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Old Oct 05, 2006, 09:30 PM
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Tom Frank's Avatar
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Salvaging cells from different but similar packs

I have a couple of 3S packs with one cell going bad in each. One is a TPPL 910 mAh 16C pack and the other is a PQ 800 mAH 20C pack. The bad cell is making these packs unflightworthy. I have no planes that can benefit from just converting each to a two cell pack, so rather than tossing or just using them for servo set up etc., I'm considering the possibility of combining the good cells from the two packs to make a new 3S pack. While the cells are different, the discharge graphs from these at the same amp draw are not very different.

It would seem that:

1. During discharge while flying, one of the dissimilar cells may be depleted much faster than the others, risking possible over-discharge.

This doesn't concern me since the packs are of little use to me any way, but this risk can be minimized by setting my LVC abnormally high, conservative timing of flights, and/or landing at first sign of low power. The packs aren't worth enough to warrant the cost of something like a CellShield device from Dimension Engineering.

2. During charging, I use an LBA6 balancer, so this would keep the cells in pretty close balance and avoid over-charging any cell.

But I know things aren't always what they seem, so:

Has anyone successfully done something similar?
What are the flaws in my reasoning?

I wouldn't even think of doing something like this with cells that are much less similar in output, but if I can get further use from these cells in the air, I will do so.
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Last edited by Tom Frank; Oct 05, 2006 at 09:44 PM.
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Old Oct 05, 2006, 09:39 PM
Southern Pride
everydayflyer's Avatar
Haralson County GA. USA
Joined Oct 2004
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With one of these it would work just fine.

http://www.dimensionengineering.com/CellShield.htm

Charles
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Old Oct 05, 2006, 11:32 PM
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Tom Frank's Avatar
United States, MA, Walpole
Joined Dec 2003
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Quote:
Originally Posted by everydayflyer
With one of these it would work just fine.

http://www.dimensionengineering.com/CellShield.htm

Charles
Well, yes, Charles. But if you read my post I mentioned I didn't feel the cost of this device was justified for these small salvaged cells, when I could buy a brand new pack for just a few dollars more than the protection device. If I was dealing with packs that cost in the three figure range it might be a different story.

FWIW, if the pack manufacturers could ever agree to standardize on a tap plug, I think it wouldn't be long before some shrewd company came out with a CellShield-like concept built into an ESC so the LVC operated at the individual cell level rather than at the pack combined voltage level. And it would not surprise me if Dimension Engineering were that company.
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Old Oct 06, 2006, 01:44 AM
Dimension Engineering
Akron, Ohio
Joined Jan 2002
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But thats the beautiful thing about CellShield. It doesn't care that companies will never standardize on taps. It doesn't even care if you plug the taps in backwards. Unfortunately that takes enough microcontroller muscle to do that its not easily shoehorned into an ESC without going to a much more expensive and larger processor, or a dual processor setup.
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Old Oct 06, 2006, 02:18 AM
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Quote:
Originally Posted by Comatose
But thats the beautiful thing about CellShield. It doesn't care that companies will never standardize on taps. It doesn't even care if you plug the taps in backwards. Unfortunately that takes enough microcontroller muscle to do that its not easily shoehorned into an ESC without going to a much more expensive and larger processor, or a dual processor setup.
I wasn't trying to downplay the capability of your CellShield, and apologize if my comments came across that way. To the contrary, you have some very useful and innovative products, like the ServoSense which I have and use.

But for the $30 to $40 small battery packs in my case here which are on their last legs to begin with, and used on a small profile 10 oz. foamy 3D plane no less with all components hanging out in the breeze, it's just not a feasible solution due to cost, weight and "real estate" needed to mount it and the connecting cables.

Different plane, different lipo packs, different story!

FWIW, if I were flying bigger planes, I would look favorably on a more capable ESC with on board switching BEC and perhaps Cellshield as well. Even if the cost were slightly more than all of the separate components combined, the savings in real estate and cable spaghetti would be worth it to me.
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Old Oct 06, 2006, 03:04 AM
Boffin
rpage53's Avatar
Victoria, BC, Canada
Joined Apr 2001
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Quote:
Originally Posted by Tom Frank
What are the flaws in my reasoning?
Going to all that trouble to save 20 bucks.

Sorry couldn't resist. Chances are you'll kill another cell after a few flights but if you want to try go ahead and let us know how it works out. Usually when one cell goes the others aren't far behind if you've used a balancing charger.

Rick.
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Old Oct 06, 2006, 03:09 AM
Dimension Engineering
Akron, Ohio
Joined Jan 2002
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Not at all, I understand completely. I was just pointing out that, in a hypothetical "ESC with CellShield" it wouldn't matter if the manufacturers ever standardize on taps. Having a standard tap spacing would be a plus, but even that's relatively minor, as an ESC would need a tap extension to get from the ESC back to the taps anyway.

I was seriously looking at doing a line of ESCs, but then the Jeti Spins came out which did a lot of what I was looking to add. I don't like doing products where we can't push the state of the art a little.

Part of making our product line (specialty and niche products) is telling people "you actually probably don't need it for your project" so thats no problem at all. Of course, in this case you DO need one to do this, but 've got my share of $20 CSRC packs too...
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Old Oct 06, 2006, 03:15 AM
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Quote:
Originally Posted by rpage53
Going to all that trouble to save 20 bucks.

Sorry couldn't resist. Chances are you'll kill another cell after a few flights but if you want to try go ahead and let us know how it works out. Usually when one cell goes the others aren't far behind if you've used a balancing charger.

Rick.
Nah, I've already got new packs on the way. Being semi-retired with time on my hands, it's more of an exercise to satisfy my curiosity. If I can get some adequate flights out of the remaining cells, it's worth a little soldering time versus the alternative. These packs have "only" 60 or so cycles on them. I don't fly to the LVC, don't over amp them, and have been balance charging them. I thought they should do better. Maybe not...
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Old Oct 06, 2006, 04:10 AM
Registered User
Slovakia
Joined Apr 2005
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all that was said here is true,
I consider using cellshield as a must-have in case of mismatched packs,
but You can use any other way to asure that You will never discharge more
capacity than smallest cell has. Timing the flight for example with high reserve.

LBA6 is very good unit with perfect reputation, but it is "voltage equalizer"
type balancer. In case of different "C" rated cells the problem that equalizing
voltages of cells under charge creates unbalance, and as current drops there is
need to rebalance pack back at quite high bleeding current. It become much
more visible than with normal good pack. I could say that this is the job where
3C fast charge capable charging system (starting as cheap as Cellpro 4s)
becomes outstanding and uncomparable.
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Old Oct 06, 2006, 07:55 AM
Southern Pride
everydayflyer's Avatar
Haralson County GA. USA
Joined Oct 2004
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Tom,

Yes it will work fine if you will stop flying when the lowest voltage cell is 3.0 volts are greater under load.
Even new packs may have cells of different capacity and in my experience most especially the cheaper LiPolys which many seem to prefer.




The CellShield save help prevent damage to any LiPoly and the lower quality or condition the cells are the more valuable the CellShield becomes.

FYI I saw your reference to Cellshield after making my reply but I still feel the same.

One point I have never fully understood. Many say that this LiPoly only cost $18 so is not worth protecting , balancing , storeing correctly and now using a device such as a CellShield. My question is how much is the airframe,receiver, ESC, servos worth and how important is is not to cause property or personal damage / injury while you are flying.?


Charles
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Old Oct 06, 2006, 08:59 AM
Permanently Banned
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just make sure the good cells R still 90% capacity
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Old Oct 06, 2006, 09:16 AM
Registered User
Slovakia
Joined Apr 2005
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Quote:
Originally Posted by everydayflyer
One point I have never fully understood. Many save that this LiPoly only cost $18 so is not worth protecting , balancing , storeing correctly and now using a device such as a CellShield. My question is how much is the airframe,receiver, ESC, servos worth and how important is is not to cause property or personal damage / injury while you are flying.?


Charles
In addition, they missed the fact, that $18 is only the very first pack, and if they will fly to LVC (as defined by default in ESC, e.g. 9V overall), the packs will fail again and again, as second, third and any more packs are purchased. Then the price becomes higher.
I think that $30 for cellshield is quite high price , but we have to consider that business counts with much different numbers than parts and manufacture costs - the useable value and allmost no competitors are the reason..
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Old Oct 06, 2006, 01:54 PM
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Quote:
Originally Posted by everydayflyer
One point I have never fully understood. Many say that this LiPoly only cost $18 so is not worth protecting , balancing , storeing correctly and now using a device such as a CellShield. My question is how much is the airframe,receiver, ESC, servos worth and how important is is not to cause property or personal damage / injury while you are flying.?


Charles
You've added some new (and valid) concerns, and I certainly wasn't advocating the carefree attitude of the "many" you speak of. I agree with and practice all the protection measures you suggest for flying, charging, balancing and storing lipos up to the CellShield in this instance! And if I were going to be flying packs of mix-and-match cells all the time, I'd likely agree on the CellShield issue, too, but not for this one-off, try-it-and-see-if-cell-salvaging-is-worth-the-bother situation.

I've read and heeded advice from many of your posts, and I've always bought (arguably?) better quality lipos such as PQ and TP expecting to get better life cycle costs out of them. But I've only been flying lipos for about 1-1/2 years and my lipos are now apparently reaching "old age" after 60-70 cycles. I think some of the deterioration is cycle related and the rest time-related, but not from charging and discharging abuse. How do you get 200 cycles out of some of yours? You must fly that many cycles over a period of several months to avoid the time-related deterioration?

I fly with both timer and proper LVC setting (3V min., higher if ESC will allow) and land when performance visibly starts to deteriorates (i use "soft cutoff" on my ESCs... another safeguard). I charge with a balancer and proper charger (ICE), have never charged with wrong settings, use a sheetrock lined, vented metal tool box to charge in, and am always in the area when charging. I fly alone in empty fields for the most part. And I would never do what I was asking about with a larger and more expensive set up.

In this instance, the YAK airframe and ESSKAY 400XT motor were Hobby Lobby specials, $40 total for BOTH. Add another $70 for the CC Berg4 RX and T-bird ESC, and $45 for servos and I've got about $185 invested not counting the battery packs, which are already shot unless I can make use of the remaining good cells. I would expect a sudden battery deterioration in flight to cause loss of power and a dead stick landing, with little or no damage to this setup other than minor airframe damage based on powered crashes I've already had with it I suppose there's a very slight chance it could go down like a flaming comet (has this ever happened?), but I fly this one over a drainage retention pond area (much of which has water in it), so this is a risk I'm willing to take, even if all the power and electronics were destroyed during "rentry" or the subsequent drowning (I've dunked my float planes in fresh water with no damage).

So I believe in minimizing risk, but no matter how many safeguards you apply, a system can always be made ever safer, to the point where it becomes cost prohibitive (or is too bulky to fly ). I carry a sizeable liability umbrella rider insurance policy for the low frequency risks beyond that point.

When I get to the point of making up a mixed cell pack (assuming I don't run into cell tab soldering problems, etc.), I will do a test charge/discharge in a safe place to see if the pack can deliver enough amps and voltage under load to even warrant a flight test.
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Old Oct 06, 2006, 02:20 PM
Southern Pride
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Haralson County GA. USA
Joined Oct 2004
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Thing about posting info here is many will read only he last one or two post,hardly ever the OP and will disregard any and all safety warnings. That said yes your Frankenstein will work. I have cobbled such packs together many times. I did it to have first hand experience. I have a 3S 1000 mAh at present which has two cell from on vendor and one cell from another. I does OK but not great. These were the best remaining three of the original 6 cells (2 3S packs).

I understand not wishing to purchase a $30 CellShield for a single pack and I agree but fact is all of your packs would be better off.

You mention using 3.0V per cell or higher if possible. Well most of the newer cells need 3.2 or higher and the CellShield provides this option with ease.

I have flown a single LiPoly over 80 flights in one month but I also have LiPolys 1 1/2 to 2 years old with between 40 and 400 plus flights that are still doing very well.

Charles
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Old Oct 06, 2006, 03:09 PM
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Comments appreciated, Charles. 80 flights on the same pack in a month!... when do you sleep?

FWIW, the packs in question here were flown on a ESC which only goes up to 3.0V/cell on the LVC. I've read your 3.2V advice for these higher C-rated packs, and was hoping the soft cutoff and prompt landing when power pulsing starts might be enough... perhaps not. I'll lower my flight timer as an interim measure for my remaining packs.
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