Li-poly failures without physical damage - RC Groups
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Oct 26, 2003, 10:20 PM
Registered User

Li-poly failures without physical damage

My li-poly batteries seem to fail from over current or perhaps forgetting to charge before flying again. I just lost a pack of 3S2P Kokam 1020s. The pack has been cycled about 30 plus times. At max throttle, it is a 4 to 5C application. The problem may have been failing to charge. Several months ago, I lost capacity in 2 packs of Kokam 2S 1500s probably from an increase in drag on the power system that resulted in an increase in current.

The failure scenario seems to be as follows:
1) Unexpected slight drop of power in flight.
2) Pack takes a significant charge
3) Much earlier into flight there is a drop in power.
4) Pack charges about 1/4 full and discharges about the same amount.
5) Pack never delivers enough power to plane again.

My questions,

1) Is there a definitive test for dead cells?
2) Does over usage always take out every cell in the pack?
3) There were some old stories about reviving Li-poly cells. I assume that they are false?

These cells are not puffy and have no physical damage.

Is there a definitive answer/source on these questions?

Thanks, Richard
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Oct 27, 2003, 07:44 AM
Old Desert Rat
Arizona Chuck's Avatar
5C is pushing them, but if they are not hot and start to swell, I would say you are OK there. Sounds more like you may have run then down too far.
I would take the pack apart and charge the cells individually, you will be able to tell if one is bad or all of them. There is always the possibility that one cell just went bad on it's own, no fault of yours.
Be very careful taking the pack apart if you want to keep all your fingers!!
Last edited by Arizona Chuck; Oct 27, 2003 at 07:49 AM.
Oct 27, 2003, 08:18 AM
Space Coast USA
hoppy's Avatar
That is a max continuous 3C cell.
Oct 27, 2003, 08:38 AM
Senior Moment Member
hoppy is right, that's too high a discharge for them.

Continuous stress will cause them to lose capacity, even if they don't fail completely. They reach the point where you can't get much in, so you can't get much back out. A short charge and they are at max, and a short discharge and they are back to minimum.

Oct 27, 2003, 10:09 AM
Registered User
More information:

The pack that failed was made of 2 sets of 3S. To look for a failed cell, I split the 3S2P back into a 3S and noticed that both of the 3S packs had the same poor capacity. Is this typical? Is it a waste of time to break apart the pack and look for some good cells to reuse in another pack?

The 1020s were used at mostly less then 3C. The plane hovers at 2.5C. However, I have probably abused these cells. Now, I try to buy high C rated cells and design/prop to never approach there max rating.

I am getting the idea that this is the standard failure scenario for li-poly.

Thanks, Richard