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Old Dec 30, 2006, 05:02 PM
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Does slow charge lipo = balanced pack?

I'm flyin' a few old packs without balance connectors and 1 newer pack that "lost" its balancing lead. Will charging at ~1/3 C produce a balanced pack. I am amazed at the lifespan of my TP3S1320 Gen 2 packs. They are over 400 cycles and see rergular duty. What was electric flight like 4-5 years ago? I started this hobby 2 years ago.
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Old Dec 30, 2006, 05:10 PM
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No, I don't think you can do that. LiPo packs are not like NiMh and NiCd packs, where you can slow charge them until the lower voltage cells catch up with the charged cells. I believe if you try this with LiPo packs, the higher voltage cells will become overcharged and damaged.
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Old Dec 30, 2006, 07:48 PM
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My experience over the last few months (admittedly, with new packs) suggests that this does work. I've run four Apex 1350 packs through a total of about 380 charges, measuring individual cell voltages on each charge, and what I've seen is that for voltage differences of less than about 0.1V, charging at about 1/4C does seem to let the low cells "catch up" with the high ones. What I've seen repeatedly for these packs is that charging for about 15 minutes at 250 mA brings all cells to within 0.01V and I can go ahead from that point with a normal 1C charge.

Will it work with your packs? No guarantees, but it seems like a pretty good gamble to me.
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Old Dec 30, 2006, 07:55 PM
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Quote:
What I've seen repeatedly for these packs is that charging for about 15 minutes at 250 mA brings all cells to within 0.01V
What you are observing is the difference in imbalance between a discharged and more fully charged pack.

When a LiPoly is discharged imbalance will be at its' worse. Most often if you check a pack with cells at 3.5-3.7 and then charge at 1C until they are at 4.0 -4.1 you will see a major improvement in balance.

Several of us have slow charged LiPolys for 10 hours or more and the only improvement in balance was due to the voltage increase.

Ni batteries are balance by overcharging at a slow rate. This does not work with LiPolys.

Snip from a Feb. '05 post of mine. Note this was a very tired 6C pack.

http://www.rcgroups.com/forums/showp...64&postcount=2

Quote:
Discharged pack to 8.76 @ 1.25 Ah on AF109.
Added taps and checked voltages.
3.67
3.45
3.67
Difference 0.22 volts

Charged at 0.9Ah (0.75C) on Triton to 12.3(Li Ion setting.
4.09 plus 0.42
4.05 plus 0.60
4.09 plus 0.42
Difference 0.04 volts.
Charles
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Old Dec 30, 2006, 10:05 PM
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Quote:
Originally Posted by lammy1000
I'm flyin' a few old packs without balance connectors and 1 newer pack that "lost" its balancing lead. Will charging at ~1/3 C produce a balanced pack. I am amazed at the lifespan of my TP3S1320 Gen 2 packs. They are over 400 cycles and see rergular duty. What was electric flight like 4-5 years ago? I started this hobby 2 years ago.
To balance a NiMH/NiCd pack, the pack must be "overcharged" so that all cells reach a voltage of about 1.5V. The cells with a higher voltage reach the full charge level first and the excess charging current is converted to heat and if the overcharge current is low enough (1/10C), the battery will not be damaged. The cells that reach the full charged level just have to get rid of the excess heat (current) while the lower cells catch up.

Lipo cells can only be overcharged by charging at a voltage higher than 4.2V/cell. Unfortunately, the cells are damaged when the voltage exceeds 4.3V/cell and therefore, they cannot be balanced like NiMH packs can. With Lipo packs, the cells have to be charged individually or in parallel to balance the pack.
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Old Dec 30, 2006, 10:56 PM
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Charles, what I've been seeing is the imbalance dropping to near zero at the end of a normal charge cycle. To me, this pack would be "balanced". If this isn't correct, then what is the actual case? I think you have a more rigorous definition of what "balance" is than I do and I'd like to know what your thinking is. I've used balancers with my packs, and I can't see that they have any more effect than this natural "floating" process that I've seen. I suspect that perhaps the difference in voltage drops between cells is what you're referring to. If that's the case, then no balancer will correct a situation in which one cell has much higher internal resistance than its mates. A balancer can give you a final charge in which the unloaded voltages are very close, but one cell may sag badly under load. Is this what you're referring to?
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Old Dec 30, 2006, 11:12 PM
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About unbalance

Quote:
Originally Posted by roidspop
Charles, what I've been seeing is the imbalance dropping to near zero at the end of a normal charge cycle. To me, this pack would be "balanced". If this isn't correct, then what is the actual case? I think you have a more rigorous definition of what "balance" is than I do and I'd like to know what your thinking is. I've used balancers with my packs, and I can't see that they have any more effect than this natural "floating" process that I've seen. I suspect that perhaps the difference in voltage drops between cells is what you're referring to. If that's the case, then no balancer will correct a situation in which one cell has much higher internal resistance than its mates. A balancer can give you a final charge in which the unloaded voltages are very close, but one cell may sag badly under load. Is this what you're referring to?
Roidspop

Please take a read of the Li Po Handbook re unbalance and how it occurs.
http://www.fmadirect.com/tech_data/techdocs/
I believe that will answer your questions. Obviously unbalance is only a problem if it occurs. If you stay out of the "zone of temptation" that is less likely. However, as a pack begins to deteriorate with age or abuse, unbalance becomes very likely. Running a pack to cut-off all the time also exacerbates unbalance.
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Old Dec 31, 2006, 10:33 AM
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That's a very useful manual. How imbalance occurs and what the consequences are aren't an issue in this case, however. I simply stated what I've personally observed; a healthy pack showing a moderate voltage imbalance at the end of a discharge recovers balance at low charging rates. Period. If a cells have large differences in internal resistance, this "floating" process probably won't work because charging in series through the discharge connector would allow cell(s) to exceed 4.2V and be permanently damaged (puffed). Even the Kokam people are saying that a user ought to monitor the condition of individual cells using a special fixture and a DVM; modern balancing chargers give you that kind of information and automatically control charging very precisely. The fellow who started this thread doesn't have one of these chargers or a DVM rig and wants to know if he can slow-charge a pack and keep it in balance. It appears to me that, if the pack has NOT been abused, he CAN do this (if his charger works properly), but he'll be working in the dark. If he has no other alternative, then (1) set up the power system to limit current draw to 70% C (2) fly by a timer and NEVER go to LVC (3) slow-charge for about 20 minutes, then charge at 1C (4) cross your fingers.
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Old Dec 31, 2006, 01:32 PM
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But the question being asked IMO is "Can a pack that is out of balance when fully charged be put back into balance by slow charging?" The answer is no. Only by individually charging the cells can an unbalanced pack be brought back into balance. Unbalanced pack being defined as a lipo pack that is out of balance when fully charged.
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Old Dec 31, 2006, 01:48 PM
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I use SLOW charging becuz it reconditions or rejuvenate lipo packs better for longer duration and I use a self tapering CURRENT VOLTAGE CLAMP which put a stable 12.6V on 3S packs and stays there indefinitely

But, remember SLOW CHARGING does not balance packs which can only be done only by charging the normal individual cells in the packs separately thru the center taps

And, its a waste of time to balance DIMINISHED CAPACITY cells
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Old Dec 31, 2006, 01:56 PM
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In iammy1000's case, he can't know what the balance state is for the old packs he's talking about. And his question was whether charging at 1/3C would produce a balanced pack. The answer is "yes" if the cells are close to the same internal resistance. The answer is "no" if they aren't.
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Old Dec 31, 2006, 02:03 PM
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Quote:
if the cells are close to the same internal resistance.

See post #4. Slow charging has nothing to do with it. It is simplie a condition of the state of charge. Discharged max. imbalance ,charged min. imbalance .
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Old Dec 31, 2006, 02:03 PM
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The answer is an unconditional "NO", you can not balance a LiPo by slow charging. The highest voltage cells will be overcharged and most probably ruined by trying this.
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Old Dec 31, 2006, 02:21 PM
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If those Gen1 or lipo packs without balance taps or missing them have never been overheated or overdischarged, it's a good idea to only SLOW charge them to avoid imbalance and to keep them in a more stable condition like balanced charged packs

And, I have found that during the last 3 years, the best way to SLOW charge is a VOLTAGE CLAMP

I charge four packs of 3S 2100 mah all at the same time using only 2 amps max and the system tapers down to 200 mah and finally ZERO when done
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Old Dec 31, 2006, 02:28 PM
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And what good does a balancer do? It doesn't change the state of the cells, it simply artificially brings the voltages to the same level...but the internal problem is still there. When the pack reaches discharged state, as Charles says, the imbalance will be at the max. Seems as if the most important role the balancer plays is at the very end of the charge, where it would be fighting to keep the high cells from going past 4.2 V.

As to the "unconditional 'NO'", I've seen otherwise. I've watched cells go from a state of being out of balance by several hundredths of a volt to being within a hundredth after a quarter of an hour at a quarter of an amp...and at the end of a normal charge past that point, they reached normal terrminal voltages without damage...as actually witnessed on my DVM. Nothing about the internal condition of the cells changed, but going by the only criteria I've got...measured voltages...those cells balanced. So the answer may not be an unconditional "no"; I'm simply speaking from my own experience and waiting for somebody with more experience and less ignorance to enlighten me otherwise...but I notice that iammy has vanished, so what's the point?
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