# 32P cells sag on 5A load?



## Sunking (Aug 10, 2009)

Well you just discovered why parallel cells are not optimum as it forces the stronger cells to do most of the work. It all revolves around the batteries internal resistance and you can measure it. You already have but did not complete the Calculation. 

Assuming the battery under test is fully charged you can measure the Ri fairly accurately with a good volt meter and current shunt or clamp. First you measure open circuit voltage of a fully charged rested battery and label it V1. Let's say 3.2 volts = V1.

Next connect a load to the battery. The more current the better the measurement. With the load connected measure the battery voltage on the term post and call it V2. At the same time measue the current and call it IL. So lets say V2 = 2.9 and Il = 10 amps.

Now calculate Ri = [V1 - V2] / IL = [3.2 - 2.9] / 10 = .03 Ohms.

Having said all that with batteries in parallel, in theory the voltage of every battery has to be equal, but that is only true with no load or light load. But lets say you have 10 parallel cells and you terminate the load to either end and it is a heavy load. The battery closest to the load will provide the highest current of the 10. Second closest will be the second highest current and so on down the line. Why you ask. Simple Ohms Law, the wire has resistance between each cell. The cell on the other end has the most resistance.

There are two ways to counter this:

1. This is the best method of the two. If you need 100 AH then by all means buy 100 AH cells and avoid parallel strings. 

2. If you are forced to use parallel cells which I cannot think of any reason to do so unless you need something like 500 AH and there are no 500 AH cells and largest you can find is 250, then use two parallel string. Keep parallel strings to a minimum and avoid them if possible. However if you must use parallel strings connect the load on a diagonal. Example positive to the closet term post of the battery, and negative to the furthest away from the load.


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## sabahtom (Mar 1, 2011)

Thanks very much Sunking. 

I've got 97 cells to bottom balance. 

If I have a drop of 1v across the paralell string of 32 cells, do you think it's worth finding out which ones have high Ri using the method you described? Or better to just paralell them with a 10A load (that's what I'm planning to start with) and monitor individual voltages for sag as I go?

Unfortunately my new programmable voltmeter has a factory defect so I need to start the balancing manually.


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## Sunking (Aug 10, 2009)

Are these LFP cells?

If so LFP is pretty easy to work with and forgiving where they can be float charged provided you do not shoot for 100% SOC. 90% is the target.

What I am driving at if you float charge them say for several hours, all the batteries will equalize. As charge current tapers off, voltage drop lessens and becomes equal. Eventually all current stops when all batteries reach the same SOC level.


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## sabahtom (Mar 1, 2011)

Yes they're HiPower 100AH LFP.

So before bottom balancing it may be worth putting an RC charger on the pack of 97 cells in paralell and charging at 5A/3.65v for a few hours to help them to equalise, then put a 10A load on to bottom balance?

I think the SOC right now is between 8% and 15%.


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## kennybobby (Aug 10, 2012)

How do you know they are at 8% SOC--maybe they are at 0% or -5%? The reason the voltage is sagging is because the cells are near the bottom and you are pulling them down with the load--stop before you damage your cells.

Do you have a written procedure for what you are trying to do or are you just riding bareback? If not then i suggest you establish your goals and objectives and develop a plan on how you intend to do this, write it up and post it for review.


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## dougingraham (Jul 26, 2011)

sabahtom said:


> I thought cells in paralell act as one cell regarding current, so I expect them to all sag by roughly the same amount, unless one is much weaker. I see that happening so I understand.


They do act as one cell as long as the cell interconnects are low enough resistance. If you are using copper bars as the cell interconnects they are probably around 0.0001 ohm or less. You shouldn't even see a difference in voltage from one end to the other with a 4.5 amp load. A 4.5 amp load measured through 64 of these 0.0001 ohm resistances would be 0.0288 volts. But that is not what you have since there is a cell at each of these interconnects.



sabahtom said:


> I don't understand why the V from cell 1 positive to cell 32 negative should show as 2.3v. Surely it should be the average V of the cells from 1-32?


If there is much of a difference in voltage from one end of the 32P connection to the other then some of the interconnects are not good. It will be close to the average of all the paralleled cells. Can you post a picture of what you are doing? Something is just not right here.


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## sabahtom (Mar 1, 2011)

Thanks Doug and Kenny. I checked the interconnects and the V from end to end is 2.8v, individual cells are at 3.18v to 3.23v. They've only been in paralell for 2 days.

The starting v before discharge of the lowest cell was 3.15v. So far I've taken out 12AH from the pack of 97 cells as a whole so I hope I'm not near the bottom of any individual cells yet.


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## sabahtom (Mar 1, 2011)

*Bad interconnects*

It was the interconnects and wrong positioning of the cables between the strings. After tightening them all I get a drop of 8mv across the whole pack.

UNfortunately the bad interconnects made it go out of balance so two strings are low (3.20v) and one is high (3.22v). 

Edit: The connectors seem ok now. The pack is balancing out after my mistake, last measured the highest cell was 3.19v and lowest 3.09v.


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## sabahtom (Mar 1, 2011)

kennybobby said:


> How do you know they are at 8% SOC--maybe they are at 0% or -5%? The reason the voltage is sagging is because the cells are near the bottom and you are pulling them down with the load--stop before you damage your cells.
> 
> Do you have a written procedure for what you are trying to do or are you just riding bareback? If not then i suggest you establish your goals and objectives and develop a plan on how you intend to do this, write it up and post it for review.


Thanks Kennybobby. I'm following Doug's post here 

http://www.diyelectriccar.com/forums/showthread.php?t=120513&highlight=discharge+op-amp

as closely as I can, stupid mistakes all my own...

Maybe I've completely misunderstood paralelling for bottom balance. I thought if I take them to 2.8v then disconnect, they should bounce up a bit. 

If they don't bounce back, and keep self-discharging, they can't be used anyway? 

So right now my cells are between 3.09 and 3.19v. I know their SOC is low but I'm not sure how much. I thought that was the reason for going to 2.8v, so I could be sure it was low enough to bottom balance?


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## dougingraham (Jul 26, 2011)

sabahtom said:


> Maybe I've completely misunderstood paralelling for bottom balance. I thought if I take them to 2.8v then disconnect, they should bounce up a bit.
> 
> If they don't bounce back, and keep self-discharging, they can't be used anyway?
> 
> So right now my cells are between 3.09 and 3.19v. I know their SOC is low but I'm not sure how much. I thought that was the reason for going to 2.8v, so I could be sure it was low enough to bottom balance?


When you get below 3 volts there is not much charge left. The rate of voltage change will increase so be careful. I took mine down to 2.5 volts. But they are your batteries and you need to be comfortable with everything you do. I would consider 2.8 volts to be conservative. And I would try to talk you out of going much below 2.5 volts.

I think you are on the right track now.


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## Sunking (Aug 10, 2009)

I am a bit confused here on what is actually taking place. 

If I understand correctly you are taking 32 LFP 100 AH cells, wiring them all in parallel, and trying to Balance Charge them?

That will work but 32 100 AH cells in parallel is a 3200 AH. I have no idea what current your charger supplies, but if I were to bet money, I would guess you are not actually doing very much in terms of making any meaningful headway, or painstakingly SLOOOOOOOW. 

For example if your supply is only generating say 10 amps, on a 3200 AH battery is only a C/320 charge rate. While some think or claim LFP does not have a Self Discharge rate is misinformed. Truth is they self discharge at 1 to 3% per month which is low. It might be you are not really charging the batteries, you are just slowing down the self discharge rate. But at least they would all be equally discharged.


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## sabahtom (Mar 1, 2011)

Sunking said:


> I am a bit confused here on what is actually taking place.
> 
> If I understand correctly you are taking 32 LFP 100 AH cells, wiring them all in parallel, and trying to Balance Charge them?
> 
> ...


Hi Sunking. I'm discharging to bottom balance them. The pack is 32P + 32P +33P, three strings. I didn't connect the strings together properly at first, which was the reason for the drop in V from one end of the three strings to the other.


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## onegreenev (May 18, 2012)

Sunking said:


> I am a bit confused here on what is actually taking place.
> 
> If I understand correctly you are taking 32 LFP 100 AH cells, wiring them all in parallel, and trying to Balance Charge them?
> 
> ...


They self discharge much slower than what you say is 1% per month. Ive had my CALB cells resting for months and the voltage has not gone down any measurable amount. At 2.6 volts and 1% per month you'd see a voltage drop. Ive not seen that yet. They do loose the ability to hold capacity but that is not the same as discharge. 

I'd take each cell down to a set voltage like 2.55 volts like I did then hold it until it won't rise much past 2.6 volts after 5 minutes. Do the exact same thing for each cell and you will have a matched pack. If you do it all at once or one at a time it will take time. If your cells are already low then it won't go so slow. My setup is discharging at 15amps and then hold until it reaches c/20 then terminates. All cells are taken at that rate and will all pretty much be within a few thousandths of each other. 

If sabahtom were to get a powerlab 6 or 8 then he could do this balance on a single cell and have it controlled much better than a rough discharge. I like the ability of the powerlab because it will shut off when done and not continue if Im not watching it. The process should be automated. With one or two cells its no issue to do fully manually. Too easy to ruin a cell manually if you get distracted.


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## dougingraham (Jul 26, 2011)

Sunking said:


> While some think or claim LFP does not have a Self Discharge rate is misinformed. Truth is they self discharge at 1 to 3% per month which is low.


Think? Claim? How about tested? I have tested the self discharge. It is as near to zero as I was able to measure over a 6 month period. For 100AH cells the resolution of my cycler is about plus or minus 75 mah.

If you play with these cells for a while you will discover this on your own. And this does not conflict with the manufacturers data sheets. They all say something like self discharge is less than 3% per month. Well 0% is less than 3%.


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## Sunking (Aug 10, 2009)

dougingraham said:


> Think? Claim? How about tested? I have tested the self discharge. It is as near to zero as I was able to measure over a 6 month period. For 100AH cells the resolution of my cycler is about plus or minus 75 mah.


Don't get your shorts in a knot, self discharge rate is not really my point. The point is with so many cells in parallel to have any real meaningful charge current is a huge task, and being further complicated by extremely low voltages. Just to get a 10 hour charge rate is 320 amps at 4 volts. Just making the point at low currents and by low I mean C/20 and less is going to be extremely and painfully slow. 

As far as LFP discharge rate goes comes right from CALB CA series specs of 3% per month as you can see for yourself. Perhaps you could take it up with the manufacture? I am certain they would like to publish lower rates.


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## sabahtom (Mar 1, 2011)

Hi Pete

After a week of this I can defintiely see the point of automating. I've got the JLD5740 voltmeter. I thought it had a defect but it reads in the LifePO 3.10v range within 10mv when I set PvH (the max expected voltage) at 11v. So I'll use that, knowing it under-reads by 10mv. 

I've got two RC chargers that I can use to finish discharging the cells, but I don't think they'll be as thorough as the powerlab. I'll have to check if I can set the cutoff discharge voltage.



onegreenev said:


> They self discharge much slower than what you say is 1% per month. Ive had my CALB cells resting for months and the voltage has not gone down any measurable amount. At 2.6 volts and 1% per month you'd see a voltage drop. Ive not seen that yet. They do loose the ability to hold capacity but that is not the same as discharge.
> 
> I'd take each cell down to a set voltage like 2.55 volts like I did then hold it until it won't rise much past 2.6 volts after 5 minutes. Do the exact same thing for each cell and you will have a matched pack. If you do it all at once or one at a time it will take time. If your cells are already low then it won't go so slow. My setup is discharging at 15amps and then hold until it reaches c/20 then terminates. All cells are taken at that rate and will all pretty much be within a few thousandths of each other.
> 
> If sabahtom were to get a powerlab 6 or 8 then he could do this balance on a single cell and have it controlled much better than a rough discharge. I like the ability of the powerlab because it will shut off when done and not continue if Im not watching it. The process should be automated. With one or two cells its no issue to do fully manually. Too easy to ruin a cell manually if you get distracted.


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## dougingraham (Jul 26, 2011)

Sunking said:


> The point is with so many cells in parallel to have any real meaningful charge current is a huge task, and being further complicated by extremely low voltages. Just to get a 10 hour charge rate is 320 amps at 4 volts. Just making the point at low currents and by low I mean C/20 and less is going to be extremely and painfully slow.


Sab is discharging to bottom balance. Not sure where you got the idea that he is charging.



Sunking said:


> As far as LFP discharge rate goes comes right from CALB CA series specs of 3% per month as you can see for yourself. Perhaps you could take it up with the manufacture? I am certain they would like to publish lower rates.


That is not a CALB datasheet. It looks like someone filled in the fields of a database and then displayed that selected entry on a form. If you have an RAR unpacker of some kind you can grab the real thing from the CALB website in china http://en.calb.cn/Product/?id-116.html. You will discover that CALB at least no longer even lists a self discharge value. I looked at the CAM 72 entry and the CALB 100 CA datasheet. And as I said in my previous post, all the older datasheets from Thunder Sky, Sky Energy, CALB, GBS, Winston, Hi Power and several more I cant think of now seemed to be copied from each other and they all listed the self discharge as <= 3% per month. I had not noticed until now that any of them had finally stopped listing a value but I am glad they have. And I do know that simply not listing a value does not make it zero. And it may not be zero but by my measurements it is less than 0.0125% on two different kinds of LiFe cells and on some Kokam LiPoly cells I tested back in 2004.

I have attached the pdf file I unpacked from the CALB china web site to save you the trouble of unpacking it.


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## Sunking (Aug 10, 2009)

dougingraham said:


> Sab is discharging to bottom balance. Not sure where you got the idea that he is charging.


Well that is where I got off track. That being the case not really any need to do much of anything in terms of hardware. Just connect all the cells in parallel and let them be for a few hours or overnight to equalize. Weaker cells will be charged up by stronger cells until all are equalized and current stop flowing. No need for a load or charger right? 

As for lithium self discharge is really a moot point. The industry as a whole clearly state self discharge is 1 to 3%, and CALB are not on the high end of that as they are not top quality LFP batteries. But again that is a moot point when the batteries are cycled daily/weekly or frequently. They will never be sitting around long enough for self discharge to amount to anything.


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## sabahtom (Mar 1, 2011)

*Difference in voltage under load between pos/neg terminals*

I can connect the 5A load at A or at B, and still keep it across the whole pack.

I see 2.85v at point B under 5A load. At the same time at point A it's 3.56v.

When I remove the load point A is 3.205v and B is 3.208v. 

The highest cell in the pack with no load is 3.214 and the lowest is 3.205. There might be some inaccuracy in the measurements there but the interconnects seem good.

Any thoughts appreciated 

Edit: Seems that 2.85v + (3.56-2.85)/2 gives me 3.205v. That's the pack average, and I guess it's not a coincidence if it's that neat.


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## Sunking (Aug 10, 2009)

*Re: Difference in voltage under load between pos/neg terminals*



sabahtom said:


> I can connect the 5A load at A or at B, and still keep it across the whole pack.
> 
> I see 2.85v at point B under 5A load. At the same time at point A it's 3.56v.
> 
> ...


Sounds like you have bad connections. Only way possible to have .4 volt difference between batteries in parallel is if you have significant resistance somewhere in the circuit with current flowing.


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## sabahtom (Mar 1, 2011)

You're right Sunking. The floor is uneven, causing a few of them to twist and reduce the contact surface.


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