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Old Dec 31, 2006, 02:48 PM
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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.
I can fully understand this however my point is that the same thing would have happened even if they were being charged at 1C.

What purpose does a balance serve? Make less than perfectly matched groups of cells connected in series safer to charge and lets one squeeze a bit more performance and life from them. A balancer can also prevent a cell or cells from being damaged by an overcharge.

Charles
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Old Dec 31, 2006, 02:58 PM
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that usually depends on how much the imbalanced cells have been overdischarged and overheated and they usually R < 80% DIMINISHED CAPACITY

now U can compare that with a normal healthy pack using a AFI watt-hour( or WATT-SEC ) meter to measure the " PUNCH " that the pack can deliver
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Old Dec 31, 2006, 03:07 PM
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Quote:
Originally Posted by roidspop
... 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. ...
From several hundredths of a volt to a hundredth of a volt is a pretty fine measurement for typical home measuring devices, so there's obviously a +/- factor to be considered. Ideally, you'd run similar tests at various charge rates to see if the variance was due exclusively to the rate of charge. It will never be as accurate as a climate-controlled lab with the best (most expensive) calibrated measuring devices. But careful home testing can yield reasonable results.

The danger, of course, is to imply to someone with no measuring devices at all that slow charging Li cells can produce a balanced pack. This could create a false sense of security and lead to safety problems. So any answer to the original question should leave no room to be interpreted as a "yes."
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Old Dec 31, 2006, 03:13 PM
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Quote:
Originally Posted by Rodney
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.
My practical experience proves this incorrect. My packs without balance taps have been charged many many times and if slow charging hasn't helped these packs then it certainly hasn't hurt them.

Also, my prolites and extremes are often "slow charged" and very rarely go out of balance. I was beginning to question my balancer until I checked packs w/DVM.
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Old Dec 31, 2006, 03:35 PM
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Possibly I can illustrate this in an opposite sort of way. I'll start out by saying that a slow charge (or any charge)will not cause the cells to self balance (like a nickel pack)

The reason they are appearing in balance at the top is because they were never out of balance at the top. They are only out of balance at the bottom. Now for illustration:

Balance the pack perfectly at the bottom (at 3.5 - 3.7 voltsper cell) When the pack is in it's discharged state and in perfect balance it will stay "in balance" until it is charged and the opposite will occur. A slow charge will now unbalance the cells when they are full. In fact any charge will unbalance the cells when they are full. This can be easily observed and repeated.

So, I can say that a slow charge will "unbalance" my series cells every time I charge them because when I am done with the run and check them they are in perfect balance.

The answer has been to either use a balancer or to use exactly matched cells. If exactly matched cells are put together in series they will be balnced at the top and the bottom. If this doesn't illustrate then I have a demo using clear cups of water of varying size to illistrate it.

My NIckel demo cups are either the same size with slightly different sizes "leaks" in them and they are all nailed to a common board simulating a series charge or discharge

The Lipo cups have tiny tiny leaks that are insicnificant, but are not all exactly the same size and will show the balance very visually.
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Old Dec 31, 2006, 04:10 PM
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1. If a lipo pack is balanced, it doesn't make any difference if it is slow charged or fast charged, it will end up balanced.

2. If a lipo pack is unbalanced, it doesn't make any difference if it is slow charged or fast charged, it will still end up unbalanced.

A pack that when charged has all cells within +/- 0.03V is a balanced pack. This is true whether the pack reached that state by itself, by parallel charging, by using a "blinky", or by using a balancing charger.
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Old Dec 31, 2006, 04:38 PM
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Charles...good point about charging an out-of-balance pack at 1C; I haven't because I know that the low-rate charge SEEMS to work. We all tend to go with what works. I'll try it the next time I have a pack that is out of balance.

Hoppy...I think we've got a semantic confusion here. You're using "balanced pack" in the sense of "near-zero difference in internal resistance/chemistry". If so, you're probably right about your point #1. The question that might be raised is "if a pack is balanced and at the end of a discharge, a voltage difference exists among the cells, it is (still balanced? Unbalanced?) Which is it? I think most of us would say it's "unbalanced"...I know my TP 205 thinks a pack in that state is unbalanced and tries to bring the high cells down to the low one. So, at least Thunder Power balancers think that voltage differences constitute an "imbalance". TP even says to start with a slow charge and then go up to 1C. Are they wrong? All I'm adding is my observation that a discharged pack with a voltage difference doesn't necessarily have to be actively balanced to get the cells within about a hundredth of a volt. Charging at 250 mA for fifteen minutes has produced cells at around 3.7-3.8V plus or minus a hundreth. I have seen this too many times to just blow it off as some instrumental error or ignorance on my part. It's a real effect.

As to Iammy's packs, the point about not being able to measure voltages and the inadvisability of charging is well taken, but he's an adult, he can take all this stuff under advisement and do what he thinks is worth doing. He knows what the risks are...anybody who's flown lipos for a while knows about burned-up cars and burned-down houses. So, he'll surely do his charging in a safe place, right? Sure. If he doesn't, well, the consequences will be his to deal with. If some noobie reads this, they'd be well-advised to take the most conservative approach to charging and flying lipos! Use a balancer! Use a DVM, too! Know how much power your setup draws at WOT and what the REAL state of your pack is! Charge in a fire-proof container! All of that. The rest of us...well, maybe we burn down a house or two...
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Old Dec 31, 2006, 04:56 PM
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There are two separate issues being discussed, and they have two different answers:

Yes, it is believed by many that conservative charging and/or conservative discharging will be less likely to cause a Li pack to go out of balance than aggressive charging and/or aggressive discharging.

No, slow charging an out-of-balance Li pack will not cause it to get back into balance.
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Old Dec 31, 2006, 05:02 PM
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Normal Cell Internal Resistance

can better be reduced and maintained during SLOW CHARGING becuz each cell in a pack will become more voltage stabilized when it reaches close to 4.2v and charging current is at zero

a VOLTAGE CLAMP will give U these results and not any standard commercial LIPO charger balancing or otherwise and that's why I use it most of the time
when not 1C charging at the field

I have used most of the common commercial LIPO chargers and not one of them will stabilize a lipo pack at 12.6V at zero charge current like the VOLTAGE CLAMP
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Old Dec 31, 2006, 05:10 PM
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[As to Iammy's packs, the point about not being able to measure voltages and the inadvisability of charging is well taken, but he's an adult, he can take all this stuff under advisement and do what he thinks is worth doing. He knows what the risks are...anybody who's flown lipos for a while knows about burned-up cars and burned-down houses. So, he'll surely do his charging in a safe place, right? Sure. If he doesn't, well, the consequences will be his to deal with. If some noobie reads this, they'd be well-advised to take the most conservative approach to charging and flying lipos! Use a balancer! Use a DVM, too! Know how much power your setup draws at WOT and what the REAL state of your pack is! Charge in a fire-proof container! All of that. The rest of us...well, maybe we burn down a house or two...[/QUOTE]


Wow. Now I have to worry about burning my house down because i am charging older, unabused lipos at <1C? Please, all experienced long term electric flyers (lipo) set this straight.
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Old Dec 31, 2006, 05:21 PM
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Yes, you sure can! You don't really KNOW what state those packs are in, do you? If you get complacent and just toss the thing on a work bench, plug it in and walk off and forget it, it's entirely possible that'll it'll give you a surprise you won't forget!

Charge in a lipo vault of SOME sort...lipo sack (on a fire-proof surface!), ceramic bunker, ammo can, whatever, but it's smart to assume the pack WILL do the worst thing possible at the worst time. That's prudent!

Keep an eye on it. Don't just forget about it. As the charge progresses, check the temps (by hand or with a thermometer).

Guys HAVE burned down houses (check around in the forum) and cars (ditto), so it's a real possibility.

BUT...the vast majority of us HAVEN'T had anything like that happen, so some of us think it CAN'T. Hey...maybe they're right. I just play paranoid and can my packs when I charge...and hope I'm paranoid enough!
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Old Dec 31, 2006, 08:09 PM
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"The question that might be raised is "if a pack is balanced and at the end of a discharge, a voltage difference exists among the cells, it is (still balanced? Unbalanced?) Which is it? I think most of us would say it's "unbalanced"..

roidspop
IMO, What the voltage differences are in a discharged condition is of much less importance as long as any individual cell does not go below say 3V as long as the pack comes back into balance on charging. The balance condition at full voltage is the critical element as an unbalance here could drive a higher voltage cell into an overvoltage condition and possibly cause cell damage and possibly a fire.

Question:
If a pack that is balanced at full charge and unbalanced at discharge is balanced while discharged, what will the balance condition be when it is then fully charged?

PS - Your safety recommendations are right on the target.
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Old Dec 31, 2006, 09:41 PM
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Er...huh? Ok, it's unbalanced when discharged...we balance it and charge. Is it balanced at the end of the charge. Right?

It could be balanced or unbalanced. I don't know how it REACHED balance at full charge.

If it started out balanced BECAUSE OF A BALANCER'S INTERVENTION, then it seems to me we've got a sick cell and unless you balance actively during the charge or right at the end, it can wind up unbalanced and very possibly puffed.

If it was balanced initially without any intervention, I'd expect it to reach full charge in a balanced state, given that we balance it before going on with a 1C charge. Now...if one cell was close to 3V and we do normal balancing (ie...bleeding), then we can kill the pack right then and there. If we do a slow charge with that initial imbalance, I can tell you that it's going to balance itself by about 3.7V, and it'll go on to balance at 4.2V without any further tinkering on our part. Charles says it would happen without the slow charge phase. He may be right...I definitely pay attention to that guy!

We agree about one thing...the very last stage of charging is where people puff cells and have "venting with flames". I can imagine a balancer/charger that goes sailing along right up to about 4V and then gets really, really picky about individual cell voltages and shuts everything off when ANY cell hits the magic number...that, or it charges the cells in parallel individually...same effect. Wait. That's a modern charger, after all, right? It does what a careful user would do if he were personally monitoring each cell during a charge. I've got a Celectra that does a good job of charging in series and shutting off when it should...but I know now, thanks to discussions like this, that if one cell lags behind its buddies just enough, the high one can get puffed! What I've seen is that, if the cells all leave 3.7V at very close to the same voltage, they're going to reach 4.2V at the same time, or within a couple of hundredths of a volt. I haven't seen one go past 4.2V yet.

Anyway, happy New Year!
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Old Jan 01, 2007, 06:50 AM
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"It could be balanced or unbalanced. I don't know how it REACHED balance at full charge."

Assume it balanced by itself. It could be a typical lipo pack - balanced when fully charged with some unbalance when discharged.

"If we do a slow charge with that initial imbalance, I can tell you that it's going to balance itself by about 3.7V, and it'll go on to balance at 4.2V without any further tinkering on our part."

But what if you balanced this pack with a blinky when it was in the unbalanced discharged state? Wouldn't that action create an unbalanced pack when fully charged?

I think it comes down to that the unbalance at full charge is the only unbalance to be concerned with.

Time to fly...... Hope your winds are less than mine.
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Old Jan 01, 2007, 08:58 AM
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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
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