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Old Jan 01, 2007, 09:25 AM
Brunswick, Ohio
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It was stated earlier in this thread but ignored that the reason Nixx cells balance when slow charged is that they reach their top voltage then bleed off excess voltage as heat. This allows the already charged cells to stay at their charged voltage bleeding heat while the lower voltage cells "catch up". This does not occur with Lixx packs, discussion ends there. If you want to discuss whether it's a good idea to use a balancer, good to fast charge, good to slow charge, they are all good discussions but non of that is relevant to the question asked in the original post.

Jim
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Old Jan 01, 2007, 09:47 AM
Space Coast USA
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Quote:
Originally Posted by everydayflyer

Now everyone balances before charging,during charging ,after charging and even during discharging and most still disagree which is correct ,best ,proper.


Charles
No problem to balance before charging, during charging, or after charging or all three as long as the balancing is done in a time frame that keeps any cell from going over 4.2V+/- .

But my point is that balancing a pack because it is unbalanced after discharge may or may not result in a balanced pack after charging. If the pack was balanced before discharging, balancing that pack in the discharged state will probably cause that pack to be in an unbalanced state after charging. The balance state after charging is the key factor.
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Old Jan 01, 2007, 10:16 AM
Pompano Hill Flyers
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Quote:
Originally Posted by everydayflyer
First there were non then there were balance at end of charge then ____________.

When balancing during the charge was first introduced by Charlie Wang ,Thunder Power many stated here that they felt that was a big mistake as everyone knew that LiPolys should only be balanced when full.

Now everyone balances before charging,during charging ,after charging and even during discharging and most still disagree which is correct ,best ,proper.


Charles
Goodbye to NiCd batteries, with their unending controversy about an alleged, vaguely-defined property called "memory". We never did settle exactly what memory is and whether or not it actually exists, and no doubt we never will.

But technology moves on, and battery care voodoo moves right along with it.

Now say hello to a whole new generation of myth and controversy about the care and balancing of LiPo packs. We can be sure this will last until long after LiPo batteries themselves become obsolete.
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Old Jan 01, 2007, 10:31 AM
Southern Pride
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Quote:
Goodbye to NiCd batteries, with their unending controversy about an alleged, vaguely-defined property called "memory". We never did settle exactly what memory is and whether or not it actually exists, and no doubt we never will.

From a recent post of mine

Quote:
Two Sanyo 2700 chosen at random. Refresh mode on La CROSSE BC-900.
Charged at 1000 mA , discharged at 500 mA to 0.9V.

First cycle 2.48Ah and 2.00 Ah
Second cycle 2.61 and 2.64 Ah
Third cycle 2.70 and 2.71 Ah.

So according to these test cell # 1 gained(?) 0.22Ah capacity and cell #1 gained 0.71Ah .
So was this memory effect being cleared / erased or was it voltage depression being cleared?


I realize your statement was NiCd and my test was on NiMH but they seem to have similar characteristics other than everyone (?) agrees that NiMH do not have / develop memory.


Charles
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Old Jan 01, 2007, 10:49 AM
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Get with the program, Charles! Memory arguments are just so five minutes ago.
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Old Jan 01, 2007, 11:13 AM
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Hoppy; In the case you referred to, the cells are in good health and the pack would be restored to balance through charging whether or not a balancer was used during charge. Active balancing won't throw it off at the end.

Charles; that's interesting about the early history of balancing...I was still toasting NiMHs at that time and missed it. I've got a theory about the contrariness of inanimate objects that this deal might fit into. Have you ever had some machine go bad, you take it apart, can't find anything wrong, put it back together and it works now? Machinery demands that we pay attention to it, see. That's all that a balancer or DVM or whatever is for...it just keeps the gremlins in the pack happy. Whatever works....
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Old Jan 01, 2007, 11:29 AM
Space Coast USA
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[QUOTE=roidspop]Hoppy; In the case you referred to, the cells are in good health and the pack would be restored to balance through charging whether or not a balancer was used during charge. Active balancing won't throw it off at the end.
QUOTE]


I'm not so sure about that if a non-balancing charger is used - It seems to me that if the low voltage cell was brought up to the level of the others before giving it a full charge, that it would end up overvoltage at the end. I might try that on one of my "unbalanced/discharged - balanced/charged" packs but I think I'll check the V on that cell near the end because if I'm correct, it's going to be over 4.2V.
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Old Jan 01, 2007, 12:08 PM
That's a funny word
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You are absolutely correct, the cell that has the charge added to it bring it up to match the others will be overcharged at the end of the pack charge. Only if the pack's cells were all the same voltage at the top. (balanced at the full mark) Chances are good that it is a cell that must be classified with slightly less capacity than it's brothers.

Some distinctions have been made to terms

balance, match, ect. It is interesting to clarify some terms.
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Old Jan 01, 2007, 12:18 PM
That's a funny word
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It may also be good to narrow the discusion down to cells that are

not overcharged by voltage or current, not overdischarged by votage or current.

If normal cells were watched on seperate graphs the imbalance of each one could be seen as an increasing differential of voltage all the way down to the discharged state and could also be repeated over and over again. It can be watched on the way UP and on the way Down. They are VERY very predictable if all is "normal" and slow chargeing or any chargeing will not "equalise, balance,make the voltage identical" for any other reason than a normal charge would.

It still seems some are misunderstanding the "slow charge balances cells" MYTH

If anything goes on outside the limits of specs, well then the predictability may really go down the tube and I have had ZERO observations there. That's a different matter.
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Old Jan 01, 2007, 12:35 PM
Southern Pride
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Thing is some are aware of OHm's Law and some are not.

If all cells are normal as in not damaged ,adnormal IR etc. the the cell(s) with the lowest voltage ( lowest state of charge) will accept more charge (mA) than the higher voltage cells and will thus tend to balance the pack. Problem is that if a cell has high IR / indenpence or reduced capacity than its' pack mates then it can be overcharged at the top of charge.


read real numbers here

http://www.rcgroups.com/forums/showp...92&postcount=4


Charles
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Old Jan 01, 2007, 12:40 PM
Pompano Hill Flyers
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Quote:
Originally Posted by everydayflyer
Thing is some are aware of OHm's Law and some are not.

If all cells are normal as in not damaged ,adnormal IR etc. the the cell(s) with the lowest voltage ( lowest state of charge) will accept more charge (mA) than the higher voltage cells and will thus tend to balance the pack. Problem is that if a cell has high IR / indenpence or reduced capacity than its' pack mates then it can be overcharged at the top of charge.
Wait a minute! We're talking about a series circuit, aren't we? The current is the same everywhere in a series circuit.

Or are you talking about something else and I missed it somehow?
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Old Jan 02, 2007, 10:35 AM
That's a funny word
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Yes, won't the high resistance of any 1 cell also affect it's "packmates"?

I thought all along they were "brothers"
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Old Jan 02, 2007, 11:26 AM
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Post # 4 has LiPoly voltages and cell imbalance numbers.

Here are A123 ,Li-Ion numbers .
3S A123 ran flat cells at 2.742 / 2.635 / 2.733 imbalance 0.107

Added (charged) 100 mAh and rechecked cells 3.098 / 3.059 / 3.079 imbalance 0.039.

Imbalanced reduced to 36% of original imbalance by adding 100 mAh or approx. 4% of the packs total capacity.

This was obtained while doing a 2C rate charge and no external balancing.

Reminder of charge on TP 1010C Rev. 2.0D set at 10A and an addational 2,179 mAh add for a total of 2,279 mAh (cells rated 2300) and this was cycle # 73.

Cells at 3.502 / 3.498 / 3.493 ( imbalance 0.009) an no external balancing used.


Charles
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Old Jan 02, 2007, 12:33 PM
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I am confused by the posts I tried to follow in this thread.., I am sorry but I may post off topic if I see it in a wrong light..

I think that You are mixing capacity with resistance in a big chaos...

But... there is a rest voltage of lipo cell, when no current is flowing at all.
Perhaps it could depend on temperature? So let we consider room temperature for now.
Rest voltage can be found on a voltage versus state of charge (=SOC) graph, pointing exactly into certain state of charge in percents of full capacity. Another name for soc could be percent charged in.
Balanced pack is the one where all the cells are full soc. Not half or any other, only "full" or 100% . (the voltage at 100% soc is a bit below 4.20V).
The same Current flows thru every part of closed circuit. No exceptions, thus no way that in a series pack could a cell suffer higher or lower current than others.
The time should flow equally until we will approach the speed of light.
Current multiplied by time is capacity, in mAh (miliAmper/hour) or in coulombs (=Amper/second) is then EQUAL for every cell in series pack. Note that Voltage is not counted in this formulas.

By discharging full balanced pack of 1250mAh nominal capacity, for example if I will discharge 625mAh from this pack, it means 50%. Thus the rest voltage per cell should be 3.90V
But alas, cell voltages are not equal but various, for example somewhere between 3.890V to 3.905V.
This means that they are discharged by 52% to 49%.
!!! Note that the differences are much lower, max few mV, for useable packs !!!
If the differences are as high as 10mV around 50% soc, the smallest cell will be overdischarged and destroyed in real life.
Voltage differences in balanced pack are much higher at deeper discharge levels, for example as shown on graphs in this thread: http://www.rcgroups.com/forums/showthread.php?t=488123
http://www.wattflyer.com/forums/atta...1&d=1141571728

The name for this difference is NOT unbalance, this pack is perfectly balanced as when it was full, no matter that voltages are not equal, - just the pack is UNMATCHED, = cell capacities are various, in theory from 1202mAh (625mAh=52%) to 1275mAh (625mAh=49%) in this example.
Charging this pack without any balancer brings it back to full SOC with exactly equal voltages.
HOWEVER, charging by 1C will temporary force higher than 4.200V onto several cells in the middle of charge (just around the point of CC to CV swich).
Charging at very low currents, as mentioned, runs at lower than 4.200V allmost till the end, and that is the reason why the cells does not suffer such overvoltage.
.

Note that this post did not mentioned and count Ohm law at all, yet. All the voltages was mentioned as no current voltages.
The Internal Resistance (IR) is easy to measure and easy to understand. In very simple point of view, You can estimate its value, and use it in every formulas where voltage under load or under charge current flow is mentioned.
Voltage measured equals to RestVoltage plus (or minus) Current times IR.
Nothing more... Just,.. if one will try to balance cells while current is flowing, he have to count the IR and use modified formulas... foolish. He could succeed only in lucky circumstances that cells are allmost equal in IR. Older or abused packs are far from balanced when they are at equal voltages..
But you should keep in mind that healthy Lipo has neglible IR, few mOhms, so the difference in voltages between 0.0352Ohm and 0.0354Ohm is far below 1mV per 1 Ampere.

There is absolutely nothing necesary with healthy and matched Lipo pack. Ordinary charger without balancer, and flying till ESC cutoff will not hurt it.. But I am sure that such pack is a dream and even if it was matched as new, it is no more matched after several flights. Old or abused ones are too complex to write that as a post in discussion. Some rare chargers are designed with such packs in mind.
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Old Jan 02, 2007, 02:19 PM
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or just VOLTAGE CLAMP em just to keep it cheap and simple
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