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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 02-24-2011, 07:50 PM   #1
Terry.0132
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Default Changing 3S to 2S on Faulty LiPo

After having had good service out of my LiPos for a couple of years, because of lack of attention on my part two fairly new (4months old) LiPos failed on my, I say again I was at fault not the LiPos.
The one is a 3S 2200mA and the other a 3S 1300mA both are 20C. What is happening is on my cell checker the 3rd cell on each battery is no longer holding charge. The 3rd cell being the one where the +(red) power line comes off.
My question is, how difficult (unsafe) is it to remove the shrinkwrap, remove the dud cell rewire the +(red) lead to the 2nd cell and then rewrap and use as a 2S, knowing that i will then be charging only through the Deans connector and possibly not having a balance lead.

How difficult is it, has anyone done it before, does anyone have hints on doing it. I really do not wish to throw away the batteries knowing that 2cells on each are still perfectly usable. A few of my fling wings would be better rated to flying with 2S.

Thanks
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Old 02-24-2011, 08:11 PM   #2
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I've done it. There is not pat answer because the particulars of lipo construction but the general makeup is the same.
I'd take the voltage down to ~20% to increase the safety margin.
Proceed carefully and slowly-makes sense eh?
If you pay attention to connections you can even get a new end for the balance port and make a 2S balance.
I've never soldered onto the lipo sacks with the aluminum soldering stuff. There has been enough of a solder tab left to work for me. If it became necessary at that point I'd chuck it myself.

If the lipo got hosed because of a broken tap wire (not uncommon), this may be a reasonable way to go. If the entire lipo (all cells) got abused what you are seeing is the weakest link. Chances are the rest are close behind and not worth bothering with.

fly
If you're going to learn to fly them, you have to learn to fix them.
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Old 02-24-2011, 09:50 PM   #3
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There may be hope if the voltage difference between Cell 3 and the other two cells was say within 0.50 volt when you took them out of storage. So before dismantling the two packs try the following. Make up a 2-pin charging patch cord. Balance charge your questionable packs at 0.50C(0.65a) to around 4v per cell and stop. Then charge Cell3 to 4.15 volts, thereby hopefully increasing its mAh capacity more than that of Cells 1&2.

Then while bench discharging check each cellís voltage around 10 volts before throttling down at 9.6 volts. Record individual resting cell voltages a couple minutes after throttle off. After a couple discharge/charge cycles the cell voltages may be close enough to balance charge to 12.4v, so that a LVC of 9.6v wonít result in Cell 3 being less than 3.00 volts. You may have to continually baby these two LiPo batteries with some individual cell attention.

Hopefully, you should be able to get more life out of these two LiPoís with some TLC (TenderLovingConditioning) without having to dismantle them.
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Old 02-25-2011, 09:31 AM   #4
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Thanks for the promt replies, Swift 428, I have tried babying the cells and trying the individual attention, but each time after leaving the batteries for 24 hours the 3rd cell has again dropped to about 1.0V and after 48 hours to 0.5V, with cells 1 and 2 remaining at 4V.
flydiver, I will try the cell separation and conversion to 2S, if I remember I will photograph the stages I go through although I will not hurry as I've a couple of builds I want to finish, during our very windy season. (had a S'Ester blowing here almost constantly (15Kts+)for last three months, makes for dicey flying but has occasionlly dropped to about 10Kts (+_20kph) which makes for exiting flying)
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Old 02-25-2011, 08:41 PM   #5
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Gota? ... Maybe you, flydiver or someone knows the answer or has a pretty good guestimate.

When you dismantle a couple 3S packs to makeup another 3S or 4S pack does it make much difference which cells are in the middle. For example let's assume one(for remaking a 3S pack) or two(for remaking a 4S pack) of the reusable cells have *slightly less capacity, bounce back recovery, or more IR or whatever (in other words they aren't quite up to par with the other two(3S pack or 4S pack).

Is there any justification/reason for placing the *weakest cell(3S pack) or cells(4S pack) in the middle of the series 3S/4S pack or vica versa the strongest cell(s) in the middle of a 3S/4S pack?

Also, when I once took apart a PZ 2200 3S with a bad cell I noticed some tiny circuitry. I just removed it, rewired and found after charging the questionable cell wasn't bad afterall. Can someone explain the purpose of that little circuitry in some LiPos ...

Edit: ... spent a few hours last night partially dismantling and rewiring two packs ... each having 4-AAA NiMHs ... out of two R.S. walkie-talkies in which two of the cells in each pack had 0 volts and neither pack would hold a full charge. Well, finally was able to fix the problem and recharge the poor cells back to health with my Vision Peak Ultra. So there still may be hope for your 3S LiPo pack.
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Old 02-28-2011, 12:49 AM   #6
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Originally Posted by Terry.0132 View Post
After having had good service out of my LiPos for a couple of years, because of lack of attention on my part two fairly new (4months old) LiPos failed on my, I say again I was at fault not the LiPos.
The one is a 3S 2200mA and the other a 3S 1300mA both are 20C. What is happening is on my cell checker the 3rd cell on each battery is no longer holding charge. The 3rd cell being the one where the +(red) power line comes off.
My question is, how difficult (unsafe) is it to remove the shrinkwrap, remove the dud cell rewire the +(red) lead to the 2nd cell and then rewrap and use as a 2S, knowing that i will then be charging only through the Deans connector and possibly not having a balance lead.

How difficult is it, has anyone done it before, does anyone have hints on doing it. I really do not wish to throw away the batteries knowing that 2cells on each are still perfectly usable. A few of my fling wings would be better rated to flying with 2S.

Thanks
I have saved several lipo packs as well as made up new ones

It is not difficult or dangerous at all if you pay close attention to the polarities of the individual cells and do not over heat the solder joints removing the connecting wires and resoldering them ( you would not want to melt or puncture the plastic bag that covers the battery) If there are tiny circuit boards I would remove them and re- assemble your packs without them.

If you lack the skill to do your own soldering then enlist the aid of somone that is experienced

Roger aka GIFLYRC

Independently Poor
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