far they are mostly at either just a tad under or just a bit over the rated 100AH for the cells. The early hi-power cells were not the best for quality if your talking of getting a batch of equal cells. The cells them selves are just magic as are all LiFePO4 cells. Just plain magic.


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## Guest (Oct 21, 2011)

5:47 Pacific time and the two cells are at a happy 3.30 volts each. My cells usually stay at about 3.30 to 3.34 being the high end of the charge. Remember that the Hi-Power cell was rated for 3.8 volts while TS at the same time frame was saying 4.2 volts for theirs. So my cells are full at this time but I will squeeze in a bit more as the cells were not quite back up to 3.65 and 4 amps when I shut down. They were back up at 3.58 and 3.59 when I shut down. So I can squeeze a tad bit more. Do I need to? No. Both cells are full. Are they top balanced? No. I did not do this experiment to top balance. I did this to see if putting cells in parallel would self balance to each other and to see if once two out of whack cells after sitting together for a good amount of time could then be charged up to full and come to that full state together. 

For the most part the answer is yes. Time consuming but yes. 

Next experiment will be to drain a cell then connect them together and and charge them up and see if the one cell will remain neutral and not charge much because it's already full giving the other empty cell a chance to catchup with no damage to the already full cell. 

Pete


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## JRP3 (Mar 7, 2008)

This all seems like a lot of work to "prove" what is already known. Our cells are already a collection of paralleled cells internally, and we know they self balance, otherwise they would not function. Externally paralleling two or more cells is simply putting a larger collection of already paralleled cells in parallel. Video taping the process does not add anything, we already know what cells in parallel look like.


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## dougingraham (Jul 26, 2011)

gottdi said:


> Next experiment will be to drain a cell then connect them together and and charge them up and see if the one cell will remain neutral and not charge much because it's already full giving the other empty cell a chance to catchup with no damage to the already full cell.
> 
> Pete


The answer to this is yes as long as you don't exceed the 3C max charging rate for a single cell. Since the full cell will be charging the empty cell you need to include that charging current in your calculations. Since you probably don't have a charger that will do even 1C you are most likely safe that you wont damage the empty cell by charging too quickly. I am not at all worried about the full cell as long as you treat it like you would a single cell for charge termination.


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## Guest (Oct 21, 2011)

JRP3 said:


> This all seems like a lot of work to "prove" what is already known. Our cells are already a collection of paralleled cells internally, and we know they self balance, otherwise they would not function. Externally paralleling two or more cells is simply putting a larger collection of already paralleled cells in parallel. Video taping the process does not add anything, we already know what cells in parallel look like.


The video will actually be short. Just lots of stuff to filter. Do you know because you have done this or just on the words of others? I am doing to document an actual process. So far all I hear is talk and no real documentation. My results are actually mixed.

I still stand on bottom balancing. Discharge if possible in series to a safe level then proceed one cell at a time until your voltage is reached static. Put back in series and charge


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## dtbaker (Jan 5, 2008)

gottdi said:


> I still stand on bottom balancing. Discharge if possible in series to a safe level then proceed one cell at a time until your voltage is reached static. Put back in series and charge


lets assume the OP has been through his pack and now has a pile of cells that accept a charge, and are resting at something above 3.2v.

ok, so lets say you discharge all cells from something above 3.2+v down until they hit exactly 3.00v and stop. 

You have a perfectly bottom-balanced pack... after you put back into series how do you suggest ending the charge cycle without a cell-level BMS to catch the first cell that gets 'full' and starts up the curve while the rest are still chugging away?


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## JRP3 (Mar 7, 2008)

gottdi said:


> Do you know because you have done this or just on the words of others?


I know it because it's inherent in the physics of paralleled cells. I charge paralleled cells every time I charge my pack, just as all of us do. The cells inside our "cells" are in parallel.


> I still stand on bottom balancing. Discharge if possible in series to a safe level then proceed one cell at a time until your voltage is reached static. Put back in series and charge


Which has nothing to do with how cells behave when paralleled.


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## JRP3 (Mar 7, 2008)

dtbaker said:


> You have a perfectly bottom-balanced pack... after you put back into series how do you suggest ending the charge cycle without a cell-level BMS to catch the first cell that gets 'full' and starts up the curve while the rest are still chugging away?


The same way I charge my pack every time, you stop charging before the pack is full, which keeps the pack at a lower SOC, preserving it's life.


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## dtbaker (Jan 5, 2008)

JRP3 said:


> The same way I charge my pack every time, you stop charging before the pack is full, which keeps the pack at a lower SOC, preserving it's life.



HOW do you (and gotti) stop it is what I am trying to get at. a timer? checking/trusting your ahr counter? 

Without a cell level BMS to stop the charge automatically with the first cell hitting target, bottom balance presents a challange in ending daily charge safely. top balance presents the 'danger' on the full-discharge scenerio, so the user has to decide which they want to live with and can manage most easily.

I don't really want to start the top/bottom argument, other than to make sure the OP understands the considerations and maybe avoids killing more cells by either over charge or over-discharge.


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## EVfun (Mar 14, 2010)

JRP3 said:


> The same way I charge my pack every time, you stop charging before the pack is full, which keeps the pack at a lower SOC, preserving it's life.


I think what he is asking is how do you select the end voltage and charge time or ending charge current with a bottom balanced pack. Will it work with a charger that doesn't have an infinitely variable max voltage set point? With a top balanced pack it is easy because the voltage will start rising faster than 1 volt a minute, even at 0.2c charging rates. This makes it easy for the charger to pick a number and either start a timer or hold until the current drops to some lower level. The exact voltage isn't even that relevant because all the cells are rising together. Its a few hunredths of a volt per cell for each volt in pack rise.


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## dtbaker (Jan 5, 2008)

EVfun said:


> I think what he is asking is how do you select the end voltage and charge time or ending charge current with a bottom balanced pack. Will it work with a charger that doesn't have an infinitely variable max voltage set point? With a top balanced pack it is easy because the voltage will start rising faster than 1 volt a minute, even at 0.2c charging rates. This makes it easy for the charger to pick a number and either start a timer or hold until the current drops to some lower level. The exact voltage isn't even that relevant because all the cells are rising together. Its a few hunredths of a volt per cell for each volt in pack rise.



exactly.... the issue that a user needs to be aware of is that with a bottom balanced pack (for less likely cell death on full discharge if you plan to run it into the ground very often), you have to take special care not to overcharge whichever cell(s) have the least capacity in a normal charge cycle. The typical charger can't sense the first cell headed up the knee because the pack voltage hasn't changed hardly at all, so you have to have either a cell-level BMS, or use a timer based on your ahr counter and avoid the top end like the plague.


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## dougingraham (Jul 26, 2011)

EVfun said:


> I did the parallel balance effort with 2 parallel cells that where close in SOC. I used a pair of 6 inch long 16 gauge jumpers and left them in place for at least 2 weeks on 2 cells that where at 3.28 and 3.30. After separating them and waiting overnight I had 1 cell at 3.29 and the other at 3.30 volts. I could measure current flow on the wires paralleling them down to about 50 milliamps (0.1 millivolt.) The measurable amp flow must have moved about 10 amp hours, and then at least another week left in parallel after I couldn't measure current flow. It still didn't completely balance the cells. Many users have proven that paralleled cells work well together so it must sort itself out effectively at both full and empty.


If you parallel two cells that are not equal voltage the cell with the higher voltage will transfer charge to the lower one. The rate at which this occurs depends on the voltage difference and the resistance. Since the voltage difference decreases as the charge is transferred the current will reduce as well. The cells will be in perfect balance about the time the universe ends. But they will be pretty close in not too much time and there is no external influence that will make them diverge in state of charge. What I am saying is that the difference will be so small that it doesn't matter in just a few days. Running the cells to full charge will escalate the transfer as will full discharge. This is because near the ends the voltage difference increases greatly as the good cell goes up the cliff face on charge or the bad cell falls off the cliff on discharge.


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## dtbaker (Jan 5, 2008)

dougingraham said:


> If you parallel two cells that are not equal voltage the cell with the higher voltage will transfer charge to the lower one.



yes.....

BUT paralleling two cells at unknown state of charge and just leaving them doesn't do much since there is only a tiny difference in voltages between two cells that could be at significantly different SOC. it would indeed take a LONG time, and would result in a 'somewhere in the middle' balance ant some unknown SOC.

to BALANCE the cells, they need to be in parallel AND either brought up to your intended end-of-charge for top balance, or discharged to your intended lowest cutoff voltage for bottom balance.


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## JRP3 (Mar 7, 2008)

dtbaker said:


> HOW do you (and gotti) stop it is what I am trying to get at. a timer? checking/trusting your ahr counter?


I get it now  I have a Manzanita PFC 30 so I can fine tune the current, the ending voltage, and the CV time. Since I don't charge to full the cells don't get into the really steep part of the curve where one will take off far in advance of the rest. My pack is also within 3% capacity of each other, it would be even easier with a more closely matched pack.


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## Guest (Oct 22, 2011)

dtbaker said:


> yes.....
> 
> BUT paralleling two cells at unknown state of charge and just leaving them doesn't do much since there is only a tiny difference in voltages between two cells that could be at significantly different SOC. it would indeed take a LONG time, and would result in a 'somewhere in the middle' balance ant some unknown SOC.
> 
> to BALANCE the cells, they need to be in parallel AND either brought up to your intended end-of-charge for top balance, or discharged to your intended lowest cutoff voltage for bottom balance.


You don't need to parallel them to do that. If you bottom balance you take each cell down into the end of charge curve below the level your going to stop. Bring each one down to a known static voltage. Any level will do but a good level for most LiFePO4 cells would be like 2.6 volts static. Then they will all be at that level if you or someone else's drives them into the bottom to get home. Not that it will happen all the time but it will happen to enough folks. Those who say they will never go that low are the exception and not the rule. Even if you say you may. Be ready. Be safe. All the cells will be at the same SOC if you do this. You can do the same by going to the upper limits and upper curve if you MUST top balance. Regardless you won't be using any more than your lowest capacity cell. No matter how you slice it to balance you must take the cell to the ends in order to know that your SOC is equal in all cells. I chose to bottom balance and no BMS. Some have chosen to top balance and use no BMS. 

With that putting cells together of the same AH in parallel works fine if you want to wait. I say don't bother but you can do that. Be sure you have your cells in parallel. That means all the POS are connected together and all the NEG is connected together making one huge AH cell. They will balance if given time. 

I gather that some here don't want to bother with the experiment so I will not bother continuing. I will finish but may not share. My current experiment is two cells out of whack and putting them together and charging . So far the voltages are the same. 

I am thinking that someone who is having trouble here has done some funky stuff to loose so many cells. 

Pete


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## dtbaker (Jan 5, 2008)

gottdi said:


> You don't need to parallel them to do that. If you bottom balance you take each cell down into the end of charge curve below the level your going to stop. Bring each one down to a known static voltage. Any level will do but a good level for most LiFePO4 cells would be like 2.6 volts static.



I am going to beat that dead horse just once more and insist that the definition of 'balance' has to mean having the whole pack in parallel and hitting the determined voltage you want at either the top or bottom.

Otherwise you are introducing error from measurement and repeatability of your process from cell to cell. 

if you MUST take a first pass to make sure cells are healthy (as this OP must do), I would argue hard that you STILL need to put the final pack back in parallel and balance at top or bottom to consider the pack truly balanced before you go back to series and hit the road.


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> I am going to beat that dead horse just once more and insist that the definition of 'balance' has to mean having the whole pack in parallel and hitting the determined voltage you want at either the top or bottom.
> 
> Otherwise you are introducing error from measurement and repeatability of your process from cell to cell.
> 
> if you MUST take a first pass to make sure cells are healthy (as this OP must do), I would argue hard that you STILL need to put the final pack back in parallel and balance at top or bottom to consider the pack truly balanced before you go back to series and hit the road.


+1, that is what I've been trying to say all along. It works. I've done it. I've done it with two cells, I've done it with 20 cells. And no, it doesn't take weeks. It also doesn't take hours of babysitting time.


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## Guest (Oct 22, 2011)

Its not just balanced voltage or balanced SOC. What you are doing by bottom or top balancing is putting the equal end on one end or another so when you either reach the top you reach the top at the same time or the bottom at the same time. It is about CAPACITY. No cell has the same exact capacity as another. One my have 108 AH another 100 AH another 110 AH another 99 AH all in one pack. Not unheard of at all. So if you want the SAFE way you bring all your cells to the SAME SOC either on the TOP or BOTTOM. Then put your cells in series and charge. If you balanced at the top then at the end of your charge but while still charging you will hit the target voltage at the same time. If you bottom balance you hit your target at the same time. That is all. It is not the same as just putting cells in parallel and allowing to come into an equalized state. What that does is put all the cells in the same neutral state as the other but each end is ragged. Your neither TOP or BOTTOM balanced. Your only in equal neutral state. So if you go with that you then need to stay off the bottom and the top. What ever way Top, Neutral, or Bottom you are still only able to utilize the lowest capacity cell or cells. Some cells being nearly exact while others are not. 

I as well as others just think that the bottom is the safest place to balance. Reason is because if you do happen to take your car to the bitter end of your charge to go that last mile your cells will be at the low state at the same time. If you top balance you can't do that without a danger of driving a low cell into oblivion. If you neutral balance then you have a little better leeway than if your top balanced but you still can't go that last mile safely. If one goes to that low end then thousands will too. Many will not be like us and will just get in and go. So give them the safe way and if they decide to go that last mile they will still have good cells. It is more dangerous to drive 200 amps through an empty cell than 2 amps through a cell that may be at 3.8 volts instead of 3.65 volts at the end of the charge. Taking a cell to reversal is fast once the pack gets to 2 volts per cell. It will literally happen in a minute. I have seen it. 

That is all. Putting cells into parallel for neutral balancing is the only way to get a proper neutral balance. The problem with neutral balancing is you still don't know how ragged your end or top will be. Some will be just fine but others with miss balanced cells will need to do a balance. Bottom, Middle or Top. You choose. There is really no argument beyond that. It is your choice. 

Pete


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## JRP3 (Mar 7, 2008)

dtbaker said:


> I am going to beat that dead horse just once more and insist that the definition of 'balance' has to mean having the whole pack in parallel and hitting the determined voltage you want at either the top or bottom.


Why does balancing have to mean putting the pack in parallel if you individually bring them all to the same voltage, top or bottom? If all your cells are at 2.6V, individually or paralleled, they are still all at the same voltage, and SOC.


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> Otherwise you are introducing error from measurement and repeatability of your process from cell to cell.





JRP3 said:


> Why does balancing have to mean putting the pack in parallel if you individually bring them all to the same voltage, top or bottom? If all your cells are at 2.6V, individually or paralleled, they are still all at the same voltage, and SOC.


I think you have to include the next sentence, quoted at top, to get your answer. You just have to have a reliable way to top or bottom balance a set of cells. Paralleling them to do it eliminates voltage measurement differences from cell to cell due to inconsistencies in the procedure.


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## JRP3 (Mar 7, 2008)

What inconsistencies are there moving a DVM from one cell to another? To be clear I've done it this way successfully, including a real world test driving my cells under load to 1.77V per cell, 2.45 resting. If they weren't bottom balanced I would have killed some for sure.


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## Guest (Oct 22, 2011)

Any reasonable digital voltage meter will allow you to accurately measure your cells to either top or bottom balance. I might be ok with taking my cells to like 2 volts per cell then let them sit for a month while I finish working on the car then putting them in the car in series and only allowing the charger to go to 3.65 volts per cell. Being nearly equally bottom balanced that way I am sure the top would be ragged but still not allow the lowest cell to drive up past like 3.8 volts. This way I'd be sure that all the cells were in the low part of the discharge knee. A good simple way but if one did this and the pack was nearly at 50% SOC to begin with then both ends of the pack would be ragged and you would need to stay off the top and bottom. It can be done. It is a matter of choice. 

It is always a matter of choice. I choose the bottom way. Some choose the top way and others stay in the middle. Oh well. 

Enough slapping the dead horse.


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## dtbaker (Jan 5, 2008)

gottdi said:


> Any reasonable digital voltage meter will allow you to accurately measure your cells to either top or bottom balance.


with ANY instrument there are limits to both accuracy and repeatability. With any given process there is yet another variable as to whether you can repeat the exact same process and start/stop at exactly the same point.

If you have cells in parallel you bypass the limits of any given instrument and the cells are by definition balanced relative to each other.

No matter HOW good or expensive a multimeter is, there IS error in absolute accuracy and repeatability.... hence any procedure done on one cell at a time will never be as 'balanced' as a pack done in parallel as a final step.

thats my only point

since you guys don't want to hear or consider that, I'm out of here and unsubscribing this particular thread as non-productive.


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## EVfun (Mar 14, 2010)

dtbaker said:


> I am going to beat that dead horse just once more and insist that the definition of 'balance' has to mean having the whole pack in parallel and hitting the determined voltage you want at either the top or bottom.
> 
> Otherwise you are introducing error from measurement and repeatability of your process from cell to cell.


I'm going to agree with Pete here, you don't need to have them in parallel to top or bottom balance. You need to reliably charge or discharge to a set point that is in the steep part of the capacity curve. The resulting difference will be some fraction of 1%.

In parallel might be more effective if you are just barely in the steep part of the curve at your chosen balance point. If I was trying to balance at 3.50 volts and 3.0 amps just picking the set points may allow the results to vary 1 to 3 percent. 

I balanced by charging my cells with shunt regulators on each of them. The voltage control was turned up so it wasn't the primary limiting factor. I turned the current way down to keep any cell from going over 3.75 volts. Pretty soon the current was turned down to no more than the regs could bypass and the cells settled at 3.65 volts each. Then I held it for a while because I couldn't check to see if each cell was taking the same current (the shunts could be bypassing different amounts.) The results got a little touch up with a 5 ohm resistor and a small 12 volt AGM battery. I could either discharge at about 0.7 amps or charge at about 1.7 amps. My buggy pack has never been in parallel.

_I want to add that paralleling does not rule out errors unless the current is zero. Even the wiring adds resistance. I could measure the current flowing between 2 cells that where 0.01 volt apart for a week! It just changes it from an instrument error factor to a wiring error factor. _


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## Guest (Oct 22, 2011)

dtbaker said:


> with ANY instrument there are limits to both accuracy and repeatability. With any given process there is yet another variable as to whether you can repeat the exact same process and start/stop at exactly the same point.
> 
> If you have cells in parallel you bypass the limits of any given instrument and the cells are by definition balanced relative to each other.
> 
> ...


Actually you don't need an accurate digital multimeter. It only need be consistent. Most you find are consistent enough for safe use in getting your cells to a safe balance point. This is where Good enough is actually good enough. Like hydraulic crimpers vs manual hammer crimpers. Both work. Manual is good enough. Really. The only reason folks go get the best is to say they have it. There is no need. Good enough is really good enough. When is perfect actually perfect? Perfect is subjective. Actually most of what is argued on the forums is subjective and mostly moot. 

A good mulitmeter is fine. 
A good hammer crimper is fine.
A good charger is fine. 
A good motor is fine.
A good adaptor is fine. 

Good does not mean a lack of quality. 

Oh crap. off on another rant again. 

It's late and Im tired and gotta be at work in the morning. 

Good news, I am going to finally get trained as an MRI tech. So that will be X-Ray, CT and now MRI.


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## JRP3 (Mar 7, 2008)

EVfun said:


> _I want to add that paralleling does not rule out errors unless the current is zero. Even the wiring adds resistance. I could measure the current flowing between 2 cells that where 0.01 volt apart for a week! It just changes it from an instrument error factor to a wiring error factor. _


Exactly this. Nothing is perfect and pursuing perfection is somewhat counter productive at some point. As Pete said good enough is good enough sometimes.


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## GizmoEV (Nov 28, 2009)

I think the point has been long lost in this thread. Multiple times in various threads including this one some have stated that should not be put in parallel to balance them. The problem was that their argument didn't pass the go out and test it routine. At no time did I say to just put in parallel and leave them alone. I have always said to put them in parallel and then charge them up into the upper knee or discharge them into the lower knee. This can be done with the cells at any safe SOC. As long as the cells are not damaged to begin with paralleling is definitely a safe and sure way to balance a set of cells.

I have never said that balancing them individually did not work and was not good enough but some have seemed to imply that if paralleling is suggested that balancing individually shouldn't be done was an automatic follow-on. This is simply not the case and is an illogical conclusion. Each method of balancing comes with its own challenges. For me and others I know, paralleling was the simplest and quickest method. This includes top balancing, which is what I did, and for others it was bottom balancing.

If I were to decide to change to bottom balancing the pack in my car I would definitely discharge it in series until it was close to 100%DOD and then I would parallel it to finish it. This would get the pack empty relatively easily with my lightbulb load bank and then I could take the pack to the bottom really slow with the resulting 4000Ah battery. Others want to work in short bursts on individual cells but then move from cell to cell. I don't want to do that.

There is nothing wrong with either method. At least someone else has done a test run to show paralleling is definitely a safe and viable method.


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## EVfun (Mar 14, 2010)

I would be a little concerned about charging a large pack in parallel to top balance. I used, as an example, a pack made of 36, 180 amp hour cells. That is 6480 amp hours. With a 10 amp charger it will take about 27 days to add 1 amp hour to each of the cells. I know this had been done, and it seems to have worked out. Still, that kind of slow charge rate should mean the pack takes a month to go from 3.5 vpc to 3.65 vpc and several days to make that last jump from 3.60 to 3.65 volts. I am concerned that such extended time at elevated voltage may not be good for the cells. That is why I used the shunt regulator function of my BMS to top balance my pack (then removed the BMS.) Others have done this with a number of individual chargers.

Perhaps such a slow charge does no harm, but it leaves the cells at an elevated voltage long enough to give me pause.


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## GizmoEV (Nov 28, 2009)

EVfun said:


> I would be a little concerned about charging a large pack in parallel to top balance. ...
> 
> Perhaps such a slow charge does no harm, but it leaves the cells at an elevated voltage long enough to give me pause.


That is definitely something which should be considered for each individual's situation. For a bottom balanced situation it doesn't look like it would be an issue, however.


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## JRP3 (Mar 7, 2008)

Not to mention that with an already installed pack parallel balancing means disconnecting everything, wiring in parallel, then wiring back up in series. I'm sure paralleling can work, and there are advantages to both, but for my setup and existing equipment series is the easiest by far.


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## ev4me (May 10, 2011)

EVfun said:


> I would be a little concerned about charging a large pack in parallel to top balance. I used, as an example, a pack made of 36, 180 amp hour cells. That is 6480 amp hours. With a 10 amp charger it will take about 27 days to add 1 amp hour to each of the cells. I know this had been done, and it seems to have worked out. Still, that kind of slow charge rate should mean the pack takes a month to go from 3.5 vpc to 3.65 vpc and several days to make that last jump from 3.60 to 3.65 volts. I am concerned that such extended time at elevated voltage may not be good for the cells. That is why I used the shunt regulator function of my BMS to top balance my pack (then removed the BMS.) Others have done this with a number of individual chargers.
> 
> Perhaps such a slow charge does no harm, but it leaves the cells at an elevated voltage long enough to give me pause.


That's 27 days for a full charge. 27 days for one amp hour means it would take you thirteen years to charge that pack.

I think we'll just do it both ways. Individual charging followed by a parallel top-off. We have a 180 amp power supply so that final step should take but a few minutes (obviously in CV mode the whole time). So far we've done ten out of a hundred cells and should be able to speed up the process 2-3x now that we've got the hang of it.


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## EVfun (Mar 14, 2010)

ev4me said:


> That's 27 days for a full charge. 27 days for one amp hour means it would take you thirteen years to charge that pack.


Oops, good catch. Thanks for the correction. It would be about 3 hours 40 minutes to add 1 amp hour to each cell. That will still make the finish charge slow, but nothing like my flawed estimate. 

By a rough estimate, a 180 amp hour cell must have around 15 to 20 amp hour that will put in above 3.45 vpc. It would be about 2.5 days for the pack (36, 180 amp hour cells) to go from a lower 3.45 vpc to about 3.65vpc with a 10 amp supply. With a faster charger this could be quite manageable.


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