Weird cell degradation

[BrianG]

Originally Posted by chrismechanic

one more fact i just thought of while i was just repairing a bouncy castle blower...DC voltage allways flows in one direction...this would explain why the packs are swollen at one end only..now i cant remember if its + to - or - to +.
durr.... the old grey matter is not as good as it used to be
chris

Most explanations of electric theory kinda assumes the flow goes from + to -, which is technically "hole flow". But electrons actually travel from - to +.

As to the OP; I still think it's more of a physical deformation than electrical. Maybe relative tightness of shrinkwrap in the center vs the ends? Maybe the straps slowly over time squeeze the middle more? Maybe an inner cell puffs under usage, and since they are all stacked up, the tightness of the heatshrink causes that bulge to squeeze the material of adjacent cells around? Since lipo cells are long/wide layers of material, any gas expansion would affect the entire surface area of the cell, and would tend to bulge in the middle.

[pinkpanda3310]

Originally Posted by chrismechanic

one more fact i just thought of while i was just repairing a bouncy castle blower...DC voltage allways flows in one direction...this would explain why the packs are swollen at one end only..now i cant remember if its + to - or - to +.

I was thinking that.

Originally Posted by BrianG

As to the OP; I still think it's more of a physical deformation than electrical. Maybe relative tightness of shrinkwrap in the center vs the ends? Maybe the straps slowly over time squeeze the middle more? Maybe an inner cell puffs under usage, and since they are all stacked up, the tightness of the heatshrink causes that bulge to squeeze the material of adjacent cells around? Since lipo cells are long/wide layers of material, any gas expansion would affect the entire surface area of the cell, and would tend to bulge in the middle.

That may be true and a contibuting factor but I don't think it is the sole reason for the 'one ended bulge'.

[_paralyzed_]

sounds and looks like what chris said to me, the internal tabs on the positive end are oxidizing first.

cut 'em open and see what's inside

disclaimer: don't really cut them open, you will die

hmmm...lipo surgery sounds like fun.

but a word of caution...lithium is alkali...and can cause burns to exposed skin.
not to mention the chance of still shorting out the pack (it will still have some charge in there).
and the fumes inside wont do the lungs much favours too...

you should see the corrosion on the lithuim...looks dull grey, turning black eventually...if you do decide to do an autopsy

it is rather odd i agree usually lipos puff the whole cell up...or at least all the ones i have seen so far...
chris

[pinkpanda3310]

If I get time this weekend I will operate.

[pinkpanda3310]

I took the top cell off today. Before doing anything I put the multimeter on it, the voltage bounced up and down like a yoyo. At first I didn't know what to think but after fiddling with the +tab (swollen end) I realized there was a poor connection inside the cell. Pressure it one way and the voltage was zero, pressure it the other and the v=~3.7. The movement within the cell seemed to better the connection cause it then held a steady 3.74v. So it is not quite as dead as I was anticipating.

How do I drain it? From memory I think your surposed to dunk it in salt water for a day or so.

yer..i read that too.
i had a 3s align 2200 heli lipo with a low cell...i took off the low cell and used the remaining 2s for my t8fg.

i put the low cell in fresh water for 2 days and it was still charged...so i stabbed it loads of times and put it back in for another 2 days...still charged .

in the end i had some fun by shorting it out, it still had enough charge to spark and make the tabs glow ....disclaimer....dont do it...it can be dangerous and cause burns......but its fun
chris