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Old Jan 20, 2004, 01:55 PM
Nimble with Gimbals
San Diego, CA
Joined Apr 2003
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Lithium Polymer Cell Imbalance: A Mitigating Solution

With the buzz going on regarding lithium polymer fires I decided it might be time to start thinking of ways to mitigate cell imbalance risks with pre-made packs from the likes of Kokam, Thunder Power, and E-Tec. I got to thinking about my own experiences with these packs and I've started to see some major differences.


First, some background. I fly with two other gentlemen that own 10 Thunder Power 2100 packs in 2s1p and 3s1p configurations. They both charge them with Astro 109's and have been doing so for nearly 6 months now. In all of that time, we've never flown these packs to less than 3.7V per cell at rest. Not once. As of yet we haven't had a swollen pack, a fire, or any reduction in capacity whatsoever.


Now, we can explain this in a few different ways:

1. Thunder Power lithium polymers are somehow "stronger" or more durable than all of the other brands. I find this somewhat doubtful.

2. Our policy of only discharging to above or at 3.7V per cell has kept these packs in excellent health and perfectly balanced. We still enjoy neck-tiring flight times but land far before we have to.

3. We've simply been lucky and a swell or catastrophic cell failure is inevitable.


Frankly, my money is on #2. Since we all seem to have difficulty agreeing on things like LVC's and minimum cell voltage, I propose that we use this practice as a guideline for good lithium polymer health.


There are a number of very knowledgeable lipoly experts on this board and I would appreciate their input regarding my proposal.
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Old Jan 20, 2004, 02:30 PM
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Arnold , Maryland, United States
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Hold on now A. In number two, you say the cells are perfectly balanced.
They may well be, but you did not say that you measured the voltage of each cell.
Without voltage comparisons, you don't know they are balanced.

I believe that undetected imbalance, may be the cause of some Lipo fires.

I also think that hard discharging, causes some balance problems.

It sure sounds like you guys are taking it easy on your cells. So I would think that your packs ARE balanced, but you don't KNOW until you measure them.
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Old Jan 20, 2004, 02:41 PM
luc
I plant balsa sticks too
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France
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sure it is better not to overdischarge the packs: it has also other advantages such as lower temp problems and no problem of cutoff, easier landing (can do another approach...)
Nevertheless, 3.7V per cell leaves 40% inside the cell which is in my view quite a lot. One can ask why having these batteries to only use 60% of them...
Personally, I leave 20% and I think it is a better compromise
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Old Jan 20, 2004, 02:52 PM
Nimble with Gimbals
San Diego, CA
Joined Apr 2003
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UglyStick:

You are correct. I am simply assuming that the cells are balanced because we haven't had any incidents yet. I would think an imbalance would show up in the hundreds upon hundreds of charges, though.

luc:

I'm proposing this as a stopgap measure until one of two things happen:

1. ALL lithium polymer packs go the way of Duralite and come with charge/discharge circuitry from the factory.

2. Pack makers offer to retrofit non-tapped packs for a nominal fee.

Either of these two meaures will remove *much* of the danger involved in lithium polymer packs. Until then, I feel confident about my proposal.
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Old Jan 20, 2004, 03:16 PM
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Thunder Power philosophy

Charlie Wang (Thunder Power) chose early on to go with close manufacturing tolerances, closely matched cells, supplied in carefully balanced packs only (no individual cells). The record shows he has been quite successful with that formula.

Nonetheless, he is moving toward safety circuitry in addition to his careful pack construction. His latest posts promise safety circuitry with at least some of his prospective newest generation cells, and Duralite packs are around built Thunder Power cells with Duralite safety circuitry.

So, I'm not surprised when I see reports about T.P. pack reliability.

- RD
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Old Jan 20, 2004, 10:14 PM
Registered User
Arlington Tx
Joined Dec 2003
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Lipo tanic taps

Just reading about Cell balance,
I just received my Tanic 2200 3s's 11 volt Lipos
and discovered the battery comes with Tanic taps
for cell access and balance charging, Had no idea Tanic
inc this with their product, comes with short 2 wire connector
that you can install a ultra deans for your lipo charger,
Now I have to get a 3d foamie built that this Battery will
Fit, Note! look at size of these packs and their weight!
They are as big as my Fuse is Tall, and heavier than I expected!
thanks, LJ
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Old Jan 21, 2004, 09:25 AM
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Arnold , Maryland, United States
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Awake, I'm pretty confident in your procedure too. With your moderate discharges, I think you will be fine for some time.

I was just trying to be helpful by pointing out that balance problems sneak up on us, and we need to check for it occasionally.

I built some e-tec 2S packs from balanced cells. After a dozen or so uses, I rechecked them. I don't remember how far they were off, .1 to .15 maybe, but they were off some. I rebalanced them, and after more than a dozen flights they are still within .04 v.

.1 to .15 is not much of a difference, but it grows with series charging.
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Old Jan 21, 2004, 03:38 PM
YVR
Sean in YVR (Vancouver)
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Forgive my newbie question, but I'm curious:

Am I right that the problem of cell imbalance ONLY arises in the case of a pack that is in parallel, or do individual cells in series only (e.g. 3s1p) become imbalanced as well?
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Old Jan 21, 2004, 03:44 PM
FLYER spelled I-squared-R
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Series imbalance IS the problem.

For example, in a 3S pack, if one cell is overdischarged and fails (won't take a charge), then you apply 12.6 (4.2x3cells) across the remaining 2 cells. These cells see 6.2 volts and burn up.

Parallel charging to 4.2v is almost inherently safe.
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Old Jan 21, 2004, 03:55 PM
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YVR,

The problem of pack imbalance ONLY arises in the case of a pack that has cells in series (e.g. 3s1p).

- RD
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Old Jan 21, 2004, 04:11 PM
Nimble with Gimbals
San Diego, CA
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So the quick-and-dirty way to solve the balance problem is to request that pack makers construct packs with a parallel-wired charging lead and a seperate series-wired discharge lead?

While the Tanic Taps are a large step forward, it seems to me that the above solution is as close to perfection as we're going to get.
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Old Jan 21, 2004, 11:48 PM
luc
I plant balsa sticks too
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Yup, that's my way of doing it (on the big packs, as this method is a bit cumbersome for small packs in tiny fuses)
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Old Jan 22, 2004, 01:01 AM
Dax
There is no spoon
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Vancouver BC Canada
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I think people are being WAY to affraid of lipos. The LHS here is a crazy old guy who has been into RC cars sor 25 years and only recently started flying electric in the last 2-3 years. I told him about the LIPO thing way before he heard about it, showed him my home made kokam 1020 packs before he could order lipos. Then one day HIS supplier informed him he could get premade Kokam 1500Mah HD packs in 2 and 3 cells. He has been ordering them NON STOP since then.

Here is the problem. They are old car guys. They know NiCd and NiMh. They are used to charging packs at 5 amps for 10 mins then throwing them back in the car and running them dead flat. They treat their Lo-Po's like any other battery. They fly till their ESC hits cutoff they charge them at 1.5c they generaly abuse the hell out of the packs, wire them in series and paralel without properly balancing, etc.

Not one pack has balooned or caught fire. They don't listen to me. I'm waiting to see a plane burn down, but I haven't seen it yet. I think unless it shorts it should be ok.
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Old Jan 22, 2004, 07:06 AM
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Arnold , Maryland, United States
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Well, just because it hasn't happened to them (yet), doesn't mean it won't happen to you.

Better safe, than sorry.

Continue to use proven, safe, procedures, and you won't regret it.
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Old Jan 22, 2004, 08:14 AM
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Germany
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Re: Lithium Polymer Cell Imbalance: A Mitigating Solution

Quote:
2. Our policy of only discharging to above or at 3.7V per cell has kept these packs in excellent health and perfectly balanced. We still enjoy neck-tiring flight times but land far before we have to.
I agree. If you charge the cells slowly and discharge only to 3.7V (no load voltage), cell imbalance takes much longer to develop.

My theorie is that self discharge and deep discharge are our main enemies.

I will change the default setting of my LiPo Voltage Monitors to 3.3V per cell (they support 2-10 cells and a threshold of 3.0 to 3.9V / cell.
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